Thursday, December 26, 2019

Using TV (F-connector) 75 ohm splitters and taps in 50 ohm systems on the amateur HF, VHF and UHF bands

I recently posted several articles about using commercially-available splitters link - and making one's own splitters - link - particularly for the HF frequencies and below (e.g. 30 MHz, down to a few 10s of kHz).  A comment was posted asking about how useful inexpensive 75 ohm "TV and satellite" type splitters might be for amateur radio use.
Figure 1:
The assortment of 75 ohm TV and satellite splitters and
taps tested in this article.
Click on the image for a larger version.

Implied by this question is the use of these devices in receive-only or small signal applications:  They cannot be used for transmit purposes as putting even 100 milliwatts through one of these devices is likely pushing its power-handling capability.

I've used these devices in 50 ohm circuits before - typically for VHF and UHF (2 meters, 70cm) where, along with some attenuators, combined the outputs of multiple signal generators to do "multi-tone" testing of receivers - but the question seemed to be a good one.  Rummaging around, I gathered a bunch of devices of various manufacturers and decided to test them for insertion loss and port-to-port isolation.

Please do not ask questions like "How well does a 'brand X' splitter work over the [fill in the blank] frequency range?"
There have been thousands of makes and models of these devices sold around the world over the past several decades and I simply am not able to find, locate, and measure more than the tiniest fraction of devices that have been sold.  The information given here is expected to be generally representative of the devices available from reputable manufacturers and distributors - but your mileage may vary.
Limitations of the measurements taken:

Because my VNA (DG6SAQ WVNA) was constructed for use with 50 ohm systems (the changing of  both internal hardware components and software would be required for "proper" analysis of a 75 ohm system) I was able only to analyze them in that context - but because the question was about using them in amateur radio service - which presumes a nominal 50 ohm system - I believe that the results are still useful within the limits noted in this article.

Because the emphasis of the question was interpreted as being for amateur-band frequencies likely to be encountered by the average user, the measurement range was limited to frequencies below 1 GHz - in some cases down to 100 kHz.  The nature of the equipment and methods (e.g. 50 ohm test equipment and cabling, the use of inter-series adapters, etc.) used to test the splitters and taps increasingly limits the usefulness and accuracy of these measurements at frequencies above that of the 70cm amateur band (above 450 MHz).

The variety of splitters and taps available:

There are literally thousands of brands and models of TV/Satellite splitters and taps available on this planet - some of them from recognizable names, but most not.  For those devices from sources that might be suspect (e.g. not "name" brands from reputable suppliers) you are on your own to determine the suitability of those devices for your purpose.

Although not intended as an endorsement per se, it has been observed that devices marketed by Holland Electronics appear to consistently meet their stated specifications and is one of the few brands that is likely available worldwide from a number or different sellers - including Amazon - and major suppliers of electronic components and TV/satellite supplies.

Over the years I have seen many dozens of brands and models of these devices - and the vast majority of them are what they are purported to be, but I have run across some devices that claimed to be splitters, but were simply a box with wires connecting the ports together.  In many cases, the casual user would not have noticed anything amiss, but using several of these faux devices in a larger system would certainly result in cumulative signal degradation (e.g. "ghosting" of analog signals, degrading of quality - but not necessarily signal strength - of digital signals).

General types of devices:

There seem to be three general types of these devices out there:
  • "TV" and/or "VHF/FM/UHF" and/or "CATV" - These devices are typically designed to operate over the range of off-air TV stations across the world and the frequencies typically found on receive-only cable TV (with no Internet), encompassing the frequency range of about 40 MHz through 700 MHz, more or less.  While useful for use on the amateur bands from 6 meters through 70cm, inclusive, their usability on HF or above this range is limited as noted in the testing, below.
  • "Satellite" splitters - These devices are typically designed to operate starting at about 900 MHz, often extending to 1500 or as high as 2500 MHz, depending on the vintage and intended use.  These devices are not usable on the 70cm amateur band frequencies and below.
  • "TV/CATV/Satellite"- These devices are of a bit more recent vintage and are designed to accommodate a very wide range of frequencies - often from about 5 MHz through and above 2000 MHz - a band that includes off-air, cable and "L-Band" satellite signals - plus the "reverse" channels (sometimes called the "T" channels) often used by "cable Internet" modems that reside below 45 MHz.  These are the most useful to amateur service and can often be used on HF through 70cm.
If you do not see a specific frequency range noted on the device itself, assume the worst-case, smallest frequency range that covers that usage category - unless you can test them yourself.

