Sunday, June 28, 2009

Standard P4 ATX to 14pin Compaq PSU Conversion

Intro: I saw some pretty cheap half-height compaq desktops (EVO d500 SFF) and decided to buy one to replace the browsing pc downstairs.

Problem: It was missing the psu, and as usual when it comes to compaq it's a non-standard proprietary PSU. In this case with a 14 pin connector. (the mobo was a 252299-001 with 90 deg. angled psu connector on the side sitting straight and parallel with the edge of the PCB, ( please note there has been a report of this circuit NOT working with a different type of 14pin Compaq mobo ) , the pinout for this is here. (looking into connector (not the one on the mobo), with tab on top, pin one is top left first pin, pin 2 is top left second pin, pin 8 is bottom left first pin) I'm not sure but I think there are some other 14pin compaq psu connectors that don't match this) Converting a standard ATX psu is mostly just pinswapping except for 1, 3.3v standby voltage 2, inverted PSON signal 3, fan control lines

I decided to ignore the psu fan control lines, having the psu fan on all the time won't bother me and the effort it would take to get it working right is just not worth it. On to the other two problems. The compaq mobo takes 3.3v and 5v standby voltages, but standard ATX psus offer only 5v. With the 3.3v missing the mobo won't be able to turn the PSU on. There's the option of hardwiring the PSON (perhaps to a switch, which would essentially be like an old AT psu functionality-wise, with the added trouble of having to switch the mobo on/off separately) , but that's highly inconvenient and pretty lame :) So the better solution is a voltage regulator. I chose the LM317 because I had one lying around and because it's easy to deal with. It's an adjustable reg, takes 2 resistors to adjust the output voltage, it's relatively high power (which is completely unneeded here as the load on the 3.3v standby is minuscule) 1kOhm and 1.6kOhm adjustment resistors set the output voltage at 3.31volts, assuming a 5v input ( from the standard atx standby voltage pin ) There's no drop whatsoever while in use, and the reg remains stone cold all the way, so the "heatsink" surface on my pcb is pretty pointless (I know it's on the wrong side, but if it got hot it would be minimally useful regardless :) ) also two caps for filtering were thrown in as suggested by the datasheet.

R1: 1kOhm
R2: 1.6kOhm
R3: 1kOhm
C1: 100nF
C2: 1uF
IC1: LM317
OK1: 4Nxx / TIL116

The other problem is the inverted PSON line, standard ATX psus power on if the PSON line is low, apparently the Compaq PSU is the exact opposite. As described here, I used an optocoupler to act as an isolated voltage controlled switch between PSON at the PSU, and ground, triggered by PSON at the mobo. I later removed the TIL116 from the socket and replaced it with a 4N25 as it had straighter pins (and because the TIL fell out and it took me a few minutes to figure out why the thing wasn't working), but pretty much any optocoupler can be used ( of course, only pin compatible ones in my pcb like 4Nxx )

14 pins (16 here with first column empty)

Final thoughts:

Drill the IN hole (one with the purple wires going into it) with a bigger diameter than the rest as you will have to fit two wire endings in it. (that or change the board design, add another wire pad)

If PC shuts down, front led blinking red It's probably the optocoupler.

If no video, check if the 4pin ATX 12v connector is connected.

Don't take my word for output voltage of the LM317, measure it. 3.2-3.5 should be fine but I'd try to get it as close to 3.3 as possible, just to be on the safe side.

Both standby voltage leds on the mobo should light up upon connecting power to the PSU if you've done everything right.

It would be wise to put this higher up on the cable unlike how I did, or it would also be possible to put it into the psu directly. your choice, (however don't use the hole that the tab of the LM317 is screwed into, to fasten it into the PSU case!! or use some insulating plastic screw as the tab of the LM is VOUT, not GND, so connecting it to GND would not be a good idea)

Eagle brd on request :-)

UPDATE 2009 01 30:

So apparently some of you are actually building this and having some trouble. I can't say why. This circuit has been working in two of my Compaqs since I built it without a single problem, and I've been using 4N25s in them.. Either way I'm including two images, hope they help.
Image 1
Image 2
They show an extra drill hole in the pcb because I added it after the 3rd PCB I made, but if you read the article you'll know what it is.

UPDATE 2009 06 28:

Below the image shows the mobo that I'm using these with, as stated, it probably won't work if your connector, and sorrounding pcb doesn't look like this:

Good luck.


  1. Thank you very much Viktor ¡¡¡
    I have an old Compaq that I want to rescue. With this tutorial I think that I can do it.

  2. where can i get this pcb in mumbai market
    can you suggest?

  3. Well.. It's not a commercial product if that's what you're asking. You could either find someone to make it for you or perhaps put it together on some veroboard?

  4. On shematic and layout (first picture) you conected C1 (100nF) on LM317, and on picture of finished board, C2(1uF) is conected on LM317?

    P.S."Sorry for bad english"

  5. No problem. Both are connected. One is on the input the other is on the output (see A I O markings on the PCB image). Eagle keeps track of the schematic when designing the board so they should be consistent even if it doesn't seem like it at first sight :)

    I bought a $16.00 PSU and followed all your directions, and guess what? I am connected and writing to you from a Compaq EVO 510 sff and the better of all is that the new psu has a very small mobo and it fit perfectly into the original compaq psu case. The most problematic was to find the optocoupler, but I found one in an old Xerox Fax.
    I enjoyed very much this project because I succeded. Now I would like to ask for your help with another project, it is a lab centrifuge that has a missing component in the mobo, it uses an ULN2803A to control 2 relays. Please let me know how can I send you the details of this centrifuge. Thank you very much and I hope that you can help me.

  7. Glad to hear it! Well I'm not sure that I'll be able to help but feel free to send to the "random" address in the Contact sidebar on the right and I'll take a look at it when I have some time. Also if you have some pictures of your Compaq mod that I could add to the article It'd be much appreciated.

  8. Thank you so much. Mi Compaq Evo was " dead " but now is working like the first day . I`m so grateful .

  9. hi, i´m on this project right now. Got some trouble with 4pin ATX 12 connector. Is a connector with 2 grounds and 2 +12V? Or is a connector with gnd, 3.3, 5 and 12? My atx got the second one

    1. That's kind of weird. The compaq mobo takes a standard P4 ATX connector in addition to the standard 12V (ATX P4) connector. You can find the pinout for it here:

      If you have this same connector with 3.3, 5 and 12 volts your are probably looking at a proprietary power supply.