Containing RF noise from a sine wave UPS

An amateur radio friend of mine (WA7X) has a cabin in the mountains.  It is not a particularly "rustic" cabin as it is festooned with radios, antennas, propagation beacons, computers and cameras and has an internet connection, but because it is in a remote location it occasionally has 3-6 hour power outages and thus it also has a UPS (Uninterruptible Power System) to keep many of these things online in the interim.

This "sine wave" UPS is a 1.5kVa unit that was cast off from by someone for the same reason most UPSs are cast off:  Its internal battery went bad.  Rather than simply replacing the battery, its previous owner simply got another UPS and asked the question "Do you want this?"

Instead of replacing the internal battery, the DC connections for the batteries were brought out and a bank of six 12 volt lead-acid batteries was wired up (two sets of three parallel batteries connected in series amounting to a nominal 200-ish amp-hours at 24 volts) to provide the needed 24 volts and a DC circuit breaker was added for safety, this battery capacity allowing the unit to run for far longer than it could have on the original battery.

While this UPS was more efficient than a previous unit and produces a fairly nice sine wave rather than the typical, ugly "modified sine wave" there was a price to pay:  RF Interference (e.g. RFI) that was present whether the unit was active or on standby.

But first, a few weasel words:

• This project involves high voltages and/or currents:  Do not attempt to construct a similar device unless you are thoroughly familiar with electrical safety and the wiring of such devices.

• If you use an external battery bank with a UPS it is imperative that you include some sort of current liming, such as a fuse or circuit breaker rated for both the expected current and battery voltage.  Such devices are available from auto-parts stores.

• If you use an external battery bank with a UPS you must determine if this battery bank is "mains referenced" internally by the UPS or not.  If it is mains referenced (e.g. connected directly or indirectly to the mains AC voltage) then the low-voltage DC terminals will pose a line voltage safety hazard and care must be taken - the least of which is enclosing the battery and any exposed DC terminals to prevent accidental contact.  This UPS's battery terminals were isolated from the mains and given that the room in which the UPS resides has restricted access, the low-voltage battery connections themselves were deemed to be "adequately safe" left exposed.

• YOU are responsible for the safety and for any liability if you choose to do something similar to what is described on this page.  You have been warned!

  

  Figure 1:
  The completed AC/DC filter.  This
  box contains a "brute" force L/C filter
  for the battery (DC) leads as well as
  separate filters for the AC mains in/out
  power connections.
  Click on the image for a larger version.

Interference:

The radio frequency "grunge" from the UPS manifested itself on the HF ("High Frequency" or shortwave) bands in several ways.  Most obvious was a loud "buzz" every 40-50 kHz on the lower (80 and 40 meter) bands, but there was also more subtle interference that pervaded these and higher bands:  A background "hiss" that might initially escape the notice of the casual listener until one realizes that this noise masked signals that should have still been perfectly audible.  If one switched the receiver to AM it would be observed that this "hiss" was subtly modulated at twice the mains frequency, 120 Hz.

To determine the extent of this sort of interference the typical procedure is to start turning things in the house off one-at-a-time (or, more reliably, the reverse:  Turn everything off at the breaker panel and then turn on one thing at a time) until the culprit is found.  This was done with the UPS and the magnitude of its "radio interfering" nature was determined.  Clearly, this "grunge" was being conducted from the power leads going in and out of the UPS.

Further experimentation revealed the true extent of the noise:  Even with everything disconnected from the UPS - that's to say, with it running on its battery, unplugged from the mains and the load disconnected - there was still detectable noise getting to the antenna and a quick check with a portable shortwave receiver proved that the remainder of this noise seemed to be being radiated by the physically-large battery bank and the wires that connected it.

While the proper application of a "brute force" AC line filter would likely quash the noise conducted in and out of the UPS on the mains power leads, something else would be required to minimize/eliminate the noise emanating via the DC lines.

"Brute force" line filters:

Figure 2:
A typical "brute force" L/C line filter typically found on devices
to minimize conduction of extraneous RF energy on the
AC mains.  For a 1500kVa UPS the filter would need an
appropriate current rating - particularly the fuse!
In some filters, two sections are used, with the components
L1, C1, C2 and C3 repeating.  This same filter
topology is used for the described DC filter.
Click on the image for a larger version.

One of the best ways to eliminate or minimize the amount of RF energy that might be conducted out of a potentially interference-generating device is to apply a combination of inductance and capacitance to that line as depicted schematically in Figure 2.

With the noise coming from the power supply (the UPS in our case) capacitor "C4" effectively "shorts" this RF noise to both sides of the power line, leaving the AC signal (pretty much) unchanged.  Inductor L1 consists of two equal windings on a ferrite core and it is practically invisible to signals that are equal and opposite, but it acts as a series choke for signals that are "common mode" - that is to say, equal on both sides of the power supply's mains leads - such as the RF noise energy.

On the "mains" side of the filter capacitor C3 reinforces the common mode again while capacitors C1 and C2 shunt any remaining RF energy from the power supply - its impedance now made much higher by inductor L1 - to ground - which would be the metal enclosure in which everything was mounted.

As it happens, these filters are available on the surplus market, and we would need three of them:

• One for the AC mains connection to the UPS.
• Another for the UPS's AC output
• A third one for the battery connection to the UPS.

With the UPS being rated for 1.5kVa, some surplus AC mains filters, rated for 16 amps, were obtained while a filter designed specifically to filter DC lines rated for at least 50 amps at 60 volts was found (at The Electronic Goldmine - item G21652 - link) to filter the power connection between the UPS and the battery:  50 amps at 24 volts is not quite 1.5kVa, but there was nowhere near this amount of power being drawn from the UPS and the added circuit breaker/disconnect would provide the required safety - and the filter should be able to handle a brief overload, anyway.

Putting the filters in a box:

Figure 3:
Inside the filter box.  Along the top edge is the main AC input.  On the bottom edge - and just below the outlet on the left side - are the short leads that conduct the "dirty" AC power to/from the UPS, each through its own, separate filter - the two black boxes.
On the far right is the high-current DC filter with stud (bolt) connections being used to make the connections between the battery and the DC input of the UPS.  Barely visible along the bottom, one of the three studs is used to connect a piece of heavy wire or braid to bond this box to the chassis of the UPS to minimize conducted/radiated RF.
In the middle is a circuit board that contains a mains transformer, a high-current mechanical relay and a small solid-state relay.  This board - added later in the design - allows one pair of the outlets to be turned on and off with a simple contact closure of an internet-connected remote power switch.  On the control cable for the relay (the thin white wire) is a ferrite device which minimizes the amount of RF energy that might possibly be conducted in or out of the box on that control lead.
Click on the image for a larger version.

Ideally, one would have put the UPS and the batteries in a large metal box and passed the power leads in and out of this box only via the filters, but this simply wasn't practical, so the next best thing had to be done:  Put the filters in a single, metal box that would be electrically bonded to the UPS chassis and make the connections in and out of the UPS using short leads.  By keeping the leads short and bonding our new filter box to the UPS, we'd do our best to limit the number and length of conductors that carried the RF interference and, most importantly, preventing RF circulating currents from finding their way on external connections.