* * *

General findings

For the TL;DR types, here is a summary of the results of the measurements described in more detail farther down the page.

Using 75 ohm devices in 50 ohm systems:

The most obvious issue is that TV-type consumer devices are almost universally equipped with type "F" connectors which means that one must use either an adapter or use a cable with an attached "F" connector.
Figure 2:
Left to right:  Two BNC female to male F connecitrs with an
F-type 75 ohm terminator on the right.
Click on the image for a larger version.

For receive-only systems, it's not too uncommon to simply use a 75 ohm cable like RG-6 - which is quite low loss and very inexpensive - to connect a 50 ohm antenna to a 50 ohm receiver.  The effects of this apparent "mismatch" are typically minimal as most receivers are only "approximately" 50 ohms, anyway.  In theory, the use of 75 ohm cable on 50 ohm devices will result in a 1.5:1 mismatch and commensurate losses, but this sort of mismatch is commonly observed on many antenna systems that are ostensibly designed to operate at 50 ohms and is usually of minor consequence.

When using an inexpensive cable like RG-6, it's worth noting that most of these cables use copper-coated steel (CCS) center conductors which may have implications for DC resistance of power is being sent on this cable (for a preamplifier, converter, controls) as this type of cable will have far more total resistance than one with a solid copper center conductor.  Copper-coated steel center conductors may also have implications in terms of skin effect at low frequencies (low HF and below) - but this is beyond the scope of this article - see, instead, this article by Owen Duffy.  There exist cables with copper-coated aluminum (CCA) center conductors that have lower DC resistance that CCS cables, but they tend to be more fragile due to the tendency of the aluminum center conductor to become brittle with flexure.

The device itself (splitter, tap) is designed primarily for 75 ohms and this means that its performance will be somewhat degraded in a system that is "completely" 50 ohms (e.g. 50 ohm cables with F-connector adapters) but these effects are largely as follows:
  • The "through" loss may be slightly higher.  In the case of a 2-way splitter, the ideal loss will be 3dB - but even at the proper impedance, it will be slightly higher than this due to component losses, typically in the area of 3.5 dB.  Practically speaking, the main effect of using a 75 ohm splitter in a 50 ohm system was a slight change (only a few tenths of a dB) in the loss.
  • Reduced isolation between ports.  The most obvious effect on splitters was that the isolation between ports (e.g. the "out" ports of a 2-way splitter) was reduced.  Compared to some specialized splitters, the isolation of inexpensive, consumer-grade "TV" splitters is lower overall.  As can be seen from the graphs, below, operating in a 75 ohm system resulted in better isolation - sometimes over 40dB at certain frequencies - but this assumes that all loads and sources are well-matched to 75 ohms, something that is not likely to be the case in a real-world installation.  Typically, isolation reduced to something in the 20dB area when operated in a 50 ohm system.  In many cases, this is "good enough".
  • In splitters and taps, resistors are major components in determining their "native" operating impedance.  For example, a 75 ohm splitter or tap, depending on design, may have a 150 ohm or 37.5 ohm (2 times and one-half 75 ohms, respectively) resistor contained internally.  In theory, changing this resistor to a value appropriate for 50 ohms (typically 100 or 25 ohms) would optimize performance at 50 ohms - but doing this may or may not be worth the trouble. 
In short:

Unless your situation requires precision, the use of inexpensive, TV-type splitters and taps of the types described on this page will yield "reasonable" performance over the design frequency range - provided that the device is constructed as described by a reputable manufacturer.

The use of a (nominally) 75 ohm device in a 50 ohm system will require using connectors that are not normally used in 50 ohms systems (typically "F" connectors) which means that adapters of some sort will be needed - the expense, bulk and inconvenience of which must be considered in the overall design.

Finally, note that the above comments are for the general case:  Remember that your needs, requirements and results may vary and that you must do your own analysis and testing to verify that such components are appropriate in your specific case.

