Making LED headlights RF-quiet

TL;DR

 If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem:  You will probably need to put the offending switch-mode LED controller in "RF Jail" as described below.  While VHF/UHF is the target of the efforts noted below, they should be equally effective at HF.

* * * * *

We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy.  These devices have proliferated in the past several decades and surround us at home, work and in our cars.

Figure 1:
The vehicle in question with LED headlights -
which are now RF-quiet on all bands.
Click on the image for a larger version.

Generally speaking, modern internal combustion vehicles are in RFI terms "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home.  For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.

Effective mobile operation is therefore contingent on a vehicle that is intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well.  These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers¹ , GPS receivers and extra lighting ², to name but a few.

Real-world case - Aftermarket LED headlights

A friend of mine recently installed aftermarket LED headlights in his older Honda CRV ³.  It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters:  Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when communicating simplex (e.g. direct - with no repeater) with a weak/distant station - turning on the headlights dramatically reduced received ⁴ signal quality and range.  As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.

LED headlights typically consist of two modules:  The LED module itself and the "controller" - which itself is a switch-mode power supply to regulate the current to the LEDs as well as switching arrays of LEDs for high/low beam - and even color temperature in some models.  Sometimes these units are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.

Even knowing that it was futile, we tried putting clamp/snap-on ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful".  This was not unexpected:  Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant:  It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!

Methods of filtering

As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed and ferrites alone are likely to be effective only when the interference is very slight to begin with.  Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations:  Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.

As discussed previously on this blog ⁵ one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every wire to prevent RF energy from being conducted and the metal enclosure can prevent direct radiation from leads themselves - particularly important at VHF/UHF but less-so at HF.  Done properly, this method can be "completely" effective ⁶ in preventing interference.

Figure 2:
Simplified diagram of the method of filtering.  The inductance - provided by the ferrite beads ("L")
provide "choking" impedance to the RF currents being carried on the wires from the LED controller
(the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize
their efficacy.
Click on the image for a larger version.

Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT").  The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor.  The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.

The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device.  By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure.  The above configuration is easily capable of attenuating RF energy from HF through UHF by 30dB or better (more likely 50-60dB at some frequencies) - a value far higher than ferrite alone.

The metal box containing the electronics and filtering offer another important benefit:  As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.

An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors.  As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy.  If the ferrite beads - or some sort of inductor - had not been installed, the shunting of the low-impedance RF energy from the noisy device would have been far less effective.

Figure 3:
An assortment of feedthrough capacitors.  The top two
rows are of the "screw-in" type, typically mounted to chassis-
walls and bulkheads while those on the bottom row are the
"solder-in" type as used in the partition in this project.
Click on the image for a larger version.

In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a sensitive device FROM RF energy from a nearby transmitter we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.

Feedthrough capacitors and the partition

A bit more needs to be said about "feedthrough capacitors".  Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past.  Figure 3 shows an assortment of feedthrough capacitors:  The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.

A feedthrough capacitor has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself.  By being constructed this way, there are no wires or leads between the "capacitor" part  and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.

Figure 4:
Solder-in type feedthrough capacitors soldered
to the brass partition.  This large sheet of metal
provides a low-impedance RF path to the
common "ground" (e.g. case) to contain RF
entirely within the metal case.
Click on the image for a larger version.

Compared to a "normal" capacitor with wires, such a capacitor is far more effective at bypassing RF energy to "ground" and it also suffers much less from parasitic issues like self-resonance - a property in which the capacitor and its internal inductance form a resonant circuit can cause it to practically "disappear" from the circuit (e.g. cease to be effective) at certain frequencies.  For this application - where it's particularly important to reduce RF interference at VHF and UHF - the use of feedthrough capacitors is - along with some inductance - a nearly foolproof method of attenuating such energy without resorting to surface-mount components and/or a specially-designed PC board. ⁷

Figure 4 shows nine feedthrough ⁸ capacitors soldered to a brass partition (the soldering to the plate is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them.  As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal.  This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.

Putting it in the box

Figure 5:
The LED controller in the box w/filtering.  By
containing RF currents within the box, both
common-mode and differential RF currents
on the leads are reduced to near zero.
Click on the image for a larger version.

To eliminate direct radiation from even the very short leads, the LED's controller which can cause problems at VHF/UHF, ferrite beads ⁹ and feedthrough capacitors are contained within a box as seen in Figure 5.

Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.

Below the partition (see Figure 5) are the wires that connect to the outside world:  On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module.  The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition and install the rather large ferrite beads.

The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering.  As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.

Figure 6:
As the original cables were very short (about 5", 13cm) longer
wires had to be spliced to allow placement of the large die-
cast box.  This shows the male headlight connector and the
LED module spliced to the cable and covered with nylon web.
Click on the image for a larger version.

At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.

The result

With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.

The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID:  The return signal was very weak and noisy - as hoped and expected - but there was no difference in the way that it sounded when the lights were switched on and off.  While admittedly unscientific, this test tells us pretty much everything that we needed to know:  Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!

* * * * *

Footnotes:

1. The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see:  "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article:  "A 'quiet' 5 volt USB car power supply" - link.
   
2. This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards.  Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
3. Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
4. A low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
   
5. Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits.  This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system.  This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
6. "Completely" eliminating conducted RF is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
7. The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation.  "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.  With the solder-in feedthrough capacitors, a brass partition was used as it was easier to drill and solder to than one of copper.  If screw-in feedthrough capacitors were used, the partition could have been aluminum.
8. Nine wires are needed for the headlights:  Three wires are from the vehicle itself (low beam, high beam and "common") with two more wires for the fan and two each wires for the pair of LED emitter arrays.  As an aside, this and many other retrofit LED headlights are polarity-insensitive (e.g. it doesn't matter if the "common" lead is at V+/battery+ or ground) since different vehicle manufacturers connect the headlight's "common" pin to either V+ or ground.
   
9. The ferrite beads used in this project were Fair-Rite 2643000801.  These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller.  This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing.  I used them primarily because they were on hand.
   

 * * * * *

This page stolen from ka7oei.blogspot.com

[END]

A (simple) WWVB loop amplifier for radio-controlled clocks

Note:

The techniques described below should work - with only minor adaptation - for any "Longwave" time signal used by these radio-controlled (non-GPS) clocks - not only WWVB, but DCF77, MSF, BPC and both JJY signals as well.

* * * * *

Last year I moved a bunch of SDRs (KiwiSDRs, RTL-SDR) and a bunch of network gear to a new shelf in my shack, but this placed them much closer to the wall on which I'd previously mounted the two "Atomic" (e.g. radio-controlled) clocks which had been there - and operating - for years.  Since then, they hadn't been able to reliably synchronize to the 60 kHz WWVB signal out of Fort Collins, Colorado.

Figure 1:
The two radio clocks surrounded by
the four-turn loop, near a number of
pieces of "noisy"equipment.
The top clock is set to UTC and
the bottom for local time.
Click for a larger version.

While annoying, I wasn't terribly surprised.  There are several switch-mode power supplies involved in the aformentioned gear and it's not uncommon for them to operate in the 30-60kHz range, offering the potential of "jamming" the receivers.  As both the location of these clocks - and the nearby gear - is convenient, I wasn't too inclined to move them again and initial efforts to "filter" the switching power supplies didn't really help - but I wasn't surprised about this, either, since it's likely direct coupling of their magnetic fields that is the culprit rather than any electrostatic field as the clocks themselves use ferrite loopstick antennas sensitive to just the H-field.

A solution

 Many years ago a friend came to me to solve a similar problem in a downtown Salt Lake office building where the WWVB clocks in a conference room never synchronized and I constructed the remote loop and amplifier/coupling system, described here:

• Getting "Atomic" (WWVB) clocks to work indoors and in weak signal areas - LINK 

In short, a rooftop loop antenna amplified the signal and it was conveyed into the room with the clocks where it was further amplified and then, using inductive loops placed in the proximity of the clocks.  This is how the WWVB signal was coupled to them.  To my knowledge, this system worked for many years (well over a decade) and for all I know, it may still be in use.