Figure 4:
The end of the box with the mounted outlets.  As noted,
one pair of these outlets is connected to a relay to allow the
connected devices to be remotely controlled.
Click on the image for a larger version.

To that end a power distribution box was found at a home improvement store (Lowes Depot, I think) for less than $25 and the "guts" removed (the box with "guts" was cheaper than just an empty box by itself!) and the filters mounted inside.

With the short-as-possible conductors between the UPS and the filter, they will have minimal ability to directly radiate RF while the RF conducted on these lines will be filtered by the circuitry in the box itself with the bonding of the two boxes minimizing differential RF currents.  The power "to" the UPS was made with a short length of "SO" cord with a female connector on it while the power "from" the UPS is via a short cable with a male connector, plugged into one of the UPS's outlets - and being the ONLY thing plugged into the "dirty" AC output of the UPS.

Installation:

Figure 5:
Perhaps a bit cluttered, this is the UPS sitting atop the filter, installed
and working.  In the lower-left corner of the picture, above two
batteries may be seen the DC circuit breaker/disconnect that protects
the DC circuit for short circuits.
Click on the image for a larger version.

Being that this is a remote location, the filter unit was installed a few weeks after it was constructed, having been tested (as best as could be done) on the workbench:  Power flowed through the various filters and the load control relay worked properly.

When installed, the filter box was placed underneath the UPS as it had a slightly larger footprint - and to minimize the length of the "noisy" DC and AC power leads from the UPS, along with the lead used to bond the two cases together.

To connect the DC, the cable coming from the batteries was effectively cut so that there was just enough of it emerging from the UPS to connected to the "output" side of the filter:  On both sides of this cable heavy "ring" lugs were attached and these were connected  to the studs of the DC filter.  To eliminate the probability of accidental shorting, the ground stud on the "input" side of the filter was removed and near both connectors a plastic wire clamp (one may be seen in Figure 6, below) to keep the positive wire in a fixed position and rotating into and shorting to the other stud.

Figure 6:
The back side of the UPS, showing the coiled power cord and the
(green) bonding wire that connects the chassis of the UPS and filter
box firmly together.  At the bottom of the picture can just be seen the
plastic clamp the keeps the positive wire lifted and away from the
the negative wire, to prevent shorting.
The ONLY thing plugged directly into the "dirty" AC output
of the UPS is the cord going to the filter:  Plugging anything else
directly into the UPS would at least partially negate the filtering! 
Click on the image for a larger version.

The AC input and output of the UPS was simply "plugged in" to the cables and the excess cordage was neatly coiled and stowed at the back end of the UPS using plastic "zip" ties:  It was important to do this because these cables are unfiltered and are "noisy" with RF meaning that they should be kept as small and as close to the metal of the cabinet as possible and kept away from any other conductors to minimize cross-coupling which would defeat the purpose of this filter.

To the remaining ground stud of the DC filter was attached a short piece of heavy (8 AWG) green wire with a ring lug on each end, this wire being visible in Figure 6.  The other end of this wire was attached to a marked grounding screw on the UPS chassis to bond the two boxes together and minimize RF circulating currents and to prevent the UPS chassis itself from being a source of radio frequency interference by direct radiation.

The result:

Taking the 20 meter (14 MHz) amateur band as an example, the UPS caused an extra 2 "S" units or so of noise above that of the typical ionospheric noise floor when it was powered up, before the filter was installed - this, being detected on a Carolina Windom antenna lofted between two trees high above the cabin's roof.  After this filter was installed the noise from the UPS was completely undetectable on any HF band, revealing other weaker low-level noises from other devices - the quieting of some of these will be discussed in later installments.

Figure 7:
A general block diagram of how the parts are interconnected.
The external UPS battery bank is protected with a DC-rated circuit breaker/
disconnect switch.  This particular UPS operates from 24 volts, hence the two
series-connected 12 volt batteries.  If the battery is inside the UPS's metal
cabinet, the DC filtering and connections are not needed.
Note that the "output" side of both AC line filters are both connected to
the UPS:  This is done because many filters are designed such that the
"output" side is that best-suited for connecting to RF-noisy circuits.
This diagram does not include the remote relay described.
Click on the image for a larger version.

Before the installation of the filter I'd placed my FT-817 (a small, portable HF transceiver) across the small room from the UPS, receiving with a short antenna and tuned to the 20 meter band.  When the UPS was operating and connected to its loads its noise was clearly audible, causing several S-units of indication on the signal meter.

After the filter was installed and the unit was powered up again with the loads connected, the noise was barely audible in the FT-817 and from across the room, it went away completely when the green wire was connected, bonding the UPS and filter chassis together.  If the radio was moved to within a foot or two of the UPS I could start hearing the "hash", but it seemed to be emanating only from the coils of AC cables "zip"-tied to the back end of the unit.   Because of the short length of the wires - and their being close to the metal case and not near any other wires into which this noise could be coupled it is unlikely that these short conductors will radiate any detectable noise at a distance greater than a few feet.

Doing the same on another UPS:

As part of his "noise abatement" strategy, WA7X did some RF sniffing around his house and discovered that a major contributor of RF noise was a sine wave UPS of a different brand.  While the UPS itself was completely different, the same things needed to be done to it as was

Figure 8:
Line input filter.  This was mounted externally, using very
short wiring, as it was difficult to insert a filter directly
into the AC mains path via the connector that was
directly mounted to the UPS's circuit board.
Click on the image for a larger version.

done to the UPS at his cabin:

• A "brute force" line filter on the AC input.
• Another line filter on the AC output.
• Because the batteries on this one were, like the other one, external there needed to be a filter on the battery DC line as well.

This UPS was a bit smaller - 750VA (about 600 usable watts assuming an average 0.8 power factor) and it, too, had been modified to use a much larger set of external batteries to maximize run time.   Like the other UPS, it produced copious quantities of RFI whether it was "UPSing" (providing back-up power) or idle.

Figure 8 shows the input filter - and its mounting represents a slight complication:  The AC (mains) input went directly to the UPS's circuit board - and no on-board RF filtering was seen.  While we could have probably removed the board, cut some traces and soldered wires to "intercept" the AC flow between the input mains cord and the circuitry, we did something a bit "cheesier" - but still effective:  Attached a filter to the outside.  Going to our local electronics surplus store we found a suitable line filter with an overkill rating of about 10 amps (6 amps would have sufficed for a 750 kVA UPS) with a built-in IEC power connector.  As can be seen from Figure 8 we cut up an IEC power cord and with minimal length, soldered it directly to the terminals of the line filter and insulated them

Figure 9:
Output line filter.  This was mounted in the empty battery
compartment.  The "dirty" AC power from the inverter
is via the black/white wire on the left and the "clean" AC
power to the outlets is on the right.  The "clean" power
was wired using green wires with black/white markings
as that was the only solid wire - used to push into the
outlet's rear connector - that was on-hand.  These two
sets of wires are kept physically separate to prevent RF from
capacitively coupling from one to the other.
Click on the image for a larger version.

with heat shrink tubing and copious quantities of RTV sealant.  Because this filter was intended to mount via screws to the back panel of a piece of equipment, it had no mounting lugs, so we soldered some to the tin-plated case and used these for the ground connection as well.  It looks a bit kludgy, but its "pretty safe"!