* * *

Plots of various devices:

Below are selected plots of devices representative of the types on-hand.  In general, devices with similar stated ratings performed in the same manner.  In all of these plots, the insertion loss is represented by the blue line while the complex impedance data is depicted on a Smith chart in the middle:  Numerical data at the frequencies indicated by markers is seen in the lower-left corner of the screen.  Again, remember that at higher frequencies, the nature of the 50 ohm test system, connecting cables and adapters will increasingly skew the results - particularly those depicted by the Smith chart.

The interpretation of a Smith chart will not be covered here, but there are many online resources that describe its use including this video in a series on this topic by W2AEW on his YouTube page.

A "satellite" splitter:
Figure 3:
The "through" loss of the HFS-2 splitter represented by the blue line
across the top.
Click on the image for a larger version.

This device - a "Tru Spec HFS-2" is representative of those intended for use on an (older) L-band system found in satellite receive systems, having on its label a "900-1500" MHz frequency range.  As noted above, the limitation of the measurement set-up made measurements above the 70cm amateur band (in the 440 MHz area) suspect - but the object here was to see if it was usable below that range.

At initial glance, the "through loss" of this device below 900 MHz (Figure 3) might seem to indicate that it worked below this frequency, but notice that at lower frequencies (below 50 MHz) indicates a loss less than 3dB indicating that it is not working as a proper 2-way splitter.  A look at the isolation plot (Figure 4) tells more of the story.
Figure 4:
Isolation between ports of this splitter.
Click on the image for a larger version.

As can be seen, at about 900 MHz and above, the apparent isolation between ports is reasonable but at 2 meters (146 MHz) it is only 3dB verifying the fact that at these lower frequencies, it less a proper splitter, but more equivalent to a device where the three ports are connected with a piece of wire.  The apparent isolation increase at low HF is more likely an artifact of its construction - the insertion loss being below 1 dB (in Figure 3) verifies this.

In short, these "Satellite only" splitters aren't really useful on TV and CATV frequencies or the amateur bands 70cm and below.

A "TV" splitter:
Figure 5:
The "through" loss of the Archer splitter.
Click on the image for a larger version.

I tested several splitters that were intended for general VHF/UHF/FM use - one of these being an "Archer" (Radio Shack) two-way splitter being typical of that type.  The implied frequency range is from at least 54 MHz to 700 MHz - the extent of the cable TV, FM broadcast, and off-air VHF and UHF TV frequencies at the time it was made.

Figure 5 shows the measured "through" loss in a 50 ohm system.  Compared to a plot done at 75 ohms (not shown, using resistive matching) the insertion loss barely changes across the frequency range.  In both 75 and 50 ohm systems, at least at 2 meters, down to 20 meters (14 MHz) seems to be "ok" - but the "dip" in the 3-4 MHz area - and the fact that the attenuation below it drops below 3dB - indicates that it's not likely acting like a splitter at these lower frequencies.

Figure 6:
Port to port isolation at 75 ohms for this splitter.
Click on the image for a larger version.
Figure 6 shows the port-to-port isolation at 75 ohms and we note that in the "low" and "high" VHF band (U.S. channels 2-13 - which more or less includes the 6, 2 and U.S. 222 MHz amateur bands, that the isolation is quite decent - well above 20 dB.

From this plot we can see that the "dip" in the 3-4 MHz area seen on Figure 5 is quite telling as the port-to-port isolation is pretty much gone below this frequency

Figure 7:
Port to port isolation in a 50 ohm system for this splitter.
Click on the image for a larger version.
The plot of Figure 7 shows what happens if the splitter is operated in a 50 ohm system.  The main effect is that the port-to-port isolation is reduced - being on the order of 15 dB or so from the 20 meter band through the 2 meter band (14 MHz - 144 MHz).

From this we can conclude that this splitter is quite usable from the middle of the HF spectrum through at least 2 meters - and is probably usable through 70cm.

A "TV/CATV/Satellite" splitter - preferred for HF use:

Figure 8:
Holland HFS-2P through loss in a 50 ohm system.
Click on the image for a larger version.
I have on hand several splitters that have on their label a frequency range that starts at (typically) 5 MHz with a high end of between 600 MHz and 2450 MHz.  The reason for this extended "low end" is likely due to their being designed for use in systems that have "Cable Internet" where the return (upstream) signal from the user's modem to the cable system are likely to be in the 5-50 MHz (or, possibly, a bit higher) range.  The plots included are those of a Holland Electronics HFS-2P which is a 2-way splitter/combiner that has a stated range of 5-2050 MHz and the results of this device are typical of that type.)