 Why revisit?

I've tackled this type of problem before - but I decided to revisit it as the circumstances are slightly different:  I already had a signal source as noted below plus I wanted to see if I could do this with more commonly-available components in a simpler manner.

While I don't have a WWVB loop on my roof, I do have a dedicated LF E-field whip antenna - a 40 year old LF Engineering LF-400B with integrated low-pass filter.  This antenna has been on the roof wherever I have lived almost continuously since I purchased it in the mid-late 1980s and with a few repairs over the years, it still works well, having been on the roof of my current house for several decades.  Its use for LF reception as described on the following page:

• A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - LINK 

The fact that I already had an LF/VLF receive antenna system meant that I had a "clean" source for WWVB, and other devices that receive signals below 500 kHz (e.g. LF receivers for 630 and 2200 meter operation and my Blitzortung "Blue" receiver) and I decided to add one more to the list.

Other types of outdoor antennas 

Note that the circuit described here should work well with other types of active antennas including E-field types such as the PA0RDT "Mini-Whip" and the DX Engineering ARAV3 - to name but two.  An amplified loop such as the Wellbrook and similar will work, provided that it is not oriented such that the desired time station's transmitter is not in its nulls.

Buffer/Amplifier

Through back-of-the-envelope calculations I figured that the already-amplified signal from the active whip needed another 15dB or so of boost and it could then be applied to a loop of wire around the WWVB clocks on my wall.  One thing that helps greatly is that the WWVB signal is extremely strong here in northern Utah - on the order of 5mV/meter or so - and connecting an oscilloscope to my LF-400B whip's signal output showed that the amplitude-modulated time code of the 60 kHz signal from WWVB was visible among the many others.

What I needed to do was to tap off the signal (e.g. "bridge" the connection) from the existing coaxial cable without affecting was was being sent to the other devices using it, amplify it. and apply it to the loop - and I did this "tap" using a BNC "Tee" connector on my antenna feed.

The circuit diagram below gives more details:

Figure 2:
The schematic of the loop buffer/amplifier/driver showing the isolation from the power supply
via L1, the buffer circuit of Q1 and the amplifier and loop driver of Q2.
Click on the image for a larger version.

Circuit description

Of high importance is L1, a common-mode choke, liberated from a failed switch-mode power supply somewhere.  This particular unit has an inductance of about 1mH per winding meaning that it has about 377 Ohms of impedance at 60 kHz and helps to prevent a ground loop and the coupling of noise from the power mains.  If you replicate this circuit I would strongly suggest that whatever you use for L1 have at least a similar amount of inductance.  On either side of L1 are electrolytic capacitors (C1, C2 - preferably of low ESR types) to offer low impedance and a degree of reinforcement of common-mode rejection through L1 while C2 and C3 provide RF bypassing for the circuit itself.

A buffer amplifier consisting of Q1 - with a high-impedance input, but no actual gain - couples the signal from the existing antenna:  Having several k-Ohm of input impedance, it is unlikely to appreciably load the existing antenna system.  On the feed from the E-field whip, I simply installed a coaxial "T" connector to allow me to bridge across the signal feed rather than split the signal, which would have been complicated owing to the fact that the DC power for the whip was also being carried on that same cable.  The connection to the amplifier in Figure 3 was made using a very short piece of coaxial cable (about 2 feet long - less than a meter) and since this whip is not used for reception above about 500 kHz, neither its presence or that of the added amplifier had any discernible effect on the other received signals.

Coupling from the existing antenna are series components L2 and C4, selected to resonate at about 60 kHz:  The resonance is extremely broad, so finding a capacitor combination precisely equal to the "ideal" value of C4 - according to the formula below actually calculating as 0.007uF (7000 pF) - is unimportant.  This series resonant circuit is probably not essential and a simple coupling capacitor of 0.01uF (10000 pF) could be used (omitting L2 entirely) but I chose built it with L2 to broadly filter off-frequency signals - something that might be important if your E-field whip antenna doesn't have a low-pass filter to remove AM (Mediumwave) signals as mine does as well as to block any stray coupling of HF signals when I transmit.

Figure 3:
The circuit of Figure 2 built on a piece of prototyping board
in the case.  Bifilar choke L1 is on the right with the BNC
connector (J1, input) and output to the loop (J2) on the left.
Click on the image for a larger version.

The buffered signal from Q1 is then passed to amplifier Q2 which is configured to have "about" 15dB of signal gain.  This circuit is, perhaps, slightly more complicated than it needs to be, but with its feedback, it is very stable and tolerant of large signals.  The use of electrolytic capacitors for C5 and C6 is, perhaps, overkill (0.1uF ceramic would probably suffice) but I used them as they were handy.

As the signal from WWVB is quite strong at this location, there is only one stage of amplification shown in Figure 2, but if I lived more distant, greater overall system gain might be required.  Replicating the circuit involving Q2 (R4-R8, C5-C6) and cascading it with the existing amplifier would add yet another block of gain to boost the absolute signal level - but this would presume that whatever active antenna you were using outdoors to pick up the WWVB signal was working well, providing a "clean" signal and that the deficit was just in signal strength at the clocks rather than than signal-noise ratio.

Due to the smallness of the project box that I chose I couldn't mount the bifilar choke "through" the prototype board so it was mounted on the edge to minimize height.  To hold it in place I used UV cured resin along the edge to prevent it from breaking the pin connections mechanically:  UV cured resin is very handy as it's about a strong as epoxy, but it is cured almost instantly meaning that it's able to be handled immediately.  For the BNC connector, the one that I found in my parts bin didn't have its matching mounting nut, but more UV-cured epoxy did the job for that, too!  As can be seen in Figure 3, I didn't bother "mounting" the board in the box, letting it hang about on its own wires.

Indoor Coupling loop

The "coupling loop" - visible in Figure 1 and shown on the schematic - is just a loop of wire - and it is used to inductively couple the signals from the outside antenna to the clocks.  In my case, I measured a rectangle that would encompass both of the wall clocks and found a cardboard box with similar dimensions and on it I wound four turns of 22AWG hookup wire.  Connecting this loop to the amplifier, I used some shielded microphone cable:  Coaxial cable would have been fine as would just some single-pair speaker wire as this frequency is not all that much higher than audio!

Neatly forming the individual conductors, I used small "zip" ties to hold them together and with four screws, attached it to the wall, placing the clocks inside the loop of wire.  Within the loop, signals from the amplifier would be strongly coupled into the ferrite loopsticks in the clocks themselves - but being very small in terms of the 60kHz wavelength, this loop is unlikely to radiate more than a few feet/meter outside it.

To improve efficiency of the coupling loop I wanted to series-resonate it at around 60 kHz as this would increase the amount of energy transferred to the loop from the amplifier somewhat, effectively providing "free" signal gain.  Measuring the inductance of the loop I found that it happened to be about 22uH and using this simple formula, I calculated the value of C7 - the resonating capacitor in Figure 3:

LC = 25330/(F_MHz)²

Where:

LC is the product of the inductance and capacitance (e.g. Capacitance in pF * Inductance in uH)

F_MHz is the desired resonant frequency in MHz (e.g. kHz/1000)

Knowing that we have 22uH of inductance in the coupling loop and a frequency of 60 kHz (0.06MHz) we end up with "LC" being equal to 7036111.  Dividing this value by the known inductance of our coupling loop (22uH) we get  the capacitance, as in (7036111/22) = 319823pF, or 0.319uF.

Figure 4:
The finished amplifier in its box, hanging
out below the loop - connected, and in
service.  (It's just visible in the bottom
of Figure 1)
Click on the image for a larger version.