For the AC mains output we disconnected the wires from the rear-mounted power socket and spliced short lengths of wires to it, sleeving the splices with several layers of heat-shrinkable tubing, connecting the output of the UPS's inverter to one side of the filter mounted in the battery compartment.  Connected to the AC outlets were short pieces of wire which were then soldered to the other side of the mains filter.  Although not seen in the pictures, there were no chassis holes that permitted the passage of these two sets of wires, so a pair of holes (protected with rubber grommets) were drilled with the unit tilted so that metal shavings (e.g. "swarf") would not end up in the circuitry - one hole for the "dirty" AC power from the inverter and another hole for the "clean" AC power to the socket on the output.  Having these two sets of holes and routing the two sets of wires away from each other prevents the RF grunge from coupling from one set of wires to the other.

Figure 10:
The DC line filter.  Too big to fit in the battery compartment,
this filter was mounted to a metal partition present in the UPS,
holes in that partition and in the chassis below to pass the
metal studs that carried the DC power.  Underneath, through
the battery compartment, was later wired the cabling for the
24 volt, fused DC input.
Click on the image for a larger version.

Finally, there is the DC filter.  Using the same surplus filter obtained from Electronic Goldmine as was used in the other UPS, this filter was mounted in an empty space adjacent to the circuit board.  There, a metal divider provided a handy mounting point for the "dirty" side of the DC connection (e.g. the input to the inverter).  A metal hole punch and a nibbling tool was used to fashion the holes necessary to pass the power studs through them with a similar hole being punched through the case into the battery compartment.  Before the filter was installed we declared which terminal was the positive side and clearly indicated it with a red permanent marker.

In Figure 8 one can see a grommet through the panel to the right of the filter:  This was the original passage of the DC power cables which were not installed at the time the pictures were taken.  When these cables were added later, appropriate DC fusing was included and the wires were carefully routed through the battery compartment, keeping them away from the "dirty" side of the filter to prevent coupling of RFI.

The result:

As with the other power supply, a portable AM/shortwave radio held near the UPS was not able to detect any noise unless it was held within an inch or two (couple of cm) where it was likely that magnetic coupling occurred.  When checked, there was no noise heard on the HF spectrum that was attributable to the inverter - only a few more "new" noises that will need to be chased down now that this UPS has been quieted!

* * *

Yet another UPS:

Some time after the above article was written Glen acquired another UPS at his house - this time, an 8 kVA unit pulled from an office.

Not surprisingly, this, too, produced RFI since it was only a Part 15 "Class A" device.  Because the AC and DC current was too high for inexpensive/surplus RFI filters, they had to be constructed using very large 31 Mix "Monster" toroids (from KF7P.com).  For the mains side, a bifilar choke of adequately-sized wire was wound on a choke and the same circuit as depicted in Figure 2 was constructed using class X and Y "safety" capacitors:  About 4700 pF for C1/C2 and 0.1 uF for C3/C4.

After the AC side was done, it was observed that noise was still being radiated from the wiring to the external battery bank.  For the DC side, a length of welding cable was procured and it was possible to wind 7 bifilar turns of this cable on another "monster" toroid - and the same types of class X and Y safety capacitors were used.

Each filter was built into its own box - inexpensive steel electrical boxes obtained from Lowes Depot - and these boxes were bolted to the chassis of the UPS itself to eliminate the possibility of noise radiating from connecting leads on the "UPS Side" of the filters.  The result - as with the others - was that the noise was undetectable with a portable shortwave radio more than a few feet away.

* * *

Links to other articles about power supply noise reduction found at this site:

• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.
• Quieting a 150 Watt Samlex Sine Wave inverter - link.  This talks about making an otherwise RF-noisy sine wave inverter quiet.
  

[End]

This page stolen from ka7oei.blogspot.com

A "quiet" 5 volt USB car power supply

For a previous, related article see the May, 2014 posting - "How USB car power adapters can ruin 2 meter mobile reception" - link.

In that post I wrote about those ubiquitous USB car power adapters that fit in a cigarette lighter and while these devices work well for power phones, GPS receivers and the like, they are terrible if you have any intention of listening on 2 meters (or other bands!) while in your car - even if you are using an external antenna.

Unmodified, I found that the unit that I had in my car had effectively reduced the sensitivity of my 2 meter transceiver - with its external, permanently-mounted antenna - by nearly 40dB.  To put that into other terms, a signal that was weak, with this adapter turned off, would have to be increased in signal strength by a factor of 10 thousand to sound the same when the adapter was plugged in with cables attached!

Figure 1: 
The completed USB car power adapter in the box.
Click on the image for a larger version.

Modifying the adapter as noted in the above link I managed to knock down garbage emitted from the adapter by 15-20 dB (a factor of 30-100) - a significant improvement - but when driving up some of the local canyons I found that repeaters that had been perfectly copyable along the entire route previously were no longer audible unless I unplugged the adapter.

Something had to be done!
 
I'd already done about as much
to the small cigarette-lighter USB adapter as I could, short of completely rebuilding it, but it was still too noisy.  The problem with this device was that of differential currents:  Between the input and output terminals of the device there were, on the circuit board, several amps of switching current floating around.

Even though there was a common "ground" shared between the input and output, this same "ground", consisting of just a few short and somewhat thin traces rather than a large, heavy and solid ground plan was suprisingly reactive at higher frequencies - such as those above a few 10's of MHz.  Even a very short length of circuit board trace can have a few 10's of nanoHenries of inductance, but if you are pushing amps of current and considering harmonic energy at 100+ MHz, you can soon see that these seemingly few nanoHenries can make what would seem like a "solid" ground plane act like the feedpoint of an antenna.  With the switching supply itself as the transmitter and the car wiring and the connected USB cables acting like the wires of a dipole antenna one can soon see where the interference was coming from!

Aside from having the voltage converter designed on a "proper" multilayer circuit board with very high quality components - not something that you'll get on a $5-$20 power adapter - the only other way to take care of the problem is to put the entire thing into a metal enclosure and filter/bypass all of the power leads going in and out.

Two approaches:

There are two approaches that I could have taken to accomplish this task.

 1)  Containing noise from the original adapter.

The easiest would have been to take the original USB power adapter and put it in a shielded enclosure and bypass all of the leads going in and out.  I would, of course, have lost the convenience of the small, self-contained device that plugged into the cigarette lighter, but I would have solved the problem.

The techniques described below could apply to filtering the original adapter, just as they did the approach that I ended up taking.

