In the old days, radio amateurs were concerned with (or should have been) energy from their transmissions getting into devices unintentionally, the classic being televisions, phonographs, telephones, hi-fi sets, and the like.
A few years ago hams' hackles were raised with the prospect of BPL - Broadband over Power Line - a system by which the already-extant infrastructure used to convey electrical power would be used to transport data all about the land. While it did work (sort of) it had the potential to cause a great deal of interference to amateur radio operators.
A lot was written and to their credit, some designers/operators even designed their systems to avoid putting energy within the HF amateur bands - to varying degrees of success. While this wouldn't have really helped the causal shortwave listener, it did still not address the fundamental problem that the power lines were simply not suitable, low-loss, low radiation transmission media for radio frequency energy.
What we really should have worried about was not BPL...
As it turns out, when it comes to worrying about devices that had the potential to clobber our HF bands, we really should not have worried too much about BPL - which, as hindsight has proven, wouldn't have gotten anywhere, anyway, but rather devices that are right under our noses: Switching power supplies - particularly the cheap, lightweight ones that are now supplied with everything that we buy and even put in our own shacks!
These inexpensive "wall warts" used to consists of a small, iron and copper transformer - often with a rectifier and capacitor. These devices would plug into the wall and operate, typically for 5-10 years until whatever it is that they were powering wore out.
Unfortunately for them, they would consume 1-5 watts all of the time just sitting there doing nothing, even when the device was "off" - the so-called "phantom loads" or "power vampires" and many locales/countries have legislated them out of existence in favor of the newer, much more efficient switching-type devices.
All would be good except for two things:
The first of these is that many of these cheap switching-type wall warts last only 12-24 months before dying - usually a victim of an inferior quality capacitor and/or poor design. What this means is that more often than not, the device to which they were attached is often thrown out as well. While this new-style switching-style wall-wart may take less power to operate, it is my guess that considering that its premature failure caused a premature product replacement, it never actually saved any energy - since it probably took a lot of energy to make the device in the first place - or saved the consumer any money!
Stepping back off the soapbox, these switching supplies - even if well-built and long-lasting (if you are lucky enough to encounter one) bring us to the second of the two problems concerning us about these devices: The generation of RFI, or Radio Frequency Interference.
Such was the case with one of these devices that I use on my TV to run a small multimedia computer. This computer, obtained surplus, did not come with its original supply so I found an OEM Dell laptop supply of reasonable quality and suitable ratings - about 19 volts and 3 amps. There was one problem: It seemed to radiate a low-level RFI signal that got everywhere on HF.
Now part of this was how it was connected - See Figure 2, above:
Whatever low-level RFI was being produced by this power supply had exactly what it needed to be conducted out into the world and cause problems: A complete path in and out of the power supply!
What is sounded like:
Typically, switching power supplies sound like a "buzz" every 30-60 kHz - the power supply's switching frequency - up and down the bands, usually worse on lower bands, but not always. This buzz is usually modulated at twice the power line frequency (120 Hz in the U.S., 100 Hz in most locations in Europe, Asia and Africa) but this modulation is usually very "dirty" and full of harmonics: If the radio is switched to "AM" mode the "buzzy" nature of the modulation becomes much more apparent!
It is often the case that the 30-60 kHz intervals at which the interference is "bunched" together are more apparent at lower frequencies such as the AM broadcast band and 160 through 80 meters (1.8-4 MHz) - that is, one can more clearly hear the distinct switching supply "carriers". As one moves up in frequency the interference may sound like it is decreasing, but this may not actually be the case as these "bunches" of energy often get spread out, changing from a fairly sharp "buzz" as you tune across the switching harmonic to more of a "hiss" and in severe cases - and on higher bands - these "bands of hiss" may actually run together. In the latter case, it may not, at first glance, sound like a switching supply at all, but rather just an elevated noise floor! In this case it may not be until one switches to AM and notices that this "hiss" has a powerline frequency AM component to it and/or that it disappears when the power is removed from the supply that it is, in fact, from a switching supply!