Figure 8 shows the "through" loss in a 50 ohm system showing a reasonable insertion loss (4 dB or below) from below 40 meters (about 5 MHz) through at least 70cm (440 MHz) - but again, the limitations of the measurement set-up make readings higher than this a bit suspect.

Figure 9:
Port-to-port isolation at 50 ohms.
Click on the image for a larger version.
Again knowing that the "isolation" measurement is the way to get the "true" story, port-to-port isolation in a 50 ohm system is depicted in Figure 9.

This verifies - to the extent that the test set-up can - the 5-2050 MHz range showing that the port-to-port isolation from 5 MHz to 1 Ghz is well over 15dB.  A port-to-port isolation measurement at 75 ohms (not shown) is slightly better (by a few dB) over the same range.

The combination of Figure 8 and Figure 9 show that this device may be usable down to the 160 meter band (1.8 MHz) provided that a slight amount of extra insertion loss (about 1dB) and lower isolation (approximately 12dB) can be tolerated.    (The Holland HFS-2D has characteristics similar to the HFS-2P down to 1.8 MHz.)

Figure 10:
The through loss, the other 7 ports being terminated with 75 ohm F-type
connectors.  The insertion loss is reasonable - between 10.5 and
11.5 dB over the range of 1.8 to 450 MHz.
Click on the image for a larger version.
An 8-way splitter:

The final splitter to be tested was the Holland Electronics GHS-8 8-way splitter-combiner.  Often, splitters with an even number of outputs greater than two contain multiple two-way splitters which means that this 8-way splitter might contain seven such devices - but I didn't break it open to check.
Figure 11:
The port-to-port isolation between two adjacent ports with the "in/out"
port and unused ports terminated with 75 ohm "F" loads.  The apparent
isolation is on the order of 35dB from 1.8 through 450 MHz - but this would
likely drop to something closer to 20dB.
Click on the image for a larger version.

Figure 10 shows the typical "through" insertion loss with the seven unused ports being terminated with 75 ohm "F" type terminators:  I don't have enough F-male to BNC-female adapters on-hand to terminate the 7 ports at 50 ohms - but if one were going to use one of these devices, it's probably more convenient to use F-type terminators on the unused ports, anyway.  The typical "through" loss is measured to be about 10.5-11.5 dB - slightly higher than the predicted "ideal" 9dB insertion loss, but typical for these devices.

The port-to-port isolation was also measured and the use of 75 ohm terminations on the other ports and the "common" in/out port likely improved this:  The isolation would likely be significantly worse if all ports were at 50 ohms, for the same reason as the other splitters tested.

Based on these readings, this device is useful down to 1.8 MHz and up through 2 meters - and probably 70cm.

Figure 12:
Coupling coefficient at 50 ohms for this tap
Click on the image for a larger version.
A TV-type signal "tap":

Likely unfamiliar to many, a signal "tap" is a very useful device in multi-drop TV installations found in hotels, hospitals and other larger buildings.  Unlike a splitter - which usually divides a signal equally to its output ports - a "tap" will siphon only a certain amount of signal off the cable and leave the majority of it intact - which is very useful for systems such as those in a hotel or hospital to distribute and split a signal hundreds of times to serve all of the devices.

In some ways it can be considered to be similar to a part of an SWR bridge where only a small amount of signal is sampled - and in only one direction - allowing the majority of the original signal to pass with minimal loss.  Several taps - all from Holland Electronics - were tested as they were what was on-hand and the "DCG-6SB" is represented in the plots. 
Figure 13:
The "reverse isolation" loss of the tap (e.g. turned "backwards") with a
50 ohm termination.
The reverse isolation is described as being the absolute amount of isolation
(e.g. that in the chart above) minus the coupling coefficient which means
that the actual forward coupling loss - which means that using Figure
12, we know that the actual reverse isolation is about 7 dB lower than
indicated by the graph above.
Click on the image for a larger version.

Figure 12 shows the "coupled" energy in a 50 ohm system:  Compared to the coupling in 75 ohm system (now shown) the insertion loss was slightly higher (about 1dB) but the frequency loss/flatness was about the same, being pretty consistent from about 1.8 MHz through 1 GHz.