2)  Build another supply.

Figure 2:
The "DC to DC Converter Step-down Voltage LED Power Module 3A"
obtained from EvilBay.  (Ignore the slightly messy soldering - that
was the fault of myself and the often  "grainy" nature
of  lead-free solder!)
Click on the image for a larger version.

If you have been following this blog or read any of the other entries, you will not be surprised that I took this route - but I did not build it entirely from "scratch", but I went to EvilBay and found some inexpensive buck-type switching regulators for under $2 each and used them as a starting point:  I couldn't even get the components themselves for $2!

The devices that I found were described as "DC To DC Converter Buck Step-down Voltage LED Power Module 3A 12V To 5V 3.3V" (see Figure 2) and are based on the LM2576 switching regulator.  These also had a fixed 3.3 volt regulator on board, but I had no need for that so I left it in place, doing nothing with it.

It is important to note that a cheap, $2 switching regulator from EvilBay is not going to be any better than the original USB car adapter in terms of RF cleanliness and it may even be suspect in terms of reliability so a few things need to be done to the $2 board before it may be deemed to be reliable and useful.

The first things to consider are the electrolytic capacitors on the cheap switching regulator boards:  They are not to be trusted!  This should be considered to be true of any cheap switching supply that you get on EvilBay!
 

Figure 3:
The inside of the converter showing the  components for filtering
and the two switching modules.
Click on the image for a larger version.

In any switching regulator, one of the most important aspects of component selection is that of the quality of the capacitors:  They must be of the low ESR type, designed specifically for switching service and even more importantly, they must be of a known manufacturer, obtained from a reputable seller.

Upon inspection of these switching regulators I saw that the capacitors onboard were rated at 105C - a good sign - but I'd never heard of the maker.  Rather than removing the original capacitors, I simply paralleled them with 100uF low-ESR Nichicon units that I'd obtained from a reputable supplier (either Digi-Key or Mouser) and used a dab of RTV ("silicone") adhesive to secure them into position.  I would not recommend the use of "hot melt" (thermoset) glue for this:  It will be in a car, which will get hot and bounce around and they will break loose!  Putting these "good" capacitors in parallel would take a lot of the stress off the "unknown" capacitors on the board, prolonging their life.

Figure 4: 
A close-up of the added capacitors and the piece of copper added
for heat sinking and mounting of the regulator board.  As can be seen, a
small piece was added to help stiffen the right-angle board and
increase the heat conductivity.
Click on the image for a larger version.

The second thing to consider about these cheap switching regulator boards are their current/power ratings.

These units are rated at 3 amps output, and a quick check on the data sheet for the LM2576 indicated that this was about right.  Going on faith that the LM2576's on board were the genuine article - and not counterfeit devices - I put one of them on the bench supply and loaded the output to 3 amps and found that they held up fine, but that the heat-sinking - such as it was - was not adequate, at least if I were to put them inside a metal box with no air ventilation.

Fortunately, the solution was simple:  Add a bit of extra heat-sinking.

Using a bit of scrap copper, I constructed and then soldered an "L" bracket to the circuit board on the back side of the board, opposite to where the LM2576 was mounted.   This bit of copper would not only conduct heat away from the LM2576, to the aluminum body of the  die-cast box, but it also provides a very good local electrical "ground" connection for the board as well:  Figures 4 and 6 show details on this bracket.  When you solder this piece of copper to the board, be careful in the application of heat as you could easily "un-solder" the LM2576 from the other side:  I did this by accident on this board that you see in Figure 2 which explains the rather lumpy solder connections!


Filtering circuitry:

The schematic diagram in Figure 5, below, shows how the input and output filtering is connected.

Figure 5: 
Schematic diagram showing the filtering and interconnections.
The "5V/3A Buck Conv." are the voltage converter modules as described/modified.
Click on the image for a larger version.

How it works:

Referring to the diagram in Figure 5, above, L1, a 22uH inductor (the toroid in the upper-left corner of Figure 3, wound with red wire) offers impedance to RF that may ingress from outside and feedthrough capacitor FT1 shunts any remaining RF to ground.  L2 (the yellow-core toroid on the far left wound with reddish wire) blocks RF that may be present from the switching converters DC inputs, also allowing capacitor FT1 to do its job.  Capacitors C1 and C2 perform "bulk" filtering of high switching currents that may be present on the DC input lines, coming from the two DC-DC converters.

There are two identical DC-DC "buck" type converters and only the upper one will be discussed:

Inductor L3 (visible on the right side of Figure 3 on edge covered in heat-shrink tubing) blocks residual switching energy and RF that are on the DC line coming out of the switching converter and these are shunted to ground by FT2, a feedthrough capacitor and additional filtering is provided by C3.

As noted above, all of the electrolytic capacitors are of the "Low ESR" types and of well-known manufacturers (I use only Panasonic or Nichicon).  Again, when dealing with switching supplies, these special low-impedance capacitors are absolutely necessary in order for proper, long-term reliability and efficient operation of such power supplies and using any other type of capacitor will inevitably result in reduced operational lifetime and/or efficiency.

Figure 6: 
The back side of the switching regulator board showing the
added capacitors and the added mounting bracket/heat sink.  The 100uF,
low ESR capacitors added to supplement the original capacitors
on the (cheap!) regulator board can be clearly seen, held in place
with RTV (silicone) adhesive.
Click on the image for a larger version.

Not mentioned in the above description are components R1, R2 and R3 which are self-resetting thermal fuses.  These typically look much like yellow disk ceramic capacitors (they may be either round or square) and when the current through them exceeds their ratings, they get hot (approximately 100C) and their internal resistance increases, effectively opening the circuit.  Unlike a fuse, when the current is removed they immediately cool down and return to their previous state and reset themselves.

These devices are inexpensive and have the obvious advantage of protecting their circuits like a fuse, but self-resetting after the fault has been cleared!

In the above circuit I happened to use "feedthrough" capacitors which may be seen on the metal barrier near the right edge of Figure 3 (the blue devices soldered into it).  While these devices are especially designed for passing DC and blocking RF, they are a bit difficult to find - but are not absolutely necessary.  Instead of feedthrough capacitors, good-quality "monolithic" multilayer capacitors (the small square ones - not disk ceramic) could be used instead, soldered to the wires with very short leads as they pass or through a hole in the solderable ground plate.

In looking at Figure 3 you will also notice that there are two metal barriers constructed of brass:  One in the upper-left corner, just above L2, and the more obvious one near the right side into which feedthrough capacitors FT2 and FT3 are soldered.  Perhaps a bit of overkill, these provide a (literal!) RF barrier into which the feedthrough capacitors are soldered,  Practically speaking, they provide a convenient place to which the important RF bypassing capacitors may be mounted to the aluminum box to which one cannot solder

Aspects of filtering - why this works:

It was noted earlier that the reason why the original USB power converter was so noisy was that there were many amps of switching currents floating around along the circuit board and even though it was "grounded" at DC, the fact that there was so much current and that the circuit board's traces had some inductance that was significant at VHF was the reason why it radiated badly!