As 0.33uF (330000 pF) is the closest common capacitor value, I used that for C7.  Again, as with C4 and L2, the resonance is very broad and precision isn't too important.  The article linked near the top of this page goes into more detail on how one would construct and resonate a coupling loop.  Based on this formula, if I wanted to resonate the same loop for use with DCF77 at 77.5 kHz I would have picked a 0.18 or 0.2uF (180000 or 200000 pf) capacitor, instead.  Similar changes could be made to accommodate longwave time signals on other frequencies (e.g. 40 kHz, 50 kHz, 68 kHz).

The formula above can also be used to calculate the value of C4 with the 1mH (1000uH) L2 inductor:  If your interest was for another frequency, such as DCF77 at 77.5 kHz, C4 would be 0.0047uF (4700 pf), instead.

It need not be said that this loop should not be placed very close to whatever outdoor receive antenna you are using - but more than about 10-15 feet (3-5 meters) should suffice:  If they are too close to each other, feedback (oscillation) could occur - but as this loop is only around 0.01% of a wavelength in circumference it does not radiate efficiently at all - and since it's inductive, its signals won't efficiently couple to an E-field antenna, anyway.

In the diagram, C7, the resonating capacitor for the coupling loop, is shown at the amplifier - but it could have been placed at the loop itself.

Power supply  

First off, do not use a switching power supply for this device!

As noted, common-mode choke L1 was used to "decouple" the power supply from the amplifier - and also from the coaxial cable of the LF antenna.  To power this loop amplifier I would strongly recommend using ONLY a transformer-type DC power supply and NOT any type of switching power supply for the simple reason that the switching power supply will be comparatively noisy, and it - its harmonic - will likely operate at/near the frequency of WWVB or whatever time signal you are trying to receive.

This power supply does not need to be regulated:  Simple capacitor filtering with low-ish ripple (a few hundred millivolts) will suffice and any voltage between about 11 and 16 volts will work which means that about any old "wall wart" in that voltage range - regulated or not - would be fine.

Conclusion

Having had the parts on hand it took only a bit more than an hour to piece this together and almost as long to put it in the box seen in Figure 4.

When I forced both clocks to re-acquire WWVB's signal for syncing they immediately set themselves to the correct time and date - and since it had been the start of daylight saving time the night before but had not been able to synchronize prior to this - they "knew" the new, correct time, too!

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

How I prevented QRM to HF reception from my solar charger and AC inverter at Quartzfest

Figure 1:
White board from Quartzfest!
Click on the image for a larger version

As it happens, I found myself at QuartzFest in Arizona in the latter half of January, 2026 where we set up some banners proclaiming the existence of the Northern Utah WebSDR (link) - but I also scribbled on a small white board the words "QRM-Free Solar is possible - Ask How!".

Between the SDR, this message and the diverse portable HF antennas erected, I have had a lot of conversations over the past several days about these and many other topics, meeting new people and re-acquainting myself with others that I've seen on and off over the past several years of my attending QuartzFest (this is year #4 for me.)

RFI-less solar IS possible 

During the "Solar Walkabout" - an on-foot tour to look at how others camping have deployed their solar panels - I volunteered to have folks look at what I'd set up:  It's nothing obviously special - a glass-panel 200 watt Renogy folding array and another Renogy "flexible" solar array - but there is one major difference:  It does NOT produce HF QRM, meaning that I can plant my portable antennas near my panels and not get any interference on HF.

As I've done some previous articles on this, what I'll present here is mostly a set of links to those articles with a quick overview, but this effectively puts that information in one, handy place.

Let's start with quieting the Renogy solar charge controllers:

Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system- Link

Figure 2:
My humble, RF-quiet solar array at 2026 Quartzfest
Click on the image for a larger version.

The main issue with Solar charge controllers is that you have a "dipole + transmitter" situation:  The panels themselves do NOT cause RFI, but the charge controller is effectively a transmitter - especially if it's a PWM and/or MPPT-type - and the legs of the "dipole" are the solar panels (possibly long wires connected to large, rectangular pieces of metal) and another set of wires going to the battery - which also find their way around your RV/campsite via the inverters, DC wires, etc.:  It is no surprise at all that RF finds its way out of these things!  By adding filtering, we are effectively "shorting out" the the RF at the feedpoint of this hypothetical dipole and preventing it from radiating.

To quiet these panels, I added bifilar-wound ferrite toroids - but also bypass capacitors:  The toroids (ferrite) alone will probably knock down the QRM by 2-3 "S" Units, but if you are getting S-9+ interference from your solar, simply knocking it down to S-6 or S-7 when you are in the boondocks - where the natural noise floor is closer to S-1 or S-1 - is still pretty bad!

The key here is adding capacitors in addition to the ferrites and this method is perfectly capable of quieting even the noisiest of solar chargers.  It is also vitally important to put this filtering physically close to the noisy device and use good-quality bypass capacitors. 

Figure 3:
Filtering on the bottom of the Renogy controller
making it RF-quiet.
Click on the image for a larger version.

While the above blog entry showed a modest (200 watt) system, the above can be scaled up for higher-power systems:  Larger wire will handle more current and larger toroids will accommodate it!

RF Quieting a Samlex 150 watt Sine Wave inverter - Link 

Another component of RV/camping with power is the inverter to run mains-voltage devices, and these can be terrible noise sources.  The article above shows how it's possible to make one of these devices completely quiet.  For the older Samlex inverter - which was terribly noisy out-of-the-box, it is now quiet enough that I can power LED Christmas lights from it that are strong from the same mast as the antenna and I get NO RFI (the LED Christmas themselves don't produce QRM).

I was fortunate that there was enough room in the Samlex's case to be able to add this filtering, but it may be added externally as well, provided that the leads are kept short.

What follows below are some methods for quieting UPSs (Uninterruptable Power Supplies).  These are very much like the inverters in an RV in that they produce mains voltage from battery power - and the same problems with RFI occur:

A high-current DC (and AC) noise filter for UPS or RV use - Link

This shows a rather extreme example (an 8kVA UPS) where high currents are involved:  Such would be the case with a kilowatt-class DC-AC inverter or even a large PV system.

Containing RF noise from a sine wave UPS - Link

This article shows the techniques involved in quieting a lower-power UPS, but it also introduces some other components:  Rather than winding your own filter using toroids and wire, you can get "Line Filter" modules from electronic parts supplies (e.g. Digi-Key, Mouser) with brand names like "Corcom" or "Delta" (among many others.)  These are self-contained modules with the components built-in - available in a wide variety of voltage and current ratings - that can do an excellent job of filtering.

 

Completely containing switching power supply RFI - Link

This is an extreme example, but it shows how one might be able to make even the noisiest switching power supply quiet - and this might be important to someone who is trying to get every device in their ham shack - whether it be at home or on the road - quiet.  This method is foolproof in its effectiveness, but it is also likely overkill for many applications, but it discusses the "how and why" these techniques can work.

* * * * * * 

I hope that this helps those who venture out in the wild with their RVs, solar power and battery system and still be able to operate HF.

This page stolen from ka7oei.blogspot.com

[END]

 

 

 

Reducing RF susceptibility for the HamGadgets "Ultra Pico Keyer" - and mimizing RF issues on portable HF stations in general

Note:

While this article describes a modification of the Pico Keyer to reduce RF susceptibility, it also talks about general methods to minimize/reduce RFI-related issues for both portable and "base" stations:  This specific topic is covered near the end of this blog entry.

POTA operation 

Over the past several years I've done a bit of POTA (Parks On The Air) operating, racking up "about" 1000 contacts as an activator in a number of parks - usually as an "activator", and mostly on CW.  Typically, I have operated from a campsite using a portable antenna - usually the JPC-7 loaded dipole (discussed in this blog entry) or the JPC-12 loaded vertical (discussed here) - but I have also used an end-fed half-wave and a simple dipole on occasion - and even the Yaesu ATAS-100 on my vehicle.

Figure 1:
Operating CW POTA from US-0004,
Arches National Park in Utah
Click on the image for a larger version.