Figure 13 shows the reverse isolation of the tap:  Rather than 6dB of coupling from the main line for signals going the "other way", the absolute is closer to 20dB - about 13dB lower.  (The actual reverse isolation is the absolute isolation minus the forward loss).  In a 75 ohm system (not shown) the reverse isolation was quite a bit better (closer to 30dB over the 5 MHz-1GHz range) - but this result is completely expected:  The reverse isolation is akin to measuring VSWR, and operating a 75 ohm device at 50 ohms implies a VSWR of 1.5:1 - a "return loss" of 14dB - very close to the values depicted in Figure 13 over much of the frequency range when the "forward" loss is taken into account.

On a tap there is yet another measurement to be taken - the loss between the in and out port.  Because we are measuring a 6dB tap - a device which siphons off about 25% of the signal - we would expect at least that amount (theoretically 1.25dB for 6dB) to be lost as it is coupled to the "tap" port. Figure 14 we can see that the measured loss is slightly higher than this between 1.8 and 200 MHz- a bit over 2dB.  Some of this "extra" loss is due to the intrinsic losses of the device, but a smaller amount is a result of the use of a 75 ohm device on a 50 ohm system.
Figure 14:
Through loss of the 6dB tap in a 50 ohm system.
Click on the image for a larger version.

This device - which is rated down to 5 MHz - may be useful through at 160 meters (1.8 MHz) - but the insertion loss goes up rather quickly at lower frequencies.

This device is NOT suitable for passing DC (e.g. for amplifiers, control signals) as it has a DC short across it - but that is not true of all taps.  For example, the Holland Electronics "HDCS" series does allow low frequency RF down to DC to flow through it - but like the DCG-6SB, its coupling coefficient deteriorates quickly below about 1.8 MHz.

* * *
General conclusions:

If you are going to use TV-type splitters for HF, make sure that you get devices that are explicitly rated down to 5 MHz.  Based on the (limited!) sample of devices that were tested, these devices can be expected to work into the 160 meter amateur band (down to 1.8 MHz).  While these devices may be usable thoughout the entire AM broadcast band (down to 540 kHz) expect performance to drop quickly in terms of added "through" attenuation and worse port-to-port isolation.

A "TV" type device - one that may indicate a start frequency of 5 MHz, or just any device that is claimed to work at TV (VHF/UHF) and FM broadcast frequencies will likely work from 6 meters through 70cm (50 MHz - 450 MHz).

Again, for general signal splitting and combining, these 75 ohm devices, used at 50 ohms, are quite usable for non-critical applications - provided that they be used at low power levels (a few 10s of milliwatts at most) and where one need not have precise 50 ohm matching and high port-to-port isolation.  Remember that most 50 ohm devices (receivers, amplifiers, filters) have only "approximately" 50 ohm source/load impedances - and filters in particular will, out of their design frequency range (outside the band-pass, on a notch frequency, above the low-poss cut-off, below the high-pass cut-off) will likely have anything but a 50 ohm characteristic impedance, so even a "proper" 50 ohm splitter/tap device would not necessarily yield any better performance in those situations.

For information about the design and use of splitters/combiners in general, a good reference is Mini-Circuits AN10-006, "Understanding Power Splitters" - link.

* * *

Far more data was gathered than was presented here, but that depicted above is representative of the devices that were on hand.

* * *

This page stolen from


Saturday, December 7, 2019

Automatically re-connecting low-voltage cut-outs for 12 and 24 volt lead acid battery systems

In a previous post I described a simple circuit that provided a low-voltage cut-out that could be used in a battery-operated system - see the article "A latching low-voltage disconnect for 12 volt lead-acid and lithium batteries".

That circuit - intended mostly for lithium-based batteries - required manual intervention to "reset" the device, intended for those situations where you wanted to provide manual intervention in resetting the circuit to prevent causing harm to the battery - and maybe the gear connected to it.