In this circuit, we have taken pains to avoid the pitfalls that would cause it to radiate and if you take your own approach using your own switching converter - perhaps putting that USB power adapter that you already own into its own, shielded box, there are a few things to consider.

In this case, it is the combination of the box itself and the inductors and capacitors that work together to contain the switching energy within the confines of the ground plane of the interior of the box.  It is important to note that it is not the shielding of the box, per se that is the magic here, but the combination of chokes in the various leads and the "solid" ground plane which assures that the circulating currents stay between the input and output leads and do not appear across them where they can radiate.

Take, for example, the output inductors, L3/L4.  The job of an inductor is to resist the change of current so it will pass DC just fine, but it will block AC which means that any RF that gets out of the switcher will hit L3/L4, be blocked by it and whatever small amount of residual energy is left will get shunted to ground by FT2/FT3 and further filtered by C3/C4.

The point here is that on the output lead, L3/L4 will block the RF currents as they leave the switcher, breaking up the path for these currents on the output leads.  This not only prevents that energy from appearing on the output leads, but it also prevents any circulating currents between the input and output as well.

What about switching currents on the input lead?

This is handled by using a good quality capacitors for C1 and C2 which will shunt the vast majority of switching energy to ground.  Residual RF switching energy is then blocked by L2 and whatever little gets through is shunted to ground by FT1 and then there's yet another inductor, L1!

Sources of components:

I happen to have a pretty good junk box of components - particularly the toroidal inductors used.  If you don't have such inductors laying around, junked PC power supplies will likely have what you need in the form of toroidal inductors and small, solenoid-wound chokes wound on ferrite.

The values given (22uH, 47uH) are not at all critical:  Anything from 4.7uH to 100 uH would likely work fine as this range would be more than enough to choke off RFI - but the higher values (22uH and higher) would be somewhat preferable if you have an HF rig in your vehicle that might be bothered.  The most important rating on the chokes to consider is that they be wound with reasonably large wire - say #18 AWG or heavier:  If a couple of amps is pulled through the choke, you don't want it to drop more than a tenths of a volt at most, particularly on the output lead!

As mentioned above, you really do need to get good quality electrolytic capacitors for this and I would recommend places like Digi-Key or Mouser in the U.S. and Panasonic or Nichicon brands. I would strongly suggest that when you go looking for capacitors that you get only those that have "low impedance" and/or "low ESR" in their specifications.  Another thing to look for is their temperature rating:  If they are only 85C, they are probably NOT low ESR or low impedance type.

The die-cast box is approximately 4-5/8" x 2-1/2" x 1-1/2" (12 x 6.5 x 3.75 cm) in size and I obtained it from Jameco Electronics, but similar boxes are readily available surplus and on EvilBay.  A suitable enclosure could be also be constructed using pieces of copper-clad circuit board material, and this would have the advantage of being able to solder directly to it, or one could use a much less-expensive folded aluminum "Bud" type utility box.  The important point is that internally, everything must be connected together on a very heavy, solid ground plane, preferably  without any mechanical joints in the box itself between those internal ground connections.

If you were to just put all of these same components into a plastic box and connect their common point grounds together with thin pieces of hookup wire, you would be risking having RF currents circulating along that thin piece of wire and there being differential voltage across it and having the problem, once again, of RFI escaping the switching regulators!  The only way to make a plastic box work for this sort of thing would be to construct a "box within a box" with the internal one being constructed entirely of metal.

Getting the power out and connecting it to the devices in the car:

Up to this point nothing has been said about getting the power out of this box.

Since the female USB connector is ubiquitous, I decided that this was a good approach so I found some inexpensive "USB Extension cables" on either EvilBay or Amazon (I forget which) and when they arrived, I cut them up, using only rather short portion of the cable with the female USB cable to minimize voltage drop, verifying the power connections in the USB connector using an ohmmeter and a USB pinout diagram that I found on Wikipedia.

It should be mentioned that some devices, particularly cell phones of various brands, particularly those named after fruit, may require that the "data" lines be connected to resistors that "program" the charging current before they will accept a charge:  Not having one of those types of phones I connected nothing to the "D+" or "D-" lines and found that my Android phone charged normally - although it may be that it would charge faster if I would have connected those lines to resistors.  Some phones will pull several amps while charging, hence the use of a pair of 3 amp converter boards!  (e.g. one to run the GPS receiver in the car, the other to charge phones...)

Other devices simply connect to the female USB sockets as normal.

One of the devices powered by this box is my Garmin GPS receiver which has a "special" power cord:  If there is not a resistor across one of the pins of its mini USB connector it will search for a computer when it powers up, delaying its start up.  Since this resistor is built into its power cord I simply removed the cord from the original Garmin power adapter and connected it to the new power converter box and it was happy, booting up immediately, bypassing the check for the computer!


How well does it work?

One of the tests that I ran on this USB supply was to connect a 100 MHz oscilloscope to the input and output leads of the power supply.  With the sensitivity of the 'scope set to maximum I can see just a few millivolts of ripple from the LM2576 regulators, but this energy is confined only to the switching frequency of these devices and the first few harmonics.  Since it is at such a low level it is very unlikely that even if I were to power a sensitive receiver from this regulator that was tuned to the switching frequency that I would even be bothered by it!

The 'scope showed absolutely no evidence of switching energy at higher frequencies so I connected my FT-817's antenna directly to the DC input and output of the supply via a 0.01uF blocking capacitor and only at the lower frequencies (say, 160 meters and lower) could I detect the harmonics of the switching regulators.  If I placed a wire connected to the FT-817's antenna port near one of the power wires going into/coming out of the box, I could hear nothing of it at all.

After all of this, it needn't really be said that this device is completely "clean" at 2 meters and 70cm as well as the FM broadcast band!

What I did hear a little bit of "grunge" from is one of the devices that I run from it (a gps-based dashcam) but breaking that device open and adding a small choke and capacitor on its DC input lead fixed that problem - but a couple of turns of the power lead through a ferrite core would have probably quashed this as well.

Using a "Linear" regulator:

It is worth noting that a much simpler 5 volt USB power supply could have been built using a linear, 3-terminal regulator such as a 7805 or one of its variants.

While this would certainly satisfy the problem of there being switching energy, it would introduce the problem of power conversion efficiency.  In the car environment, wasting a few watts of electricity isn't too much of a problem, but the difficulty is getting rid of heat.  For example, if you were charging your telephone and it was pulling 1.25 amps, running the numbers tells us:

14 volts (typical vehicle voltage) - 5 volts (output) = 9 volts to drop across the regulator

9 volts * 1.25 amps = 11.25 watts of heat to dissipate

11 watts of heat does not sound like much, but it actually takes a fairly large heat sink to do this - and this heat sink must have free air circulation around it.  In other words, if you build a power converter based on this, if you put it into a box, the heat sink cannot be enclosed within the box unless there are a lot of holes and the box itself is located where there can be good convection air circulation!