In the recent past there has been a revolution in portable power sources in that a LiFePO4 battery - which can supply 20-ish amps - is both light enough to be practical and fairly inexpensive.  For those instances where I may be staying at one location for several days the advent of inexpensive solar to maintain the power budget - and the solar controllers can be made to be RF quiet to make it compatible with HF operation (see this article).  With this in mind it's practical to operate the transmitter at 100 watts much of the time, something that makes it as easy as possible for those who wish to work me.  Despite the ability to run 100 watts, I have occasionally operated QRP (5 watts or less) - again, usually on CW.

A memory keyer

Having used a number of different radios for POTA operation (Yaesu FT-100 and FT-817, Icom IC-706MK2G and even a RockMite) - none of them with a memory keyer - I decided that an "Upgrade" was in order so I got the Ham Gadgets "Ultra Pico Keyer" (Link here).  This device is small, powered by a single CR2032 lithium coin cell and costs about US$40 as a kit (not including shipping) including a (partially) 3-D printed case.  For portable use, I couple it with the "Outdoor Pocket Double Paddle" (with magnets!) from CW Morse (link).

This is a nice, little device in that it provides a consistent interface to the user - no matter which radio you might use - and it has a number of message memories (up to eight), perfect for an activity like POTA where a message (e.g. "CQ POTA") may be repeated many, many times during the course of the operation.

Getting "stuck" 

Figure 2:
The Ham Gadgets "Pico Keyer" (left) along with the
CW Morse Outdoor Paddle.
Click on the image for a larger version.

While the Ultra Pico Keyer works as advertised, I did notice a problem on the first trip out while using a portable antenna:  It would get "stuck".

Clearly, this was an RF susceptibility issue - verified by reducing transmit power and observing that it no longer happened.  In short, at 5 watts there was usually no issue, but at 100 watts  the radio would stay keyed continuously after the first Morse element whether it was sent from a stored message or via the paddle:  While it was "stuck", I could still hear the sidetone - via the keyer's internal speaker - sending the message or what was keyed via the paddle indicating that it was not the microcontroller that had crashed but the circuitry that keyed the radio that was the problem.

Further testing showed that when the unit got "stuck" due to RF and simply unplugging the paddle from the back of the keyer would cause it to release (get "un-stuck").  The fact that this happened using a portable antenna provided further evidence of potential RF sensitivity.

Analyzing the problem

As I'm wont to do, I decided to take a look at the Pico Keyer's schematic to see if there was something about its design and construction that might make it more susceptible to RF interference - and I was surprised at what I found.  Here's the diagram found in the manual that is freely available online on the web site (link):

Figure 3:
Annotated diagram of the Pico Keyer with RF current paths shown.
The components in question are Q1 and Q2, in the upper-right corner.  The lines highlighted in yellow are those through which RF currents will flow (between the radio chassis and the paddle/cable) if no bypass capacitor is installed. 
The added capacitor is shown below the "OUTPUT" jack near the upper-right with the resulting RF current path around Q1/Q2 shown in magenta.
Click to get a larger image.

While there are protection capacitors on the paddle input (C1, C2) my eye was immediately drawn to the output keying (upper-right) where I was, at first, confused as to the arrangement with an N-channel MOSFET in both the keying line and the "common" (ring) of the "OUTPUT" connector (e.g. Q1 and Q2) - but then I remembered that the manual stated that this device would key both positive and negative voltages, explaining the "unusual" arrangement.

While admittedly clever, I could immediately see a susceptibility issue here - the problematic RF current path highlighted in yellow in Figure 3, above:  The "OUTPUT" jack more or less will "float" compared to the "ground" of the keyer itself, which is also connected to the "ground" lead of the cable to the paddle as well as the external paddle itself.  This configuration almost guarantees that there will be at least some RF current flowing from the radio and through the keyer's output circuit for several reasons:

• If you are using this in a portable situation, the radio will surely have some RF on its chassis.  As noted in the final section of this blog entry, it's almost impossible to prevent all RF current from getting onto the feedline - even if you do use a common-mode RF choke and a very nearby antenna is likely to immerse the radio and its interconnecting gear in a rather strong radio-frequency field.

• The paddle and the cable that connects it to the keyer should be considered as part of an antenna - and this situation is made worse if one is sitting at, say, a metal table and also if you, the operator, place your hand at/near the paddle/cable, further encouraging a "through" path for RF.
  

What this means is that there will be at least some RF current flowing from the radio chassis, through the keyer and then, as indicated by the yellow-highlighted lines - via transistor Q2 (and Q1) and then through the cable to the paddle.  I didn't really investigate the exact mechanism by which RF current through this path was causing the keying line to get "stuck" - but here are a couple of possibilities.

• RF may be coupling from the drain of Q2 into its gate - and subsequently into Q1's gate as well, which is tied in parallel with it with the peaks of the RF voltage turning on the FET.  Even if RF through the FET was causing it to conduct only on half of the RF cycle, this would surely be enough to key the radio.  It's also possible that the transistor was turned, on average, only "partially" on by the RF energy - not enough to completely shunt out the RF, but enough to key the radio.

• The RF could also be getting into the output pin of the microcontroller via the FET, causing its totem pole output to get "stuck" on while it was present.

Figure 4:
The added capacitor(s) can be seen soldered between the
"sleeve" pins of the "OUTPUT" and "PADDLE" jacks,
on the bottom of the board.  As you can see, I've made this
modification to both of my Pico Keyers!
Click on the image for a larger version.

Regardless of the cause, the fix was clear:  Add a capacitor to bypass RF current around Q1 and Q2 and the output pin of the microcontroller.  In Figure 3, above, the magenta highlight shows how the added capacitor conducts RF currents around the sensitive components.

When this occurred, I happened to be on a POTA activation, but I had my "electronic toolbox" in the car which included a number of useful items such as a soldering iron and a smattering of useful electronic components (a some common resistors, capacitors, etc.).  Grabbing a 1000pF capacitor, I connected one end to the "sleeve" (ground) pin of the "PADDLE" jack and the other end to the "sleeve" of the "OUTPUT" jack - effectively providing a bypass to RF energy on Q2's drain to the circuit "ground" to eliminate any RF voltage potential between the cable connecting the radio and that going to the paddle.  

This modification completely solved the problem:  It is my opinion that this capacitor should be supplied with the kit.  Additionally, Q2 could be eliminated completely and its source/drain leads jumpered if negative keying is not needed.  ^{See Footnote 1}

Since the topic of "RF on the rig" was already broached, the rest of this article will describe how to reduce it.  It's worth noting that the susceptibility of the memory keyer was such that even with the measures described below, it was affected at 100 watts.

* * * * *  

Suppressing RF on the gear and connecting cables

Some readers of this may immediately say "You are obviously doing something wrong with your set-up if there's enough RF on your gear to cause a problem".  

The problem of RF going somewhere other than out the antenna has been known for many decades and is sometimes referred to as "Hot Mic", a situation where there is enough RF on the radio - and the microphone - that the operator can even get an RF burn from touching the gear.  When this happens RF can get into the radio itself and cause undesired operation (malfunctions, distorted audio, etc.) but accessories connected to the radio - most notably sound interfaces, computers and even keyers - can be adversely affected.

While in the case above there was apparently some RF present on the gear to cause a problem, there isn't anywhere near enough to cause issues with the radio itself, and the radio+microphone (when running SSB) seemed immune.  Some types of antennas - typically ground-plane verticals, random-wires and end-fed half-wave antennas can, by their nature, put RF on the feedline - and thus the radios - unless extra steps are taken to minimize this problem in addition to properly installing/configuring the antenna, namely:

• Common-mode choke on the feedline.  Typically placed near the antenna, this usually consists of coaxial cable wound on a ferrite toroid - typically 6-12 turns on an FT240 or FT140 core with either Mix 31 or Mix 43 as the material - the latter being generally more useful/preferred for portable operations where the higher bands (40 meters and up) are most likely to be used.  Sometimes operators wish to have the feedline itself act as part of the counterpoise/ground - something that can risk a "hot mic" situation and in this case placing the common-mode choke farther along the coax - often near the radio - is the better choice.  (Some operators will put a choke at the antenna and near the radio.)
  