The latter was the case of the power supply depicted in Figure 1: On 160 meters it could be heard every 60 kHz or so as a "dirty" buzz, but on 40 meters it was an indistinct rise in the noise floor of about 2 S-Units that was about 10 kHz wide while on 20 meters it just seemed to raise the noise floor by 1-2 S-Units everywhere that, to the uninitiated, didn't even resemble noise from a switching power supply!
It should be pointed out that I'd already modified this power supply to reduce its conduction onto the AC and DC power leads and that had solved one problem - bothering a receiver that was located next to it - but the lower-level, HF frequency energy that was induced across the power supply between its AC input and DC output was much more difficult to manage as that was not a matter of either shielding or direct power line conduction.
Since I'd already gone out of my way to add bifilar chokes to both the AC and DC leads of this power supply, I'd likely reduced its potential to emit energy by a significant amount, but here, we are talking about residual amounts that are being coupled into what amounts to antennas that are connected to my TV system and being picked up by a sensitive HF receiver.
Before we continue on, let me say a few things about what won't work to fix this.
What will NOT work to solve this problem:
Ferrite beads and snap-on chokes!
Ferrite beads and snap-on chokes will not likely solve this sort of problem because what is needed is to prevent the egress of the RF energy from the switching supply one or more of the following:
Ferrite beads and snap-on chokes are better at minimizing the ingress of energy to reduce its potential of being bothered by external RFI than they are at eliminating the emission of RFI in the first place. In other words, the reactance that they add to the interconnect leads gives whatever built-in RFI immunity the device already has more of a chance of working. They are much less effective in quashing the emission of RFI by that device in the first place.
To get enough inductance to present a high inductive reactance at the lowest desired frequency it is often required that many turns be wound on a piece of ferrite, but the size of the core, the diameter of the wire - and even the length of the wire - usually precludes putting more than a turn or two on all but the largest core.
In our case, I'd already have installed additional filtering in the power supply - and it wasn't enough - so we are going to attack this problem using the second of the above techniques: Shunting the RF energy to a common path.
I knew now that I had to do the complete"filter job" on this power supply.
Having had to do this before on other power supplies, I gathered the necessary parts - this time, documenting my efforts for this blog:
Before I go on I must spout out a few weasel words of warning:
Gathering parts:
The first thing to do is to gut the PC power supply, leaving in the case the connector for the power cord and the on/off switch if it has one.
Please be aware that the input capacitor of the power supply may retain voltage even if it has been powered down for a long time - check and discharge it if necessary.
The picture shows several of the parts that you will need from the power supply:
Comment:
Warning:
Constructing the filter:
The schematic diagram of the filter is shown below.
The filter is of the so-call "Brute Force" type and it is a common-mode low-pass filter that removes high frequency content from the AC power line. Because our main goal is to contain the RFI within the box, any RF energy from the switching power supply first hits the common-mode capacitor which forces it to be equal on both sides of the AC power line. The RF energy then hits the bifilar RF choke which then cancels out any energy that is equal on both sides of the AC power line - a condition that was just enforced by the common-mode capacitor.
Any RF that managed to make it through the bifilar choke will now be greatly dimished and it is now shunted by the two capacitors to the metal case to ground while the (optional) common-mode capacitor on the AC input side reinforces the equilibrium of any RF energy that might be on that common-mode choke.
The filter shown was built on a piece of phenolic prototyping board, maintaining at least 1/2" spacing (12mm) between any two points that carry mains voltages or between a mains voltage and/or a ground point. On the bottom, short pieces of solid bus wire were used to interconnect components and to make the loops used to solder the interconnecting wires.
As can be seen, the power supply's original fuse was retained and used on the "line" (hot) side of the AC input of the filter as a matter of safety.
The phenolic board was mounted on the side of the PC power supply case, but to protect it from items protruding into a vent hole and causing a short or electric shock, a piece of heavy plastic larger than the perfboard was cut out and mounted against the case. This piece of plastic was cut from a discarded "blister pack" that had contained items bought at the store and was fished out of the trash can: It just so-happened that there was a large enough portion of flat plastic to accommodate my needs and it made a nice, durable and free shield!