Alternatively one could build the regulator into a metal box, using the enclosure itself as the heat sink.  Still, it would be a good idea not to bury it somewhere where it could not get some air across it or was exposed to engine heat or in the direct path of air from the heater vent.

In contrast, the aforementioned switching power converter is capable of approximately 6 amps and if one assumes that it is 85% efficient - a reasonable value - one can see that it would not produce much more heat than the above example, worst case!


Final comments:

I could have used the above techniques to clean up the original cigarette-lighter USB power adapter, if I had:

• Removed the circuit board from its case.
• Put it in a metal box, grounding it firmly.
• Supplied DC input power via the L/C (coil/capacitor) filtering as done above.
• Filtered the DC output power via the L/C filtering as shown above.  I would not have been able to use the USB connectors of the original power adapter, directly, but rather I'd had to have wired in female USB connectors as was done above.

 It would have taken less effort, but it would probably have worked just as well!


[End]

This page stolen from "ka7oei.blogspot.com".

Quieting high-current switching power supplies used in the ham shack.

Over the years I have acquired several switching supplies that I use in the shack and in portable operation such as field day.  These power supplies (two Samlex model 1223's and a Radio Shack #22-510) were designed to be "RF Quiet" compared to more typical switching supplies that might be used for computer or industrial applications.


It's all in the filtering...

What separates a typical, industrial power supply (and most computer-type supplies) from one that is intended to be "quiet" (RF-wise) is largely filtering, as what is contained within the box comprising the switching supply is essentially a high-power transmitter!  It's pretty easy for low-level harmonics of, say, a 300 watt switching power supply (oscillator!) to leak out, and since even a few billionths of a watt at the input of a receiver make a signal that is annoyingly strong, one can appreciate the need for proper containment!

Fortunately, most switching power supplies used in this application operate in the 30-60 kHz range which means that, by virtue of the large frequency difference, the harmonics - those buzzy, raspy things that often appear every 30-60 kHz across the HF bands - are already weakened considerably, but one needs to do more to submerge them below the noise floor!

It should be no surprise that it's largely the AC input and DC output leads that conduct this energy out of the box so some fairly good filters are required.

AC Line filtering:

Take as an example the circuit depicted in Figure 1, below:

Figure 1:  2-stage "brute-force" line filter using bifilar inductors.  The AC power comes in on the left and is delivered to the "guts" of the switching supply ("load") on the right.

This is a typical filter found on the AC power line of better-quality power supplies that are designed to be "RF quiet" and what does the most work are the two bifilar inductors.  How this works is that at capacitor Cd, where there are strong RF components of the switching energy, the two sides of the AC power line are "shorted" at RF frequencies (e.g. made to be "common-mode") so that when equal amounts of RF pass through bifilar inductor, they get canceled out and "choked" by the inductance.  The first of these inductors (the one on the right) isn't able to do all of the work, so another stage of filtering consisting of capacitor Cc and another bifilar inductor is applied.

For an article describing what is meant by a "common-mode" signal, read here:  http://en.wikipedia.org/wiki/Common-mode_signal

Finally, at the power line we have capacitors Ca and Cb and these not only help reinforce the "common mode-ness" and the effects of the bifilar inductor, but shunt the remaining small amount of RF from the switcher to the metal box containing the switching power supply so that it does not escape to the power line.  Typically the values of Cc and Cd are in the range of 0.01uF to 1 uF and the higher the capacitance, the better - but at the cost of the component itself (larger capacitors are more expensive) and the fact that as you increase the capacitance, it will draw more and more current from the AC power line on its own due to capacitive reactance.  (This leakage current will not generate heat since it is very reactive - but that's another discussion altogether.)

The values of Ca and Cb can vary, but it's common to find anything from 0.001uF to 0.47uF, but some safety laws limit the values owing to the fact that at this midpoint (ground) there will be approximately 1/2 the AC mains voltage (with respect to either side of the AC mains) should the ground be disconnected:  The value of these capacitors and their reactance will dictate how much of a shock hazard (current flow) that this might present should accidental contact occur.

These capacitors must also be appropriately safety-rated since failure could put the full mains voltage on the safety ground and pose a lethal shock or fire hazard.  Typically, these capacitors are blue - sometimes yellow - and have imprinted on them their specific AC rated voltages and have an "X2" marking on them as well as having symbols indicating the various safety and regulatory bodies by which their use is approved.

The inductors are the most expensive components in this filter since they use fairly heavy copper wire for to handle the multi-hundred watt load as well as fairly pricey ferrite material.  They are fairly large and heavy so it is not too surprising to find an off-brand or counterfeit power supply where all of these filtering components (inductors and capacitors alike) are omitted to cut costs:  Such power supplies radiate lots of noise and do not meet regulatory (or even safety) requirements in most countries!


DC output filtering:

The other place where RFI might escape is the DC output.  Take the example of Figure 2, below:

Figure 2:
Simplified diagram of the DC output of a switching power supply.
Capacitors Ca and Cb are typically large electrolytic units that remove high-frequency ripple of the switching supply from the power supply's output voltage.  See the notes below regarding capacitor Cc.

Here, we have the switcher's output circuit:  A high-power oscillator running in the 30-60 kHz range feeding a transformer that converts the voltage from the 150-300 volts of rectified and filtered AC line input, down to the 13.8 volts while also isolating the power mains from the DC output.  This is typically a center-tapped transformer with a full-wave rectifier followed by bank (usually 2 or more) of good-quality (hopefully!) filter capacitors represented by "Ca."  Even with the best capacitors there is still residual switching energy, so inductor "L" is typically used to filter it further followed Cb which consists of another capacitor or two in parallel to knock it down even more.

If the circuit board has been laid out properly properly and good-quality components have been used, the "V+ Out" line will be pretty clean - but notice something else:  The "ground" to which the transformer center-tap, Ca and Cb are connected is different from that of the chassis (case) ground in that that they aren't even connected directly to each other!

There are several reasons for this.  First of all, it is often desired that the case ground - which is usually connected to the AC mains safety ground, as well, be isolated (DC-wise) from the DC output of the power supply, this being done to prevent "ground loops" - that is, power finding its way along more than one lead and back to the same place.  In extreme cases this can cause hum or, in the case of faulty mains wiring, put a shock hazard on the metal case of the gear being powered.  The use of a capacitor such as Cc "connects" the two at RF, but not at DC or at mains frequencies.

In the cases depicted below, plastic capacitors rated for at least 250 volts are used which is adequate for 120 volt mains.  This seemed to provide adequate bypassing - even at fairly low radio frequencies - and the value used still presents reasonably high reactance at AC mains frequencies (>800 ohms at 60 Hz, >960 ohms at 50 Hz for 3.3uF) to afford a reasonable degree of safety, minimize circulating currents (hum) at those frequencies while eliminating the possibility of any DC ground loops.

Common problems with "noisy" power supplies:

First, some warnings:

• Do not perform any modification described here unless you are familiar with the techniques involved in high voltage and high current circuits.  Accidental contact with mains voltages can be lethal!