• Use of a "balanced" antenna.  A balanced antenna like a dipole is generally more likely to induce less RF current on its feedline than a purely end-fed antenna (a vertical is included) as it contains its own counterpoise - but having a perfectly-balanced antenna is not really possible and the feedline itself will usually participate in conducting/radiating RF along with the antenna to some degree.  A high-impedance antenna like an end-fed half-wave can sometimes reduce the probability of RF currents on the gear, but note that current can peak at every odd-numbered quarter-wave interval along the feedline and if the radio happens to be at one of these current nodes, issues are more likely to arise:  Placing a common-mode choke at a current node can help.
  

• Counterpoise/ground plane at the radio.  If you are operating in a metal vehicle it's less likely that RFI will be a problem as one is likely to be surrounded (e.g. shielded) - plus the fact that the shield of the coaxial cable feeding the antenna can be electrically bonded to its chassis.  Barring being in a Faraday cage like a vehicle, having a counterpoise connected at the radio (particularly if it's 1/4 wave long at the operating frequency - and if there is more than one of them) this can siphon off some of the RF that might be present owing to its lower impedance.  The use of a common-mode choke prior to the counterpoise at the radio will help to raise the impedance of the conducted RF and will usually improve the efficacy of a counterpoise/ground plane.

• Ferrites only go so far.  At HF, a simple "snap on" choke will probably do very little for the simple fact that there does not exist a common ferrite material that will offer a reasonable degree of choking impedance at, 14 MHz with just one turn (e.g. wire passed through it).  What is required is that multiple turns of a conductor be passed through the device (snap-on choke, toroid, etc.) as the impedance/inductance is proportional to the square of the number of turns.  Even so, there's a practical limit as to the choking impedance of a piece of wire around a ferrite (probably in the hundreds of Ohms for a "casually-wound" device).  As in the case of the keyer, I chose to use a capacitor, instead:  It is a tiny, inexpensive device able to fit inside the keyer rather than a large lump in a cable and it directly addresses the issue at hand by making the circuit intrinsically RF-tolerant.  In other words, it's the correct component for the job!
  

• Place the antenna far away from the radio.  As noted, this isn't always practical - or even desirable during portable operation.  In my opinion, equipment used with a radio transceiver should already have a modicum of resistance to stray RF energy so that even small/moderate amounts of RF on the gear will not cause any problems.
  

If you are operating portable, there's one thing that you probably aren't going to get very faraway from:  The antenna itself.  Almost by definition, portable operating implies being near the antenna owing to the need to have a feedline of manageable length and also due to practicalities of not wanting to lug a long feedline along or taking up more real estate than necessary.  What this means is that it's likely that you and your radio will be immersed in a rather strong RF field - and this also means that anything made out of anything that is conductive (the radio, power cables, microphones, interconnect cables to your paddle and keyer - and even you) are likely to intercept RF energy this will get into everything.

* * * * *

Footnote

1. A 1000pF capacitor has theoretical impedance of about 23 ohms at 7 MHz and it did the job here, but a 10000pF (e.g. 0.01uF or 10nF - ideally about 2.3 ohms at 7 MHz) capacitor would to just fine as well.  For positive keying (which is what likely what any modern radio uses) values as large as 0.1uF (100nF) would work as well - but this large of a value may cause issues with radios that use negative keying (e.g. high-impedance lines on some vintage radios).

 If you never plan to use a radio with negative keying, you could simply short together the source and drain leads of Q2 together to reduce RF susceptibility.

This kit is actually supplied with an "extra" capacitor:  The user can select between a 0.01uF (10nF) and a 0.047uF (47nF) capacitor (C3) on the "headphone" jack to set the loudness.  As I installed the 0.047uF capacitor, I had the 0.01uF left over.  Unfortunately, the specific capacitor supplied was thick enough that it prevent the board from sitting in the bottom of the case, raising it up and preventing the lid from fitting properly.  I could have probably connected this capacitor to the same circuit points on the top side of the board, but as I was home when I made this modification to my second keyer I simply found a lower-profile capacitor that didn't interfere with the board clearance.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Charging LiFePO4 batteries from a vehicular electrical system - the problems and a solution.

Figure 1:
The Renogy RNG-DCC1212-20 - an isolated
and current-limited battery charger, intended
for use with vehicle electrical systems.
Click on the image for a larger version.

There are times - usually on a camping or road trip - where I would like to charge my LiFePO4 batteries en-route, from the vehicle.  The "need" is largely the result of having one of those coolers with a built in compressor:  It runs about 10-30% of the time at normal room temperature and pulls up to about 3.5 amps when doing so - but it's also advantageous to be able to keep a battery topped off in the event that you didn't start the trip with a fully-charged battery in the first place.

To do this, one may be tempted to connect the battery directly to the vehicle's electrical system, as might have done in days past with a lead-acid battery.

DO NOT do this with any lithium battery - at least not directly.

In short, you cannot and should not parallel a LiFePO4 battery with an existing charging system intended for lead-acid batteries.  The biggest issue with doing so is that unlike a lead-acid battery, a LiFePO4 battery will attempt to charge with all available current, likely resulting in blown fuses, heated wires and burnt-out alternators. A secondary issue has to do with the BMS (Battery Management System) of the LiFePO4 simply disconnecting abruptly when the battery is fully-charge, potentially causing voltage spikes capable of damaging vehicle electronics and possibly, the BMS itself.

See the section "Why you need to treat LiFePO4 batteries differently" in the "tl;dr" section near the end of this article (link) for more details as to the problems that can occur.

A solution

The solution to the issues noted above lie largely in limiting the charging current.  One way to do this would be resistively - perhaps with the use of intentionally small-gauge wire and/or resistor or incandescent automobile headlamp in series.  This will, by its nature, generate heat as it's inefficient - and it can generate quite a bit of heat (potential fire risk here!) - but this is the way one might have accomplished this in years past.

This sort of limiting may occur unintentionally if one charges via, say, a cigarette lighter/accessory plug connected with light-gauge wire, but this is sort of a "kludge".  One issue with this is that it can cause frequent blown fuses as the current isn't regulated and if the user attempts to circumvent this by using a higher-current fuse, damage to the electrical system (or even fire) can result.  If the connection is made to a power source that is switched on/off with the ignition, a connected battery can "back feed" the electrical system which can result in the battery being discharged when the vehicle is off or, worst case, damage to both the vehicle and battery.

These days one would use a current-limited and regulated voltage DC-to-DC converter with its power source connected as close to the vehicle battery as possible.

One of the many devices out there that will fit the bill is the Renogy RNG-DCC1212-20 (pictured above), available at the time of the original posting of this article for around US$100:  Using DIP switches, the type of battery (lead-acid, Lithium-Ion or LiFePO4 - I configured for the latter) may be selected along with the charging profile/voltage - and the device will limit the maximum charge current to just 20 amps, selectable to 10 amps with the addition of a jumper wire to the "LC" terminal.  What this means is that no matter the charge state of the LiFePO4 battery, the current being pulled from the vehicle's electrical system will be limited - very useful if one expects to avoid blowing fuses, destroying alternators, or burning up wiring.  (Note:  I have no vested interest in Renogy, they just happen to make one of the readily-available devices that is appropriate for this task.)

Additionally, it's rated to operate from between 8 and 16 volts while maintaining a constant output voltage (once the output current has dropped below limiting) that is independent of the voltage from the vehicle's electrical system. The Renogy is also an isolated DC-DC converter in that there is no electrical connection between the input and output terminals:  By being isolated, circulating currents (through the chassis or other "sneak paths") can be completely avoided which may be helpful for some sensitive equipment and/or to minimize/eliminate alternator "whine".

This particular unit is rated for up to 20 amps output current.  Rated at about 90% efficiency, it will take more power on its input connections than it will output, producing a bit of heat (which is why it has internal fans).  Also note that the current pulled by the unit will vary depending on the voltage input despite the fact that the output voltage and current - and overall power consumption - may remain constant.