The board was mounted using 6-32 screws and spacers as standoffs to hold it about 1/4" (5mm) or so from the side of the case. For a ground connection, a ring lug was put under one of the screws and soldered to the ground connection on the filter - and also soldered to the ground connection on the AC power plug which, itself, was also connected to the case.
Using the original on/off switch and wire from the scrapped power supply, the filter was wired to the AC mains and then over to the power supply. Some push-on pins were found that mated snugly with the power supply's AC input connections and connected to its AC input, but it would have been possible to cut off the original AC power cord and wire it in. Some RTV ("silicone") adhesive was then used to secure the push-pins on the power supply's AC input as well as to hold it to the bottom of the case - either of which could be removed later, if necessary.
DC output filter:
With the AC line input now being completely filtered, we still have to isolate the other end of the path through which the low-level RF currents can flow - the DC output.
Inspecting the PC power supply again, I noticed that there were two toriodal inductors and I removed them both. One of them had several different wire gauges and was set aside, but the other consisted of a pair of wires wound in parallel, connected in parallel on the circuit board - and a quick check on the inductance meter showed its value to be around 43 microhenries - plenty good for our purposes.
Had neither toroidal inductor been suitable as-is, I would have picked the one with the heaviest-gauge wire and removed all but the winding with that wire: Most of these power supplies use toroids with yellow or green cores and a dozen or two turns on these typically yield inductances well above 10 uH - more than enough to block HF energy when bypassed with good-quality capacitors!
On a terminal strip I mounted the inductor and two low-ESR electrolytic capacitors, as shown, bypassing each one a 0.1uF monolithic ceramic capacitor. The use of these low-ESR capacitors rather than "normal" capacitors is important as these types are specially-designed to remove the high-frequency components. Once you get above a few hundred kHz, however, many electrolytic capacitors start to lose their efficacy so monolithic capacitors such as the ones shown take over, shunting the RF to the case ground.
Important construction notes and comments:
Note that if you use ordinary 0.1 uF disk ceramic capacitors instead more modern monolithic ceramic units be aware that many of these can have rather low voltage ratings (e.g. 16 volts) unless otherwise marked. Also note that they can lose effectiveness at high frequencies so they should be bypassed with 0.001uF capacitors.
This terminal strip was mounted to the case using a 6-32 screw and a "star" washer. The DC output cable of the power supply was then cut and wired to the terminal strip, using the ground lug as a "common" and passing the DC through this filter which effectively shunts any RF to the case ground.
Finishing it up:
Once all wiring is completed, ohmmeter tests should be made to verify continuity (or lack thereof) as appropriate and stick-on feet should be applied to the bottom to prevent it from sliding around and scratching whatever surface it rests on.
How well does it work?
At HF frequencies this filter's effectiveness is seemingly absolute in that the power supply within cannot be detected from outside the box, even with a portable shortwave radio held within a few inches!
It should be noted that it is not the "shielding" of this box to which one would attribute its effectiveness, but simply the fact that the AC input and the DC output share a solid, common RF ground. Any RF currents on the AC input and DC output simply circulate on this common ground (e.g. the metal case) rather than radiate on the AC power leads and/or the wires connected to the DC output - or the things connected to it!
Were this same circuit arrangement constructed on a flat piece of metal without a shield cover, it would have worked nearly as well: The 100% cover of the case is there mostly to prevent accidental electric shock and shorting of the otherwise exposed AC mains connections. With the cover removed, the RFI can radiate from the power supply, but unless a wire/conductor is placed within the box such emissions would go only a very short distance.
This blog entry may be considered to be a follow-up to the December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link
On a related topic, see also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link
A few years ago hams' hackles were raised with the prospect of BPL - Broadband over Power Line - a system by which the already-extant infrastructure used to convey electrical power would be used to transport data all about the land. While it did work (sort of) it had the potential to cause a great deal of interference to amateur radio operators.
A lot was written and to their credit, some designers/operators even designed their systems to avoid putting energy within the HF amateur bands - to varying degrees of success. While this wouldn't have really helped the causal shortwave listener, it did still not address the fundamental problem that the power lines were simply not suitable, low-loss, low radiation transmission media for radio frequency energy.