• You must make absolutely certain that all components that you use are rated for the voltage/current involved.  In particular, any capacitors that bypass from the AC (mains) input to the chassis must have the appropriate voltage and safety ratings to prevent the accidental imposition of potentially lethal mains voltages on the chassis/ground of the power supply and connected equipment!

• In this article, some of the filtering components are depicted as being added prior to line fusing.  In all cases, such components must be appropriately safety-rated for the voltage and current.  In some areas (such as the EU) it may be permitted that such components are connected only after line fusing - and that line fusing is required on both leads of the AC power connection.  In any case, take sensible safety precautions!

• Be certain that any inductors used are rated for the current involved and that their insulation is capable of withstanding the voltage applied.  For the bifilar chokes on the AC input, these must be rated for at least 4-5 amps while the output choke ("L" in Figure 2) should be heavy enough to handle 23-25 amps with minimal voltage drop.

• Some of the techniques described in this article may not meet safety regulations in certain countries.  Examples might include:  The placement of RFI/EMI components before the fuse, the types of capacitors, the values of capacitors and the amount of leakage current that they would consume and/or place on the chassis ground, etc.  Please be aware of these issues and address them in a manner appropriate.  I thought that I'd mention that twice...

• There are likely other things not mentioned here.  You have been warned!

For this discussion we are assuming several things about our power supply:

• It is contained in metal case.  The case doesn't provide "shielding" as much as it provides a common, low-impedance point to which all filtering that helps remove switching energy can be connected.  Doing this prevents the formation of differential currents between the AC input and DC output leads which could impose low-level switching supply energy onto those leads!

• It includes at least some of the above features to filter out switching components.  If this power supply was for, say,

Case study #1:  An older style model Samlex 1223

The first example is an older Samlex model 1223 power supply, a 23 amp, 13.8 volt unit that I bought in the late 1990's.  It was intended for use in, among other places, amateur radio stations, and is an inexpensive, yet fairly well-designed power supply.  Despite this, I noticed that it produced some low-level - yet annoying - spurious emissions on the lower HF amateur bands (160-40 meters) that were weakly audible at even higher frequencies.

In disassembling the unit I noticed immediately that it had just one AC input line filter.  Fortunately, there was enough room to wedge into it another bifilar choke (scavenged from a junked power supply) and the necessary bypass capacitors.

Figure 3:
Added bifilar choke on the AC input side of the old style Samlex 1223 power supply having been attached to the rear wall using RTV ("Silicone") adhesive.  In the foreground (lower-right) a pair of capacitors were added to the power line on the power cord receptacle, represented by "Ca" and "Cb" in Figure 1, above.  Because this power supply can
draw 300-400 watts, be certain that the added choke is rated for the expected current.
Click on the image for a larger version.

In Figure 3 you can see this modification with the added bifilar choke attached, using RTV, to the back wall of the power supply with the added capacitors (see "Ca" and "Cb" in Figure 1, above) soldered directly onto the IEC power cord receptacle.

I also put an oscilloscope across the DC output terminals and noticed that even though the DC output itself was quite clean - just a few millivolts of residual switcher energy - I saw few hundred millivolts of switcher energy when I measured between the chassis of the power supply and either of the DC outputs:  See Figure 4, below.

Figure 4:
The waveform present between either DC output and the chassis of the unmodified power supply.  The frequency/time noted in the box in the lower left is measured between the two purple vertical lines.
Click on the image for a larger version.

The magnitude of the "square" portions of the waveform are on the order of 130 millivolts with the extents of the high-frequency spikes going out to at least 268 millivolts:  It is this energy that is going to cause us the most grief!  This waveform looked the same whether I measured between the V- terminal and ground or the V+ terminal and ground, but this was not surprising since I already knew that from measuring across V- and V+, the waveform was quite clean.

At this point I had a choice:  Should I simply short the V- to the chassis ground and risk a ground loop, or install a capacitor?  Preferring to NOT subject myself to the possibility of a ground loop and the possibility of induced AC hum in the future, I rummaged around in my capacitor collection and found a large, 3.3uF plastic capacitor with a 200+ volt rating - probably something scrapped from an old switching supply or computer monitor.  When I connected this between the V- lead and the chassis of the power supply, I got the waveform in Figure 5, below:

Figure 5:
The output of the power supply after adding the capacitors to the output with the same vertical/horizontal scale as the plot in Figure 4.  Notice that only a fraction of the original "grunge" remains!
Click on the image for a larger version.

As you can see, there is a significant improvement!  The narrow spikes are much lower in amplitude (about 66 millivolts peak-peak rather than 268 millivolts) and, although it is a bit difficult to see in the above trace, the pulses are also much slower in their rise/fall time.  This last point (pun intended!) is important since it is the rate of change (dV/dT) of these pulses that dictate how much harmonic content they have, so between their reduction in amplitude and their being "slowed" considerably, this power supply was now VERY much "quieter."

Figure 6, above, shows the modifications made to the power supply and here are the steps:

• I found a small piece of glass-epoxy circuit board material and cut it to fit the empty space above the DC output terminals.
• Flipping the power supply upside-down, I drilled a hole for a 6-32 machine screw through the case and piece of circuit board material.  Flipping the case upside-down ensured that metal cuttings would not fall into the power supply.
• After de-burring the holes with a drill bit (also done with the power supply upside-down) I bolted the piece of circuit board material to the case using some "star" washers to ensure a solid, electrical connection.
• Between the piece of circuit board - which is now connected to the metal chassis ground of the power supply's box - I soldered a 3.3uF plastic capacitor between it and the V- lead.  Any value of 0.47uF and up would be fine, but 2.2uF-4.7uF is better.
• I also soldered a 2200uF, 25 volt low-ESR (switching supply-type) electrolytic capacitor between the V- and V+ terminals using short pieces of heavy (#12 AWG or larger) wire.  This wasn't really necessary, but it did knock down those small "spikes" in Figure 5 a bit more.

Figure 6:
The modifications of the DC output of the (older) Samlex 1223 switching power supply showing the added
capacitors.  The orange unit on the left is the plastic capacitor that suppresses the voltage differential between
the case and power supply output that contained the switching energy seen in Figure 4, above.
Click on the image for a larger version.

Putting the cover back on and testing it - even using it a few times during Field Day over the years - I have not observed that this power supply has caused any detectable interference, even when being placed next to a balanced-wire antenna tuner.

A Radio Shack model 22-510 power supply:

A couple years after getting the Samlex 1223 I noticed that the Radio Shack 22-510 power supply was on sale and grabbed one.  Rated for 25 amps, it is almost identical in size and shape to the Samlex 1223 and it had a permanently-attached power cord rather than a detachable computer-type cord.  Popping the cover I could tell that it was better filtered than the old Samlex in that it already had a 2-stage AC input line filter that strongly resembled that depicted in figure 1.  As with the Samlex, I noted that across its DC terminals the output was fairly clean, but like the Samlex, I observed a waveform that was nearly identical to that in Figure 4 between the chassis and ground.