For example, let's say that the unit is outputting 20 amps at 14.5 volts, representing a LiFePO4 battery that is nearly fully-charged representing an output power of 290 watts:  Assuming 90% efficiency, the unit will actually consume 322 watts with the difference (32 watts) as heat.   At an input voltage of 12.0 volts,  322 watts is 26.8 amps, but at 14.0 volts, 322 watts is just 23.0 amps.  The fact that it can pull more current from the source supply than it is outputting - particularly when the input voltage goes down - must be taken into account when sizing the wire and selecting the fuse rating.

You can't just connect it and walk away!

The Renogy has a "D+" terminal that, when connected to a voltage source, will activate it.  The intent is that this wire is connected to some part of the vehicle's electrical system that is likely to be on when the engine is running to charge the battery - such as the "accessory" circuit.  The reason for this is that the Renogy itself has no useful low-voltage disconnect:  If you connected it to the vehicle's electrical system with the engine off, it will happily attempt to charge the battery to which it's connected - and if the battery being charged is a large, discharged LiFePO4 battery, it will likely run the vehicle's battery down completely in doing so.

For a permanent installation in a truck, van or RV, finding a wire that is only active when the engine is running (or, perhaps, the ignition is just "on") makes sense - but in my case I have no need for a permanent installation of the unit - plus, I don't have room to mount the unit and am unwilling to connect/disconnect an "ignition on" wire from the electrical system every time I install/remove it.

One way around this would be to monitor the battery voltage:  If it's above about 13.2-13.5 volts, one can be assured that the engine is running, but it will drop fairly quickly when the engine is off as the lead-acid starting battery's voltage drops.  Unfortunately, the Renogy's only means of low-voltage cut-off is set to 8 volts (a very "dead" 12 volt battery!) which requires that I come up with another way of enabling/disabling the device. 

Another issue is that whenever  the unit is on (the D+ line is active) but unloaded (e.g. no battery connected to the output) it consumes about 250 mA at 14 volts - increasing to over 500mA at 10 volts - and more than this if its cooling fans are running:  This sort of load may run a battery dead in a few days at best, so there had to be a way of completely disabling it and eliminating current draw.

A voltage-controlled switch

In poking around, I noted that without the "D+" line connected to a voltage source, the Renogy drew no detectable current meaning that I could leave the high-current input leads connected full-time:  By switching just the D+ lead I could enable/disable the device as needed without the need of a heavy-duty relay.  (Judging by the "clunk" that one hears when applying power to the D+ line, the Renogy probably has such a relay built into it.)

As the D+ line itself drew very little current (only about 3 milliamps) and anything above about 4 volts seemed to reliably trigger it, it would take almost nothing to drive it so the circuit could be very simple as the diagram below shows:

Figure 2:
Schematic diagram of the low-voltage cut-off circuit with hysteresis.
This circuit provides an "on/off" control of the converter to the "D+" line based on the
voltage at the "V+" and "V-" connections.
Click on the image for a slightly larger version.

How it works:

The "V+" and "V-" lines are connected across the unit's input terminals to monitor the voltage applied to it.  Resistor R1 scales the input voltage to a lower value to apply to the top of R2, a 10-turn trimmer potentiometer, that is used to divide the voltage down to the 2.5 volt threshold of U1, a TL431 "programmable Zener" via its "reference" terminal.  Capacitor C1 connected across the top of R2 provides a degree of filtering to reduce the probability of the circuit from responding to noise on the electrical system.

Figure 3:
The prototype, built on a scrap of proto board.  This uses
uses through-hole components, but could have been built to be
much smaller using surface-mount devices.  The capacitor has
been lifted up to allow a better view of the components.
Click on the image for a larger version.

Resistor R3 limits the current into U1 and R4 limits the current into Q1 while R5 keeps the emitter-base voltage of Q1 high when U1 isn't conducting, turning it off, resulting in no voltage on the "Out" lead and in this state, with the Out lead connected to the Renogy's "D+" connection, the unit would be powered down and draw no current.  Resistor R6 offers protection to the circuit in case the "Out" terminal is momentarily shorted to ground.

If the voltage on the reference terminal on U1 exceeds 2.5 volts, it turns on, pulling the bottom of resistor R3 toward ground, turning on Q1 and causing the "Out" lead to go high, enabling the Renogy via the "D+" line. When this voltage goes high, resistor R7 feeds back a slight amount of current into the junction of R1/R2, very slightly increasing its voltage, lowering the circuit's turn-off voltage slightly but leaving the turn-on voltage unchanged:  The value of 270k shown causes this voltage difference between "on" and "off"  to be about 0.9 volts while a value of 680k results in a threshold difference of about 0.3 volts.

This threshold difference between turn-off and turn-on (a.k.a. hysteresis) is very important to the stable operation of this circuit.  If the voltage applied to the circuit were just above the threshold (by a fraction of a volt) the "Out" lead would turn on and activate the Renogy.  When this happened, the Renogy would start drawing current, causing the voltage to drop slightly through wire losses and load on the electrical system - but if this voltage dropped below the threshold, the "Out" lead would turn off again and the current consumption would stop, causing the voltage to rise again and turn it back on, causing an endless "on-off" cycle.  

By adding such hysteresis - and making sure that the voltage drop under load was comfortably less than the hysteresis amount - the unit will reliably turn on at the high voltage threshold and will not turn off until/unless the voltage drops below the low voltage threshold.  It is also imperative that this unit be connected as close to the battery (with appropriate fusing!) with as short and heavy leads as practical:  Too-light wiring will cause the voltage to drop under load, possibly causing it to trip out due to low voltage - only to be re-enabled immediately (e.g. the "on/off" cycling mentioned above.)  The need to minimize voltage drop is one reason why the power source should be connected as near the battery/alternator as practical.

Figure 4:
The completed unit in heat-shrink tube.  There are no
exposed electrical connections - just the adjustment at the end.
Click on the image for a larger version.

Enabling the Renogy by voltage detection alone isn't quite as reliable as having a connection to the ignition circuit of the vehicle, but it will work "well enough" and prevent the vehicle's battery from being flattened by the unit staying on all of the time, when the engine is off.

Figure 3, above shows the prototype unit, built on a small piece of prototype board.  R2, the 10 turn potentiometer is the blue device on the far right with U1 being the black object to the left of it with Q1 being on the far left.  In this photo, capacitor C1 is bent up, out of the way to allow a view of the components underneath where it will be laid over.

Figure 4 shows the same circuit covered with some yellow heat-shrink tubing to hold the components together and to protect it from external short circuits.  The end of the adjustment resistor, R2, protrudes from the end of the tubing so that it is accessible.

Installing within the unit

Figure 5:
The circuit within the converter.  The DC output
terminals (to the battery being charged) are in
the lower part of the image.
Click on the image for a larger version.

Not wanting to have any more of a maze of wires outside the device than necessary I installed the circuit inside the Renogy unit itself as seen in Figure 5.  Using some "Shoe Goo", a strong rubber adhesive (do not use "hot melt" glue!) the encapsulated board of Figure 4 was mounted in the upper-right corner of the "output" side of the unit, set back by about 3/8" of an inch (10mm).  The location is such that the voltage threshold adjustment is accessible via one of the ventilation holes:  Setting it back prevents it from obstructing air flow and makes the precise alignment between the screw of the potentiometer and the hole less critical.

The "V+" and "V-" wires from the circuit are soldered directly to the bottom of the board on the DC input terminals and the "out" terminal of the circuit (the blue wire in Figure 5) is routed through another hole near the green "D+" and "LC" terminals.

Figure 6 shows how these wires are routed.  In addition to the connection to the "D+" terminal from the circuit, another wire and a switch was added that optionally connects the "LC" terminal to the "D+" to set the Renogy to the "Low Current" mode by pulling it high when the switch is closed - in this case, limiting the maximum charge current to 10 amps, which may be useful if you are connecting the unit to a current-limited power source (e.g. "cigarette lighter" plug) that cannot supply the 25-ish amps current input that the unit may draw when charging at 20 amps output.