Figure 1:
The completed 24 volt low-voltage cut-out.  R5, the cut-off voltage
adjustment, is the blue 10-turn potentiometer near the middle and
R8 is the black, single-turn potentiometer near the top, to the left
of Q1 and its heat sink.  U1 is the lower transistor-looking device
near the left side of the board.
This device was installed in an outdoor enclosure that houses a relay
point to bridge a wireless Internet connection over a hill that has
a 24 volt back-up battery system.  In the event of an extended
power outage, this device prevents damage to the batteries,
particularly in the winter where this device will
prevent the batteries from being damaged by freezing.
The small heat sink is adequate for this FET and an operating
current of 5 amps.
Click on the image for a larger version.
This circuit is different from that described in the link above, intended for lead-acid battery based systems where an automatic reconnect of the battery is required when the battery voltage rises above a threshold voltage after application of charge current.  An example of where this would be useful is a system in which the battery is charged by a main-powered DC supply that is used to keep the battery charged where the battery will power the load in the event of a power failure.

Why would we worry about this?

Related to this, let's briefly talk about maximizing longevity of lead-acid batteries:
  • For best longevity, the shallower the discharge, the better.  A classic example of this is a solar-powered system where there is a daily discharge.  If the battery were sized too small such that it was discharged from "full" to 30% every day, it might last only a year or so at best, but by increasing the battery size such that it was only discharged to 75% worst-case, the cycle depth would be much shallower and the battery would last much longer.  This assumes, of course, that there is enough charging capacity to do this, which can be an issue for a solar charge system in the winter and/or periods of weather where there is little direct sun.
  • Lead-acid batteries can handle being run completely down - as long as you don't keep them in that state for very long (a few days at most - as little time as possible) and don't do it very often - and the more charge you can leave on them at the deepest point of discharge, the better.  In other words, if you run an otherwise healthy lead acid battery completely dead and immediately recharge it, little immediate damage is likely to have been done other than taking a bit of life off it farther down the road as most lead-acid batteries can only handle a few hundred of such cycles before they are significantly and permanently degraded.  (The previous statement applies only to "Deep Cycle" lead-acid batteries:  Automotive "starting" batteries do not fall into this category - They can be permanently damaged by as few as dozen or two "complete" discharge cycles in some cases.)
  • In cold-weather environments, while the degradation (primarily sulfation) caused by the battery being dead for extended periods is dramatically slowed, extremely deep discharge also reduces the specific gravity, raising the electrolyte's freezing point and increasing the possibility of the battery being damaged/destroyed at very low temperatures if it does get cold enough to freeze the electrolyte.  Preventing too-deep a discharge by disconnecting the load while there is still some capacity left can reduce the probability of this happening.  (This can happen with both flooded cell and AGM batteries.)
  • Important:  A general rule of thumb is that above approximately 70F (approx 20C) for every 10F (approx. 5.5c) increase in temperature, the expected lifetime of a lead-acid battery is approximately halved.  Carefully consider this if the battery is to be in a location that might get warm/hot.
For several of the reasons mentioned above, a low-voltage disconnect circuit is essential in systems where there is the distinct possibility that the battery could be completely discharged - both to prevent the loss of battery capacity due to over-discharge (e.g. sulfation if the battery is left "flat" for a long time) as well as preventing freezing of the electrolyte when the battery is deeply discharged where/when that is a concern.

A simple low-voltage cut-off circuit:

While there are many ways to build such a circuit, the circuit below - designed for a 24 volt battery system (values are included for use with a 12 volt battery) - demonstrates a simple, yet flexible device.

Figure 2:
A low-voltage battery disconnect, designed for 24 volts.  It has a useful adjustment range of between 22 and 28 volts.
For 12 volts:  R2=4.7k, R4=62k, R7=47k, R11=47k, R12, R13, R14=10k.
 Components LED2 and R14 are optional, used to indicate that the circuit has disconnected the battery from the load.  (Errata:  LED2 is shown connected backwards.)

Not shown is appropriate fusing on the battery side of the circuit to protect against short circuits and/or catastrophic component failures.
Click on the image for a larger version.
This circuit, somewhat similar to that described in the other article linked above, works as follows:

The battery voltage is measured using R4, R5 and R6 which form an adjustable divider to take the battery voltage down to the 2.5 volt comparison voltage required by U1, a TL431 "programmable" zener diode.  When the voltage at the wiper of R5 goes above 2.5 volts, U1 will conduct.

When U1 conducts it pulls down the voltage at the base of Q1 via R3 causing it to turn on.  This voltage, via R12, is fed to the gate of Q1 - an N-channel power MOSFET in the negative battery lead. In this way, as long as the voltage is above the threshold set by R5, Q1 is turned on, connecting the load to the battery.  While U1 is turned on and pulling the voltage at the bottom end of R3 close to ground, the voltage at the gate of Q3 is low, keeping it turned off.