What we really should have worried about was not BPL...
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| Figure 1: The computer power supply making RF noise up and down the HF bands. |
These inexpensive "wall warts" used to consists of a small, iron and copper transformer - often with a rectifier and capacitor. These devices would plug into the wall and operate, typically for 5-10 years until whatever it is that they were powering wore out.
Unfortunately for them, they would consume 1-5 watts all of the time just sitting there doing nothing, even when the device was "off" - the so-called "phantom loads" or "power vampires" and many locales/countries have legislated them out of existence in favor of the newer, much more efficient switching-type devices.
All would be good except for two things:
The first of these is that many of these cheap switching-type wall warts last only 12-24 months before dying - usually a victim of an inferior quality capacitor and/or poor design. What this means is that more often than not, the device to which they were attached is often thrown out as well. While this new-style switching-style wall-wart may take less power to operate, it is my guess that considering that its premature failure caused a premature product replacement, it never actually saved any energy - since it probably took a lot of energy to make the device in the first place - or saved the consumer any money!
Stepping back off the soapbox, these switching supplies - even if well-built and long-lasting (if you are lucky enough to encounter one) bring us to the second of the two problems concerning us about these devices: The generation of RFI, or Radio Frequency Interference.
Such was the case with one of these devices that I use on my TV to run a small multimedia computer. This computer, obtained surplus, did not come with its original supply so I found an OEM Dell laptop supply of reasonable quality and suitable ratings - about 19 volts and 3 amps. There was one problem: It seemed to radiate a low-level RFI signal that got everywhere on HF.
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| Figure 2: Configuration showing the interconnects and where the RF circulating currents are flowing. The conduction of RF currents onto the AC power, speaker and outside antenna leads assured that it was being radiated far and wide! |
- The power supply was connected to the AC power line.
- The power supply was also connected to the TV through the video/audio cables.
- The TV was connected to the high-power stereo system which, in turn was connected to speakers in different parts of the room.
- The TV was also connected to a coaxial cable that went to the rooftop antenna.
Whatever low-level RFI was being produced by this power supply had exactly what it needed to be conducted out into the world and cause problems: A complete path in and out of the power supply!
What is sounded like:
Typically, switching power supplies sound like a "buzz" every 30-60 kHz - the power supply's switching frequency - up and down the bands, usually worse on lower bands, but not always. This buzz is usually modulated at twice the power line frequency (120 Hz in the U.S., 100 Hz in most locations in Europe, Asia and Africa) but this modulation is usually very "dirty" and full of harmonics: If the radio is switched to "AM" mode the "buzzy" nature of the modulation becomes much more apparent!
It is often the case that the 30-60 kHz intervals at which the interference is "bunched" together are more apparent at lower frequencies such as the AM broadcast band and 160 through 80 meters (1.8-4 MHz) - that is, one can more clearly hear the distinct switching supply "carriers". As one moves up in frequency the interference may sound like it is decreasing, but this may not actually be the case as these "bunches" of energy often get spread out, changing from a fairly sharp "buzz" as you tune across the switching harmonic to more of a "hiss" and in severe cases - and on higher bands - these "bands of hiss" may actually run together. In the latter case, it may not, at first glance, sound like a switching supply at all, but rather just an elevated noise floor! In this case it may not be until one switches to AM and notices that this "hiss" has a powerline frequency AM component to it and/or that it disappears when the power is removed from the supply that it is, in fact, from a switching supply!
The latter was the case of the power supply depicted in Figure 1: On 160 meters it could be heard every 60 kHz or so as a "dirty" buzz, but on 40 meters it was an indistinct rise in the noise floor of about 2 S-Units that was about 10 kHz wide while on 20 meters it just seemed to raise the noise floor by 1-2 S-Units everywhere that, to the uninitiated, didn't even resemble noise from a switching power supply!
It should be pointed out that I'd already modified this power supply to reduce its conduction onto the AC and DC power leads and that had solved one problem - bothering a receiver that was located next to it - but the lower-level, HF frequency energy that was induced across the power supply between its AC input and DC output was much more difficult to manage as that was not a matter of either shielding or direct power line conduction.