Figure 6:
Looking on the "output" side of the Renogy, this shows how
the "out" wire from the circuit routes out of one of the air
to the "D+" terminal.  Also shown is a switch that optionally
connects the "D+" to "LC" terminal for just 10 amp max.
Click on the image for a larger version.

This "modification" - since it does not involve drilling any holes - is "reversible" if desired as the circuit and wiring could be easily removed.

In-vehicle testing and use

High/low voltage turn-on/turn-off

Prior to testing the modified unit in my vehicle I set the "cut-in" voltage to about 13.65 volts which resulted in a disconnect voltage of around 12.7 volts - a voltage below which a 12 volt lead-acid battery will quickly drop when charging is stopped.  As expected, the unit did not get turned on until a few seconds after the engine was started, the voltage rising due to charging by the alternator:  If the battery had been heavily discharged and a lot of accessories were running (headlights, blower, wipers) it may take longer than this for the voltage to rise above the threshold.

The voltage dropped below the 12.5-12.7 volt shut-off threshold within a few 10s of seconds of turning off the engine with the entire unit drawing only about 0.5mA (all of that being from the added circuit) in that state - far lower than the vehicle's own quiescent current, and probably lower than the vehicle battery's self-discharge rate.  So far, I have found no tendency for the unit to cycle on and off while the engine is running - even if the headlights, heater blower and windshield wipers are on.  (As noted in a sidebar below, cycling did start to occur, but this was traced to the adjustment potentiometer setting having drifted upwards by about 0.4 volts, likely due to vibration.)

Of course, the voltage thresholds mentioned above are only valid for a healthy (and properly functioning) conventional charging with lead-acid batteries as part of the chassis electrical system:  If your vehicle somehow has a different type of electrical system than the conventional "alternator + lead acid" configuration it'll be up to you to determine how and even if a solely voltage-referenced on/off system like this can be done.

RF Noise generation

Being an amateur radio operator, I was concerned that this unit might produce an excess of radio frequency interference as it contains a high-power oscillator in its power converter.  While visual inspection of the Renogy (with its end covers removed) showed that it does have some filtering of its own in the form of series inductors and capacitors across the input/out leads and to the metal case (to suppress common-mode and differential RF energy) it would be unusual for even a well-designed commercial device to go to extremes in reducing radio frequency energy to the point of extinction. 

Using a "Tiny SA" Spectrum analyzer I connected directly to the input and output leads - using a 0.002uF capacitor to block DC and protect the analyzer - I measured the amount of RF energy being differentially emitted from the unit.

This measurement is important in that if the instantaneous RF voltage on the output leads is different than on the input leads, the in/out cables will necessarily conduct RF energy to the outside world, into whatever is connected at both ends, including the wiring itself, which may radiate like a dipole antenna and/or conduct radio-frequency current through the unit and into other wiring and/or equipment.  A plot from the spectrum analyzer showing the produced RF energy up to 10 MHz is shown below:

Figure 7:
The spectrum of RF energy as measured directly between the voltage in and out terminals across the range of 0-10 MHz with no filtering.  If a receiver's input terminals were connected directly to the DC terminals, the signal level at 40 meters (7 MHz) would be bit more than "10 over S-9.

Without any added filtering, I tested it in my vehicle - powering the 100 watt HF transceiver directly from the Renogy (with no battery) - something that I probably would not ever do in normal use:  If the converter does have the tendency to produce RF interference, connecting the radio directly to it and putting conducted RF energy on its power leads - and its chassis - would represent a "worst-case" scenario.  On 40 meters (7 MHz) and 12 meters (24 MHz) I could just hear the switching frequency's harmonics near the noise floor which indicated that it was pretty quiet - but not completely so.

Since the spectral switching components were just audible I decided to add a modicum of filtering on both the DC input and output leads - four bifilar turns of #12 AWG (e.g. the input/output power cables) each on their respective T140-43 ferrite cores as seen in Figure 9.  In most situations I would prefer to include bypass capacitors in the mix (see figure 4 in the article "Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system" - link) to (significantly!) improve performance, but I decided that even a modest reduction in conducted emissions would likely reduce them to the point of inaudibility.

A spectrum analyzer plot of the noise generated by the unit with the added filtering using just the bifilar-wound T140-43 cores is below:

Figure 8:
The spectrum of RF energy as measured between the in/out terminals with the bifilar inductors between the measurement point and the converter - also over the range of 0-10 MHz.  If a receiver's input terminals were connected directly to the DC terminals the signal level at 40 meters (7 MHz) would be a bit less than "S-9" - for a reduction of about 15dB, or nearly  3 "S" units.

As can be seen Figure 8, the bifilar chokes alone reduced conducted RF by a significant amount above a few MHz, but from as noted in the linked article mentioned above, the addition of the capacitors would have improved the attenuation of the conducted RF energy by another 20 dB or so, but including capacitors is a bit awkward as it involves baring wires and adding additional jumpers.  One issue related to lacking capacitors is the response peak around 2 MHz - likely due to a broad resonance of the bifilar inductors themselves - but this effect diminishes quickly as frequency increases on amateur bands likely to be used in a vehicle.  While not shown in any of the included plots, between 10 and 30 MHz the attenuation afforded by the bifilar chokes, alone, remains at 20dB or better for much of that range.

Note:  At HF, a simple "snap on" choke with a single wire running through its center will not offer enough impedance to provide good attenuation - particularly below 20 MHz.  As the choking inductance is proportional to the square of the number of turns through the ferrite device (e.g. 16-fold with four turns) it is only by being able to put multiple turns through it that we can effectively attenuate frequencies in the HF spectrum.

Figure 9:
The Renogy charger with 5-turn bifilar-wound 12 AWG
chokes wound on the DC input and output leads.  For best
results, always place the inductors as close to the noise-
generating device as practical.  Not visible is a fuse on the
input lead to provide protection to the device and wiring.
Click on the image for a larger version.

If interference from this device were to persist after adding the bifilar inductors, I will go through the trouble of adding the aforementioned capacitors.

Can it be scaled up?

The Renogy RNG-DCC1212-20 is "only" a 20 amp converter/charger, but higher current devices are made by Renogy and others.  While I don't own a higher-current Renogy device, those units seem to operate in exactly the same way:  The "D+" terminal may be used to power it on/off and the "LC" terminal, when pulled high, sets the output current to half of the unit's rating.

If RF interference is considered to be an issue, the higher-current units would require proportionally larger wires and likely larger ferrite cores (say, FT240-43) to accommodate a reasonable number of turns of that larger wire.

I cannot speak to how other brands or dissimilar models from Renogy might be powered down via their equivalent of the "D+" terminal to minimize quiescent current consumption:  That must be left as an exercise by the reader.

Real-world useage Shortly after originally posting this article I went on a rather long road trip.  I had along with me three 100 aH LiFePO4 batteries and I was using them to power not only my refrigerator/cooler, but also my 100 watt HF transceiver. The reason for powering the transceiver from the batteries was due to not wanting to pull more than about 30 amps from the connection to the battery, which itself is fused for 40 amps:  At full charge current, the Renogy could pull about 26 amps from the vehicle to deliver 20 amps to the battery, but the addition of the transceiver would have added another 20 amps, peak to this, the the desire to "average" out the current.  To monitor, I put a voltage and current meter on the "vehicle" side of the Renogy. For the most part, things work perfectly:  I heard no QRM (interference) from the Renogy across the HF spectrum and the 20 amp charge current was more than enough to keep up with the loads, recharging the batteries within an hour or so even after running the refrigerator for a couple of days, in the car. The one issue that I had was that at night, with the headlights and with the heater running was that when the vehicle's engine cooling fan would kick on, the electrical system voltage would drop just enough that my circuit for the Renogy would drop off - then the voltage would increase and would kick back on, repeatedly cycling.  At the time I simply flipped the switch (see Figure 6) to the "10 amp" position to reduce current and the related I*R drop:  This lower current was still more than enough to maintain the batteries - even with the refrigerator and the HF transceiver running/being used.  This issue was later found to be due to the voltage threshold having drifted upwards by about 0.4 volts - likely due to mechanical vibration of the potentiometer.