Figure 7:
The modification to the output of the Radio Shack 22-510 power supply.  The orange capacitor is a 3.3uF unit connected between V- and the case while you can see a 1000 uF, 25 volt capacitor connected directly across the DC output terminals.
The added, series inductor - from the high-current output of a junked PC power supply - is contained within the
insulating piece of yellow heat-shrinkable tubing seen in the upper-right corner of this picture, above the head sink.
Click on the image for a larger version.

Figure 7 shows the modification to clean this up, but in this case I used a screw with a ring lug to make the connection to ground and in the picture you can see the 3.3uF capacitor connected between it and the V- output terminal.

Since I had noticed a small amount of switching noise (not bad, but not as clean as that in Figure 5) I rummaged around and found a small choke on the 5 volt, high current output of a junked PC power supply that had been wound with #12 AWG wire and would thus be capable of handling 25 amps without much voltage drop.  Clipping the red (V+) lead, I soldered this inline and it is shown in Figure 7, insulated with yellow heat-shrink tubing.  Across the V- and V+ outputs I attached a 1000 uF, 25 volt capacitor and the combination of these two components made its output at least as clean as that shown in Figure 7.  I probably would have been fine not doing this, but since I was already working on the power supply, anyway...

After I did all of the above I noticed that there was still some low-level noise on the power supply that was not at the switching frequency, but rather in the range of a few hundred Hz:  It wasn't strong enough to be a problem, but it annoyed me that it was there at all and I was curious as to its source.  What I soon realized was that this extra noise was coming from the small cooling fan on the chassis:  Unlike the fan on the Samlex power supplies which are thermostatically-controlled with an electronic heat sensing circuit, the fan on this power supply always runs and is connected across the DC output.

One insidious problem with these brushless DC fans is that they seem to have the uncanny ability to put some of their electronic commutating noise onto their power supply leads despite the fact that they draw only a hundred milliamps or so and are, in this case, connected to a high-current power supply with lots of filtering!  The fix for this is quite simple, however:  A series 10 ohm resistor and a 220uF, 16 volt capacitor.

Figure 8:
The added filtering for the fan supply to keep its "whine" out of the DC output, consisting of a 10 ohm resistor in series with the positive lead and a 220uF capacitor on the "fan" side between the fan's V+ and its ground.
Click on the image for a larger verion.

Figure 8, above, shows this modification with the capacitor connected across the "fan" side of the resistor on the fan's power supply leads.  Because it was convenient to do so I used RTV ("Silicone") adhesive to attach these added components to the back wall of the power supply, but I could have also enclosed them in heat-shrink tubing.  With this modification the fan electrical noise was completely removed from the power supply's output and the fan ran slightly slower due to the voltage drop across the 10 ohm resistor and would likely last a bit longer - but it still moved more than enough to keep it cool under full load.

When I was done this power supply, too, was now very "clean".

A newer Samlex 1223 power supply:

Earlier this year I spotted a brand new, in-the-box Samlex 1223 for a really good price at a swap meet and couldn't resist getting it.  When I opened it up I could see that since my older '1223 had been built, they'd made some improvements:

• Like the Radio Shack unit, it now had a 2-stage input line filter.

• They'd changed the output connector from binding posts to screw-type compression terminals.

• Both the V- and V+ output leads were routed through one large ferrite bead.

At this point I'll mention, again, the ferrite bead:  While it is a common technique to run power supply leads through such a device, simply passing wires through one of these will not likely add enough reactance to provide a significant degree of RFI suppression at lower HF frequencies!  What's more, the effect of this added reactance is not well-utilized unless you add some capacitors to the output as well as shunt the residual RFI to the chassis.

To be sure, I had not tested the unmodified power supply against the others that I'd modified to see how "clean" they were in terms of causing interference to HF operations but reports indicate that these newer Samlex 1223's are better than the older version in that regard but were still known to cause objectionable interference in some cases.

Interestingly, when I placed the oscilloscope between the V- and the chassis of the power supply I got almost exactly the same waveform as I'd gotten with the older Samlex 1223 and the Radio Shack 22-510 power supply depicted in Figure 4 so I knew what I had to do.

Figure 9:
Modifications to the new version of the Samlex 1223 power supply.  On the left can be seen the orange 3.3uF capacitor along with a heavy (#12 AWG) wire running from the V- lead to the added 1000uF, 25 volt capacitor.
Click on the image for a larger version.

As can be seen in Figure 9 I did exactly the same modification as was done on the Radio Shack power supply in Figure 7 - the only difference was that didn't need to add the extra choke in the V+ lead and I also had to route a piece of insulated heavy copper wire from the V- terminal across the top of the terminals to the added 1000uF, 25 volt capacitor since there wasn't enough room to locate it elsewhere.  In this case it was important to use a heavy-gauge (#12 AWG or heavier) wire for this connection since not only was the capacitor's lead not long enough to reach in the first place, but its small gauge lead (perhaps #22 or #24) offered enough resistance/reactance that the capacitor's suppression of some of the residual switching energy was degraded:  This just goes to show how, when dealing with high frequency switching supplies and high currents, how even a little bit of extra wire can cause a difference in performance!

Conclusions:

I've made these modifications to these power supplies over the years as I've acquired them and was somewhat surprised to see that they all have the same issue in common:  Significant switching energy between their cases and their DC output lines.  Fortunately, the "fixes" outlined above seem to be very effective and add minimal safety risk to their use and these three switching supplies that no longer cause any noticeable RFI, even when placed very close to the feedpoint of an HF antenna.

In addition to keeping these power supplies clean at HF, I also wanted to make sure that they caused minimal disruption at MF, LF and VLF frequencies (e.g. those below the AM broadcast band - where there are amateur allocations at 600 and 2200 meters) where I occasionally listen.  Because of these lower frequencies it is much more difficult to keep them from causing interference for several reasons:

• Rather than being several 10's of times higher than the switching supply frequency, I might actually be listening on the switcher frequency - or on one of its first few harmonics.

• At these lower frequencies the amount of inductance and capacitance in the filters may not be adequately high to effectively remove enough of the switching energy.

It was for this reason that I used the fairly large (3.3uF) case-to-V- coupling capacitor as well as adding the 2nd bifilar choke to the older Samlex power supply - not to mention the extra 1000 uF capacitors across the output leads of the supplies themselves.

Most of the time I don't even notice any of these power supplies causing interference, but on those occasions when I do (e.g. if I'm listening around 30-300 kHz, I may hear it) I can just shut them off for the duration.

Unfortunately, I also have other power supplies around the ham shack and the house (for the computer/monitor, the DSL modem, in the compact fluorescent and LED lighting, etc.) that are far "dirtier" and, at some point, these will require some action to clean them up - but that's another article!

Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:

• Containing RF noise from a "pure" sine wave UPS.   Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.

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This page stolen from ka7oei.blogspot.com