Since I'd already gone out of my way to add bifilar chokes to both the AC and DC leads of this power supply, I'd likely reduced its potential to emit energy by a significant amount, but here, we are talking about residual amounts that are being coupled into what amounts to antennas that are connected to my TV system and being picked up by a sensitive HF receiver.
Before we continue on, let me say a few things about what won't work to fix this.
What will NOT work to solve this problem:
Ferrite beads and snap-on chokes!
Ferrite beads and snap-on chokes will not likely solve this sort of problem because what is needed is to prevent the egress of the RF energy from the switching supply one or more of the following:
- Very high series reactance to block RF energy
- Shunting of RF energy to a common path on the input and output of the power supply to prevent it from circulating elsewhere.
Ferrite beads and snap-on chokes are better at minimizing the ingress of energy to reduce its potential of being bothered by external RFI than they are at eliminating the emission of RFI in the first place. In other words, the reactance that they add to the interconnect leads gives whatever built-in RFI immunity the device already has more of a chance of working. They are much less effective in quashing the emission of RFI by that device in the first place.
To get enough inductance to present a high inductive reactance at the lowest desired frequency it is often required that many turns be wound on a piece of ferrite, but the size of the core, the diameter of the wire - and even the length of the wire - usually precludes putting more than a turn or two on all but the largest core.
In our case, I'd already have installed additional filtering in the power supply - and it wasn't enough - so we are going to attack this problem using the second of the above techniques: Shunting the RF energy to a common path.
I knew now that I had to do the complete"filter job" on this power supply.
Having had to do this before on other power supplies, I gathered the necessary parts - this time, documenting my efforts for this blog:
- Dead PC power supply, complete with case and power cord.
- Two brand new low-ESR electrolytic capacitors of suitable voltage for the DC power supply, capacitance between 100uF and 1000 uF.
- Two monolithic 0.1uF ceramic capacitors of suitable voltage for the DC power supply.
- Terminal strip.
- A piece of perforated prototype board.
- Misc. screws/hardware for standoffs.
- A piece of plastic for a shield - see text.
- Four self-adhesive rubber feet.
- Some soldering skills.
- A bit of common sense!
Before I go on I must spout out a few weasel words of warning:
- This project involves hazardous/lethal AC power/mains voltages! DO NOT undertake this project unless you have experience with such voltages and the necessary safety procedures in dealing with them!
- Please observe the safety regulations and requirements for your locale noting that the methods described here may not be suitable for your area!
- You MUST make certain that the components that you use are rated for the voltage/current at which they will be operated!
- YOU are responsible for your own safety. I cannot be held responsible for damage, injury, accidents or even death that might occur by following - or failing to follow - any instructions or recommendations on this page!
- If you do not feel comfortable working with high voltages and currents or do not have familiarity with wiring procedures and safety related to such, PLEASE do not even think of doing so!
- YOU HAVE BEEN WARNED!!!
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| Figure 3: The discarded PC power supply case, stripped of its insides leaving only the power receptacle and the on/off switch. Click on the image for a larger version. |
Gathering parts:
The first thing to do is to gut the PC power supply, leaving in the case the connector for the power cord and the on/off switch if it has one.
Please be aware that the input capacitor of the power supply may retain voltage even if it has been powered down for a long time - check and discharge it if necessary.
The picture shows several of the parts that you will need from the power supply:
- Bifilar input choke. This could either be toroidal, or look like a transformer. Make certain that you identify the two "halves" of the inductor: AC power will flow through each half, separately. These inductors will have values of 100uH to 50 mH per half, depending on the source. Those depicted in Figure 4, below, measured about 4.5 mH per half.
- Common-mode capacitor. This will typically have a value between 0.047uF and 0.22uF and will be connected directly across the AC line - usually located right next to the bifilar input choke. In the U.S., these capacitors are typically 0.1-0.22 uF.
- Two identical high-voltage bypass capacitors: These connect from each side of the AC supply and go to the case ground. These are typically blue or yellow and have values from 1000pF to 4700 pF (e.g. 1nF to 4.7nF).