If the divided battery voltage drops below the low-voltage cut-out threshold, U1 turns off and its cathode is pulled high by R2 with resistor R1 being used to allow Q2's gate to go high and turn it off.  When Q2 is turned off, the gate voltage on Q1 goes away, being pulled to its source potential by R11.

At this same time, the rise in voltage at U1's cathode also causes Q3 to be turned on and the voltage at the junction between R4 and R5 is pulled slightly down, requiring a higher battery voltage to turn U1 back on than was required for it to be turned off, thus providing a bit of hysteresis.  It is by adjusting R5 that the amount of hysteresis (e.g. the amount by which the turn-on voltage must be higher than the cut-off voltage) may be set.

There are some "extra" components included to make the circuit more robust:
  • C1, across the reference pin of U1, reduces this circuit's susceptibility to being triggered by power glitches (e.g. lightning) and stray RF energy.  If the circuit tends to trip out when a load is activated (e.g. voltage "dip" caused by battery/lead resistance)  this capacitor can be increased in value to slow U1's response - up to several microfarads.
  • Capacitors C2 and C3 are across their respective FET's gates to provide RF energy protection as well as to smooth transients that might cause false triggering or damage.
  • ZD1 is placed between Q1's source and gate to protect it against excess voltage that would damage the device - possibly caused by transients.  The maximum gate-source voltage rating of a typical power FET (Q1) is around 20 volts, so this is a required component for the 24 volt version.
  • Finally, C4 is placed across the drain and source leads of Q1 to provide both resilience to stray RF energy as well as to suppress transients that might appear across it when the power is off.
Also included in the circuit is the "power on" indicator consisting of LED1 and R13 which provides an indication that voltage is present between the "Load +" and "Load -" terminals.  The optional circuit containing LED2/R14 shows when Q1 is turned off (e.g the load is disconnected) by detecting when there is a voltage drop across it.  (Note:  LED2 was accidentally drawn "backwards" in Figure 2.)

  • Important:  To simplify the design, it is the "Batt -" lead that is switched to disconnect the load.  What this means is that the negative battery lead must not be connected anywhere else or the battery disconnect will not work properly.  It is possible to design the circuit to use a P-channel FET in the positive lead, but high-current N-channel devices are more plentiful and less expensive.  Instead of a power FET, a slight modification will allow a relay to be used, instead - but at much higher quiescent current owing to the required current for the coil.
  • LED1 and LED2 may indicate correctly only when there is a load connected across the "Load +" and "Load -" terminals.
  • Even when the load is disconnected this circuit will draw several milliamps of current due to the quiescent load of U1, its voltage divider and LED2, the "Battery Disconnected" indicator.  Keep this in mind if you are going to use it in an application where there is a reasonable probability that battery charging may be unavailable for very long periods of time.
  • The current capability of this circuit is limited by the current-handling capability of Q1, the power FET, and the amount of heat-sinking attached to it.  When choosing a power FET, not only should the current rating be considered, but also the specified "on" resistance.  For example, if a 50 amp FET is used and it has an ON resistance of 0.01 ohms, at 20 amps it will dissipate at least (20 * 20 * 0.01 = )  4 watts and will drop (20 * 0.01 = ) 0.2 volts.  The heat sink shown in Figure 1 is capable of dissipating that much power under all but the hottest ambient temperature.
Comment about Q1, the main power FET:

The power FET is not specified, as practically any appropriate N-Channel power FET will work - and here are a few selection guidelines:
  • Pick a device with a current rating of at least 3 times the maximum that you expect to draw.  100-150 amp devices are fairly inexpensive, so don't be afraid to pick a far "heftier" device than you might otherwise select.  Note that while a particular FET may have a "150 amp" rating, if there is a short circuit downstream, it is possible that connections inside the FET itself may act as a quicker-acting fuse than an actual fuse intended to protect the circuit.
  • Pick a device with a voltage rating of at least 4 times the battery voltage, which would imply at least a 125 volt device for 24 (really around 28) volts and 75 volt device for 12 (really around 14) volts - and there is little penalty in going higher than that given a choice of devices with the same rated current.
  • With a properly functioning circuit, a heat sink is likely not required, but a small heat sink doesn't hurt - particularly if the circuit will be exposed to high temperatures.  While the "on" resistance of a saturated power MOSFET can be very low, it is not zero and at maximum expected current, the FET can produce a some heat.
    • Note that a FET's "on" resistance may be a fraction of an ohm, but it is not zero so some voltage will be dropped - and heat generated - by the FET.
    • Remember the formula - Power in watts = I*I*R where "I" is the current and "R" is the "on" resistance of the FET - and if power is more than 2-3 watts, consider a larger heat sink for Q1 - and after it is built, make sure Q1 isn't getting "hot" under the expected load.
The amount of current that can be handled by this cut-off-switch is essentially the limit of the chosen FET, the fusing, and the interconnection wiring.
Examples of FETs:
  •  The IRF540 is rated for 25+ amps at 100 volts and this would be an excellent device for a disconnect that was intended to handle 5-8 amps at 12 volts.  This device is inexpensive and readily available - even from surplus outlets, less than $1.50 from Electronic Goldmine.
  • The 45N20 is rated for 35 amps at 200 volts and would be a good choice for 10-12 amps and either a 12 or 24 volt disconnect - and is also available from Electronic Goldmine as the "SSP45N20".
(The Electronic Goldmine is used as an example of surplus electronic suppliers.  These parts are readily available from suppliers such as Digi-Key, Mouser, etc.  Always buy parts like this from reputable suppliers as counterfeit power devices are quite common.)


Important:  Verify that the circuit works and is properly adjusted before placing it into service!

The procedure is as follows:
  • Set R8 for maximum resistance. 
  • Connect "Batt +" and "Batt -" to an adjustable DC power supply that is capable of providing between 20 and 30 volts.
  • Connect a known-accurate voltmeter to the DC power supply to measure the "Battery" voltage.
  • Connect a load to the output terminals ("Load +" and "Load -").  A small lamp of the correct voltage or resistor (1k at 1 watt or more) will do. 
  • Increase the voltage to 30 volts:  The load will turn on. (15 volts for the 12 volt version.) 
  • Decrease the voltage slowly:  The load will turn off below a certain voltage.
  • Increase voltage again to turn the load back on and then adjust R5 so that the load turns off at the desired voltage  The lowest cut-off voltage that is suggested is 1.92 volts/cell for a state-of-charge of 20-25% at "room temperature", which translates to:
    • For a 12 volt lead acid battery, 11.5 volts
    • For a 24 volt lead acid battery, 23.0 volts
    • Note:  At very low temperatures, the output voltage of a lead acid battery will drop given a constant charge level.  This means that in the case of the 12 volt battery that is cold (32F, 0C) a 11.5 volt cut-off would imply that the actual charge level of a battery would be higher than the 20-25% level indicated by this same voltage of the battery were at room temperature.
  • This concludes the adjustment of the cut-off voltage.  Adjusting R8 should not affect the cut-off voltage.
  • Set the power supply voltage below the cut-off voltage and slowly increase it until the load turns back on, noting the voltage at which that happens.
  • Adjust R8 so that the cut-in voltage is adjusted as desired.  The cut-in voltage is much less critical, but should be well above the cut-out voltage.
    • A typical cut-in voltage for a 12 volt lead-acid battery is 13 volts, or 2.17 volts/cell, but it may be set lower - to 12.5 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
    • A typical cut-in voltage for a 24 volt lead-acid battery is 26 volts, or 2.17 volts/cell, but it may be set lower - to 25 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
    • Note:  A higher cut-in voltage may delay restoration of the output load, but it will allow the battery to charge more before the load is restored.  An example where this may be important is in a solar-powered system where a low output from the PV array may be sufficient to slowly charge the battery, but may not be able to sustain the load:  This can happen in the morning, when the panels are covered with snow, and/or on very dark and cloudy days.
  •  Re-check the cut-off and cut-in voltages:  When adjustment is needed, remember to set the cut-off voltage first and then the cut-in voltage.

This device could be used for similar-voltage Lithium-based batteries with appropriate adjustment of the cut-in and cut-off voltages.  Note that any Lithium-based battery system should already have a "protection" circuit that protects against over and under voltage.  Particularly with LiFeO4 batteries, a circuit to equalize the voltage across the cells should be used.
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