ATX Computer Power Supply repurposed

The PC type desktop computers seem to have a universal type ATX or the newer ATX12V power supply. These are multi-voltage units that are pretty much standardized in various power output versions. Schematics for individual models may be proprietary but there are many representative schematics online if you need a more detailed understanding of their generic topology.

They are quite versatile even outside the PC chassis and they make pretty useful universal or bench top power supplies for many applications. They have built-in over voltage, over current and over temperature shutdown protection. A perfect basis for an ATX Power Supply hack!

They seem to be everywhere, probably 1000’s are shredded at E-Waste Recycle centers every day, just for the ounce of copper or the 3 micrograms of gold that may be inside. You can even buy new ones online for pretty cheap.

Don’t throw it away – repurpose it! These power supplies are a solution looking for a problem. Countless posts on the Interwebs about putting them to other uses. So here is what I have recently done.

Project “Light Up the HUEY”: Our local Vietnam Veterans of Diablo Valley (VNVDV) organization owns a UH-1H “Huey” helicopter. It is complete but in non-flying condition and mounted on a custom trailer for events, displays, parades etc. Lots of community interest when it is out and about; it draws a curious crowd who climb aboard for quick missions.

Its last service was at Travis AFB being used as a training aid for teaching crews on how to load helicopters aboard C-141, C-5 and C-17 cargo aircraft. It was then declared surplus and “demilitarized” by the cutting of several major cable bundles (groan). It last flew in SoCal with the US Army.

Although it is mostly complete, its flying days are over. So I took on the fun task of powering up the navigation lights to give it some “life” for night time display or even for daytime “road marches”. Here it is outside the Veterans Memorial Building in Danville CA for Veterans Day.

I wanted to power the port and starboard upper and lower navigation lights, the tail light and the rotating red anti-collision beacon and eventually some other things.

The DC power buss in the Huey is 24 volts like most aircraft. Absent its 24 volt battery or DC generator, this project needed 24 VDC for the lighting systems but I didn’t have a suitable 24 V power supply. Also, several of the (very expensive) 24 volt, semi-hemispherical reflector lamps were missing or broken. The 24 volt rotating beacon motor was seized due to water intrusion. Tango Uniform.

So I changed those lamps from 24 volt “Mil Spec” to 12 volt auto tail light types and isolated that circuit. The red anti-collision beacon can operate in that circuit now via a 12 volt auto tail light lamp and turn signal flasher module. Improvise, adapt, overcome.

We can power the lights via a trailer-mounted 120 VAC gas generator while moving or via shore power when parked near a building. It just needs 12 Volts for the new lamps, drawing less than 8 Amps total. Maybe an ATX?

I had an old ATX power supply in the junk box and tested it, all is OK with incandescent lamp loads. I clipped off the many unnecessary output wires and connectors, just leaving the 12 VDC wiring now connected to terminal posts on the chassis. Since the color codes for ATX supplies are pretty standard, it was an easy task. The external 12V wiring is Yellow:

  • Black: Ground
  • Red: +5V
  • Yellow: 12V
  • Orange: +3.3V
  • White: -5V (not present on some new supplies)
  • Blue: -12V
  • Gray: power on indicator
  • Green: Ground to turn on ATX

Seen here mounted inside, underneath the port side seats and powered by a 120VAC generator/shore power cord. I added a power-on switch, a green LED “on” light and an inline 10 amp fuse holder plus a bag of spares.

Below: Looks better when the building lights are turned off, we do get plenty of night time visitors! You can see the red anti-collision beacon peeking out from behind and above the engine exhaust.

The hacked ATX power supply works great, mission accomplished. The next step is to power the red interior cabin, cockpit and instrumentation panel lights and eventually the (24 volt) VHF radio gear but that’s another story.

(Note to self: ARC-131 antenna is the AS-1703 5985-00-082-3991)


Then I was cruising around at a local Ham radio swap meet and spied an ATX12V type computer power supply sitting unwanted on the “Free” table. Probably fried but there is lots of colorful output wires suitable for use as generic hook up wire. Seemed like a good deal for the wires. I took it home but then tested it:

It works just fine with a 12 volt auto headlight for a dummy load; the other 4 voltages are also present. This particular supply is the ATX12V(*) type with the 24 pin mother board connector. It is a high powered Supermicro PWS-865-PQ, an 865 watt unit with thermostatically controlled push-pull cooling fans. It is capable of lots of power for general purpose lab bench work. See specs below: (German is so much fun!)

12 volts at 70 Amps could come in handy for powering lots of things, including radio equipment.

This unit is very densely packed with several boards containing myriad SMT parts, big torroid inductors, transformers, big capacitors etc. all crammed into the box. This made it impossible to just access and clip off the unwanted peripheral wires at the PC boards.

So I chose to selectively clip many off externally and then “isolated” them with heat shrink tubing and zip-tied them to the case, out of the way. Improvise, adapt, overcome.

The multi voltage bundle to the 24 pin connector was kept intact. Many of the 12 volt and ground wires were paralleled and terminated in the 30 Amp-rated Anderson Power Pole connectors which are pretty standard around here at N6CC. (Beware of counterfeit “Anderson” Power Pole connectors being peddled online, they can be incompatible.)