- The safety fuse(s) from the power supply - if they are not blown. In the U.S., there is typically only one fuse found on the "Line" (black wire) side of the AC input.
- Another Common-mode capacitor. If you have another PC power supply to scavange - or if the power supply that you have has one, get from it another common-mode capacitor of the same description as above. This is is optional.
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| Figure 4: Parts needed for the AC input filter, found on the discarded PC power supply: The fuse, the common-mode capacitor (the yellow block), the common-mode choke (the toroidal inductor with two halves) and the two blue disk- ceramic capacitors. Click on the image for a larger version. |
Comment:
- It has been noted that some REALLY CHEAP and/or "suspected origin" power supplies have been spotted that have none of these RFI suppressing components - or even a fuse - even though their cases had a "UL" and "FCC" certification sticker on them! In this case, it was probably just as well that the power supply was pulled out of service as their were neither safe or compliant with regulations!
Warning:
- All of the capacitors should have on them explicit AC voltage ratings consistent with those of the mains voltage in your area.
- DO NOT use any capacitor unless it has printed upon it the proper voltage rating! The capacitors typically used for these applications are usually blue or a light yellow color and have printed on them an AC voltage rating.
- You may be able to find a pre-built filter unit that has a standard IEC (e.g. "Computer Plug") connector on it that you can mount to the power supply case, saving you the trouble of building a filter. These units may be found both new and surplus. You can also find such filter units pre-built as depicted Figure 5, below
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| Figure 5: Various styles of bifilar inductors that may be found in scrapped switching power supplies - plus a complete, self-contained AC RFI filter built into an IEC power connector in the upper-left corner. Click on the image for a larger version. |
The schematic diagram of the filter is shown below.
The filter is of the so-call "Brute Force" type and it is a common-mode low-pass filter that removes high frequency content from the AC power line. Because our main goal is to contain the RFI within the box, any RF energy from the switching power supply first hits the common-mode capacitor which forces it to be equal on both sides of the AC power line. The RF energy then hits the bifilar RF choke which then cancels out any energy that is equal on both sides of the AC power line - a condition that was just enforced by the common-mode capacitor.
Any RF that managed to make it through the bifilar choke will now be greatly dimished and it is now shunted by the two capacitors to the metal case to ground while the (optional) common-mode capacitor on the AC input side reinforces the equilibrium of any RF energy that might be on that common-mode choke.
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| Figure 6: The completed AC input filter, constructed on a piece of phenolic prototype board. This one has a common-mode filter on both the input and output. Click on the image for a larger version. |
The filter shown was built on a piece of phenolic prototyping board, maintaining at least 1/2" spacing (12mm) between any two points that carry mains voltages or between a mains voltage and/or a ground point. On the bottom, short pieces of solid bus wire were used to interconnect components and to make the loops used to solder the interconnecting wires.
As can be seen, the power supply's original fuse was retained and used on the "line" (hot) side of the AC input of the filter as a matter of safety.
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| Figure 7: Schematic of the AC input filter. Note: Typically, a 100k-1Meg "safety" resistor is connected across the mains (on either side of the inductor) to discharge the capacitors should it be disconnected while the AC sine wave is at either peak. This is not shown in the above diagram since this resistor was already present in the power supply to which it was connected. |
The board was mounted using 6-32 screws and spacers as standoffs to hold it about 1/4" (5mm) or so from the side of the case. For a ground connection, a ring lug was put under one of the screws and soldered to the ground connection on the filter - and also soldered to the ground connection on the AC power plug which, itself, was also connected to the case.
Using the original on/off switch and wire from the scrapped power supply, the filter was wired to the AC mains and then over to the power supply. Some push-on pins were found that mated snugly with the power supply's AC input connections and connected to its AC input, but it would have been possible to cut off the original AC power cord and wire it in. Some RTV ("silicone") adhesive was then used to secure the push-pins on the power supply's AC input as well as to hold it to the bottom of the case - either of which could be removed later, if necessary.