Conclusion

This unit - and the modification - have worked as expected:  The unit gets turned on and off with the running of the engine automatically with no connection required other than that of power.  When traveling, 20 amps is enough to provide a reasonably fast charging rate to a modest bank (say, 200aH) of LiFePO4 batteries while even the "Low Current" 10 amp limit is more than enough to keep the batteries topped off with a moderate load such as a refrigerator-type cooler or a 100 watt HF amateur transceiver occasionally used for transmitting.

With the added filtering using the ferrite cores on which multiple turns are wound, no interference from the Renogy is audible on the HF transceiver in the vehicle.

 * * * * *

The TL;DR part

Why you need to treat LiFePO4 batteries differently

In the "old days" of lead-acid batteries, you could probably get away with putting it in parallel with the vehicle's electrical system - possibly with the use of an "isolator" (e.g. diode, FET pack, a relay or contactor that connected it in parallel with the starting battery when the engine is running) to prevent the drain on the auxiliary battery from depleting the vehicle's starting battery when the engine was off - but this CANNOT and SHOULD NOT be done with LiFePO4 batteries.

A LiFePO4 battery will attempt to pull "infinity" current when charging

The reason for this has to do with a fundamental difference between the two chemistries.  A healthy lead-acid battery is somewhat self-limiting in the amount of charging current it will take - at least when it's nearly fully-charged:  The charge current will gradually taper off as it asymptotically approaches full-charge.  Additionally, on a typical lead-acid battery the internal resistance of the battery and evolution of gasses at the plates often leads to intrinsic current limiting.

A healthy LiFePO4 battery is closer to that of an "ideal" battery in that unlike a lead-acid battery, where the current will gradually taper off as it approaches "full-charge" voltage (which isn't well defined in that chemistry), a LiFePO4 battery will attempt to consume as much current as it can until it is fully charged.  Practically-speaking, the current is actually limited by internal resistance of the battery - which can be in the milli-Ohm range - and the resistance of the wiring between the voltage source (the alternator) and the battery - and since heavy-gauge wire is typically used, this current can be very high.

In the case of a large (100aH or bigger) LiFePO4 battery, it's likely capable of consuming as much current as the alternator will put out - and this could easily exceed its actual ratings.  Short-term overcurrent conditions on an alternator - such as those that might occur immediately after starting the engine, particularly if accessories (lights, wipers, heater) is on - are tolerated, but they cannot withstand a continuous overload - such as that which might occur with a discharged LiFePO4 battery - without overheating - particularly in hot weather and/or if the vehicle is moving down the road quickly and providing air movement.

Another potential issue with a LiFePO4 battery has to do with its BMS (Battery Management System).  If the charge current exceeds the rating of the BMS, it will disconnect to prevent overcurrent that could damage the cells by charging them too vigorously.  At best, this would cause the BMS to disconnect/reconnect the battery (called "load dump", which is a problem as noted below) and at worst it could cause overheating and damage to the BMS - not to mention the alternator and other vehicle systems as well.

The dangers of alternator "load dump"

Another issue with LiFePO4 batteries that does not exist with Lead Acid is that they can abruptly "dump" their load (e.g. disconnect).  While a lead-acid battery's charge current will gradually taper off, if a LiFePO4 battery attains full charge, its BMS (Battery Management System) will abruptly disconnect the battery once any of its individual cells get to full voltage - something that can happen if the cells are all fully-charged and the current is minimal (the preferred situation) or if high current is still flowing, perhaps due to too-high charging voltage - a much worse case.  The result of an abrupt drop of a large current flow is that the voltage from the alternator will briefly skyrocket, its voltage regulator unable to compensate quickly enough.

While this can happen in a vehicle using a lead-acid battery when a load is suddenly removed (e.g. fan cycling, headlights being turned off) a healthy lead-acid battery is quite good at suppressing such voltage spikes and protecting the attached electronics as it functions much like a large capacitor - but voltage transients high enough in voltage to cause damage can still occur, perhaps cumulatively, particularly if the lead acid battery's condition is poor:  If there is no lead acid battery at all to buffer such transients (e.g. only a LiFePO4 battery) such a voltage spike can damage other devices connected to that power source as described in the example below.

(Note:  As the BMS "disconnect" voltage of a four cell LiFePO4 battery is typically around 14.6 volts, a "load dump" may not regularly occur in many automotive applications as the voltage may never get that high - at least under typical conditions.)

Lead Acid and LiFePO4 batteries don't use the same voltages

A third issue is that the full-charge voltage of a typical "12 volt" LiFePO4 battery is 14.6 volts, precisely, whereas a lead-acid battery is quite forgiving, allowing anything between 13.5 and "14.something" volts as a full charge.  The implication of this is that a vehicle's electrical system is not precise enough to either avoid under-charging (e.g. too low voltage, preventing full charge) or over-charging (e.g. causing the BMS to connect/disconnect/reconnect).

Maintaining a precise voltage near the maximum voltage of a "12 volt" LiFePO4 battery (14.4-14.6 volts) for extended periods (a few hours) - at least occasionally - is also necessary for the BMS (Battery Management System) equalize the individual cells within the battery.  Failure to do this every so often will allow individual cells to drift apart in their charge states as inevitably, one or more cells will discharge more quickly - and if never fully recharged, those cells will seem "weaker" and the battery will appear to lose capacity.

"Equalization" as done by the BMS of a LiFePO4 battery is typically done by "leaking" current across fully-charged cells to top off those that are not - but this will only happen effectively at/near the battery's maximum voltage.  Depending on the degree of this "inequality", it may take hours of holding the battery at this high voltage to fully equalize the battery's cells.

Note that the equalization mechanism for LiFePO4 cells is NOT compatible with that which might be done for Lead-Acid - see the battery's manual or other references for the technical details.

Real-world case

I've seen the above issues play out on a friend's RV:  The original "chassis" battery to run the engine and charge the engine starting battery was augmented by a second and completely separate "coach" alternator which was dedicated to charging the LiFePO4 battery bank and running the devices in the living quarters (lights, TV, pumps, microwave oven, inverter, etc.)  In this case, the secondary alternator was adjusted to produce higher voltage than would be necessary for lead-acid batteries to allow full charging of the LiFePO4 system.

Built by Thor onto a Mercedes chassis,  several alternators were destroyed (one of them lasting only minutes!) by overheating due to the the lack of current-limiting in the battery-charging regimen:  One of them lasted longer than the rest only due to several of the rectifier diodes going open-circuit almost immediately, crippling the ability of the alternator to produce output, limiting current - but putting very high ripple voltage/current onto the coach battery's electrical system.  Additionally, equipment connected to that circuit (namely a $1200 amateur radio transceiver) was destroyed by the high-voltage spike when a "load dump" occurred at the instant that the LiFePO4 battery disconnected  upon full charge do to the intrinsic inability of the alternator's voltage regulator to act quickly enough.  

It is fortunate that this vehicle had two separate alternators so the integrity of the "chassis" electrical system responsible for powering the vehicle itself was spared any problems - and no damage to its components (engine and transmission computers, etc.) was possible.  Without a functioning "coach" alternator to recharge the LiFePO4 battery he was still able to make his trip, but had to stop every couple of days and camp somewhere where he could plug into a mains outlet and use the onboard charger to top it off.

Ultimately this friend ended up taking his rig to a company that specialized in RV power systems and the system was upgraded and reconfigured - at significant expense - to avoid the issues noted above.  A quick perusal of online RV forums will reveal many similar stories - some being a result of the manufacturers apparently being unfamiliar with the requirements of LiFePO4 batteries, and others from individual owners' botched retrofits.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]