A Break Out terminal board: I found an interesting PC board from an Amazon vendor. It’s a simple but clever device for just this application: An ATX12V Hack for a general purpose bench supply, plug-n-play. But there is good news and bad news here.

The good news: Cheap at $8.00, a good idea, good design, nice fiberglass double-sided PCB with solder mask and silkscreen, well laid out, individual fuses on the 4 main voltages, compatible 24 pin input connector, a switch to turn on/off the ATX12V and an LED pilot.

The bad news: A good idea, poorly executed. The binding posts will not accept standard banana plugs nor will they grab stripped wires through the post sides. The fuse clips are some soft metal that has no springiness to tightly grab the 4 amp fuse ends; loose, poor contacts. The traces are pretty thin and would not carry much more than a few amps. Oh well, you don’t get what you don’t pay for…

Making it all work: These supplies produce well regulated voltages as advertised but 12.0 volts is a little low for my radio transmitters, 13.6 or a little more would be better.

The ATX includes a voltage load sense circuit/wire. That is connected to the 12 volt, high current computer load to detect the voltage at the load, under load, to account for the voltage drop in the connecting wiring.

It then adjusts the Voltage Regulator circuit voltage upwards as necessary. It is the smaller gauge yellow wire in the group so I connected that to the Power Pole connectors to give proper feedback, at least at that point. (It could go on to the actual load but the 12 volt power supply may have hit its upper design limit by then.)

So to trick the VR circuit into running the output voltage up, I put two 1N1007 diodes in series with the sense lead to trick the VR into thinking the 12 volt load is only getting 12- (0.7+0.7) = 10.6 volts or so. With this lash up the supply now delivers about 12.6 volts under load, more better.

(The current drawn by the VR sense circuit is very low, hence the voltage drop across each diode is lower than the usual 0.7 volts for silicon diodes.)

Above, testing this scheme. A third diode did not make further improvement so I permanently wired 2 of them in the sense line and a bit of heat-shrink tubing makes it happen.

Also, several Interweb hackers stated that the 5 volt (or another) circuit needed to be dummy-loaded otherwise the ATX switching circuit would not “start” on power up. I tried a 10 ohm 10 watt resistor to ground; it turned out it was not needed with this PSU so I did not incorporate it. Maybe the 2 cooling fans provide an adequate “load”. YMMV.

After cleaning up the wiring bundles I installed a 4-circuit Power Pole terminal block, feeding one pair with the +12 VDC supply via an inline 25 Amp auto-type blade fuse. Now I can connect 3 additional loads even if I don’t want to use the break out board for more mundane tasks. In that case I just unplug the break out board when not used. Easy.

I also installed a green “Power On” LED on the housing as well as an SPST switch to ground the green wire to turn the unit on. (The stock red LED indicates a cooling failure, the red button resets the alarm.)

The breakout board could be mounted on the supply or maybe a bench panel somewhere else. Its On/Off power switch is now in parallel with the SPST toggle I mounted on the chassis. Either works.

This makes for a powerful, cheap and handy bench supply for many projects. Looks bad, works great!

Some Cautions: This lash up is certainly not a lab-grade bench power supply. If you use this board (versus a better one) I’d throw away those terminal posts and replace them with quality posts that accept banana plugs and stripped wire ends through the sides. I’d also remove the fuse holder clips and replace them with some decent ones that grab the fuses tightly.

The existing connector, copper traces or fuse holders certainly won’t handle 70 or even 20 Amps!

Note that in a 24 pin ATX molex connector each pin is only rated for 6 Amps max, hence the 4 Amp fuses. This terminal board is OK for simple, low power hobby applications like TTL, CMOS and linear audio circuits and probably most of Arduino projects too.

But I am primarily interested in using the high current 12 Volt output directly via the Power Pole connectors “out the back”. Easy access to those other voltages is just “gravy” if needed.

The PSU draws about 25 watts when plugged in and turned on, but with no load connected. Seems high but in its intended use in a desktop computer it would never be “on” but completely unloaded.

Noise performance: Like any switcher based power supply these things can radiate enormous broadband RF noise into adjacent equipment, like radio receivers. (FCC radiated RFI noise compliance certifications for imported equipment notwithstanding!)

This one radiated copious broadband white noise into a transistor AM radio when held within about 6 inches of the chassis, one foot away it was quiet on the AM BCB frequencies.

Further testing in my radio shack with HF and VHF receivers operating I could not detect any noise problems, I was a bit surprised. So no additional shielding or filtering necessary at this point. But I will keep an eye out for any broad or discrete spurious emissions as I cruise the spectrum.

When powering a 100 watt SSB transmitter the load regulation is very good. The transmitter achieves full RF power output on SSB voice peaks as witnessed by a scope indicating good dynamic regulation. The fans either remain off or are otherwise very quiet/slow. It barely gets warm to the touch indicating good efficiency.

A winner for my applications in powering my military and some commercial radio equipment as well in general purpose work.