DC output filter:
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| Figure 8: Schematic diagram of the DC output filter. The "ground" of this filter was firmly attached to the metal power supply case using the ground lug of the terminal strip seen in Figure 9, below.. |
Inspecting the PC power supply again, I noticed that there were two toriodal inductors and I removed them both. One of them had several different wire gauges and was set aside, but the other consisted of a pair of wires wound in parallel, connected in parallel on the circuit board - and a quick check on the inductance meter showed its value to be around 43 microhenries - plenty good for our purposes.
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| Figure 9: Output filter components mounted on a terminal strip with the ground lead of the capacitors being wired to the mounting lug.. One of the two yellow monolithic ceramic capacitors can just be seen behind the closest terminal strip. Click on the image for a larger version. |
On a terminal strip I mounted the inductor and two low-ESR electrolytic capacitors, as shown, bypassing each one a 0.1uF monolithic ceramic capacitor. The use of these low-ESR capacitors rather than "normal" capacitors is important as these types are specially-designed to remove the high-frequency components. Once you get above a few hundred kHz, however, many electrolytic capacitors start to lose their efficacy so monolithic capacitors such as the ones shown take over, shunting the RF to the case ground.
Important construction notes and comments:
- Again, use ONLYLOW ESR capacitors for the output filter. These capacitors are almost always rated for 105 degrees C, so if the capacitors that you have say "85C" on them, they are probably not low ESR - but their having "105C" on them that doesn't guarantee that they are low ESR, either!
- While the output capacitors of PC power supplies are (ostensibly) of the low ESR type, it is often the failure of these capacitors - along with the fan - that causes these power supplies to fail, so don't count on a failed power supply to be a usable source! Unless you have an ESR meter, don't count on a capacitance meter to tell you if a capacitor is any good, either: It can still read the proper value, but have terrible ESR!
- If a terminal strip cannot be found, a small piece of copper-clad circuit board could be used, instead, with the components mounted "dead bug" on it, using the copper itself as a ground plane. The circuit board materal would then be mounted using screws, to the metal case, assuring a solid ground connection.
- Be sure to ground the output filter directly to the case near the point where the DC cable exits the case rather than run a wire to the AC input filter's ground point! One of the ways to maximize the effectiveness of the filter is to minimize the length and impedance of its ground/common connection, and the best way to do this is to utilize the case itself!
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| Figure 10: The input filter mounted on the wall of the power supply case. Note the clear plastic shield behind the phenolic board to prevent accidental shorting/contact through the vent holes. Click on the image for a larger version. |
This terminal strip was mounted to the case using a 6-32 screw and a "star" washer. The DC output cable of the power supply was then cut and wired to the terminal strip, using the ground lug as a "common" and passing the DC through this filter which effectively shunts any RF to the case ground.
Finishing it up:
Once all wiring is completed, ohmmeter tests should be made to verify continuity (or lack thereof) as appropriate and stick-on feet should be applied to the bottom to prevent it from sliding around and scratching whatever surface it rests on.
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| Figure 11: Filters and power supply mounted within the case. The power supply itself was affixed using RTV ("silicone") adhesive. Click on the image for a larger version. |
At HF frequencies this filter's effectiveness is seemingly absolute in that the power supply within cannot be detected from outside the box, even with a portable shortwave radio held within a few inches!
It should be noted that it is not the "shielding" of this box to which one would attribute its effectiveness, but simply the fact that the AC input and the DC output share a solid, common RF ground. Any RF currents on the AC input and DC output simply circulate on this common ground (e.g. the metal case) rather than radiate on the AC power leads and/or the wires connected to the DC output - or the things connected to it!
Were this same circuit arrangement constructed on a flat piece of metal without a shield cover, it would have worked nearly as well: The 100% cover of the case is there mostly to prevent accidental electric shock and shorting of the otherwise exposed AC mains connections. With the cover removed, the RFI can radiate from the power supply, but unless a wire/conductor is placed within the box such emissions would go only a very short distance.
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| Figure 12: The completed, enclosed, power supply. Click on the image for a larger version. |
This blog entry may be considered to be a follow-up to the December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link
On a related topic, see also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link