Not Your Typical ATX Power Supply Hack

Power supplies are essential for at home tinkering and electronics hacking. Unfortunately, they’re really quite expensive, and a bit out of reach for most hobbyists. Computer ATX power supplies are a cheap alternative, although they usually tend to lack the features of real bench power supplies… unless you hack yours like [Mark Schoonover]!

When [Mark] set out on this project he wanted to use as many recycled components as possible, but still come up with an extremely functional bench top power supply. He snagged a 500W ATX power supply from one of his kid’s old desktop PC’s, grabbed some old wall-warts for individual current limited supplies (apparently ATX PSU’s don’t have 5V rails anymore?), and put it all into a nice big project box.

He’s even thrown in a voltage regulator with current metering and a nice set of 7-segment displays!

atx power supply front

We’ve seen tons of nice ATX power supply builds, and they all have a slightly different design to them — it’s great! 

36 thoughts on “Not Your Typical ATX Power Supply Hack

    1. The -5VDC rail was for the ISA slots, all new motherboards do not support ISA slots so the -5 is left out of the PSU. HOWEVER you can buy a new motherboard with an ISA slot, you just have to look for them.

    2. I can’t believe you got me to actually check.

      PSUs have both +5V and -5V rails (the one lying closest to me will do 0.3A on the -5V and 29A on the 5V). According to the article, however, the meters don’t like being connected to the lines they’re measuring, hence the need for dedicated supply lines.

      1. The wall-warts are galvanically isolated from the outputs of the ATX supply by their own little transformers, so they’re floating in voltage in respect to one another and can be connected in series whichever way you want.

        Whereas trying to connect one +5 wire in series with another +5 will result in a burned out PSU.

    1. He likely could have powered the meters from the standby rail on the PSU.

      A more useful thing that he didn’t add would be a current limit/cut-off for the PSU rails – ATX PSU can deliver 20A+ on 5V rail before the internal protection kicks in, that could make a nice fire already. This is a pretty essential thing in a bench supply.

  1. PC PSUs usually have +5V standby voltage which is separated from main 5V rail since it needs to be active even when computer is off. Maybe that line can be used to power voltmeters.

    1. The standby +5V still shares the same ground reference. These days the main PWM is operating from the secondary side from the standby supply. In the old days (AT) the PWM chip ran off the primary side 170V DC rail with bootstrap and an extra winding.

      It is no longer possible to separate the standby supply.

  2. I know it’s soooo tempting to hack a computer power supply to use as a bench top supply. If you are just getting stated – do it – but…….there is no replacement for a voltage limiting, current limiting, adjustable (or even programmable) “real” bench top supply.

    Entry level units start at less then $100. $120 (USD) will get you a korad 3005p – it’s no $1000 PS, but it’s 100x better then using a ATX PS. It has it’s oddities, like any bench PS. But it’s a real bench meter will allow you do to so much more. Just my 2 cents.

    Just for fun if you want to see a early model 3005p blow up…..see here (they fix the issue now – so I can still recommend it)

    1. Its a really nice powersupply BUT you will find the old ones still on sale for dirt cheap ($75) there tempting BUT DONT DO IT they will fail high and smoke up blowing anything you have attached … but if you get a proper one there REALLY nice power supplies with proper options and computer control … i use 3 next to eachother better than my more expencive 3 rail supply

    2. I will have to agree with George. Having a fully custom ATX for a number of years, It was my go-to power supply until I ended up shorting it a few weeks ago during an experiment and it failed to go into shutdown mode, melting the wires and smoke and all the mess that implies. I got the Korad 3005D and works like a charm. The old custom ATX earned a spot outside my bench for now.

    3. That top of the line $1000 bench supply you mentioned, I got one for $50 that instead was made by a tiny company called HP. and yes it’s about 3 decades old, but it works fine. At the same time I got a 300mhz scope for $35.00.

      find the swaps and do whatever you can go get to them via car so you can drive the big heavy stuff back home.

      1. A lot of this is what I was going to mention as well. Many hobbyists are willing to save about $400.00 for an o-scope (which isn’t completely essential, at least when beginning – a multimeter is the essential tool), but won’t plunk down a hundred or so for a decent PSU. Granted, when you get beyond a couple of adjustable outputs, price goes thru the roof…

        In a recent issue of Nuts and Volts magazine they detailed how to make a fairly nice PSU using one of those cheap chinese buck/boost converters (as well as how to fix a couple of issues with it); it has full current and voltage regulation, both adjustable, plus an LCD output, all for well around $50.00. Something to look at for the future, at least.

        As you found out, though – if you can make it to a hamfest or similar, you can find some nice older equipment being sold for near nothing – as long as you are ok with heavy stuff. I bet your items were either meant rack mounting – or on a cart; doesn’t matter, though – if you have the space and the strength, you can’t get much better than HP for test equipment.

  3. I’ve messed around with the inside of these supplies. They aren’t really suited to being used as general purpose supplies, except in special circumstances.

    They *are* highly hackable, though. The transformer output windings can be re-purposed, the output components can be rearranged and the voltage and current regulation can be modified. All of the above requires some sort of schematic, which probably won’t be for the exact supply you’re trying to hack, but might be close enough, as all these supplies seem to be built around a few PWM regulator ICs. If yours isn’t one of them, go pick up another. If you blow it up while trying to hack it, pull another from the junk bin. They are great learning tools. Don’t be afraid, but please realize that the AC input is rectified to 300-450V DC for the primary, and this CAN KILL YOU if you don’t respect it.

    My hack involved an old AT supply, and ended up giving me 400 watts at 13.8 volts, to power a commercial two-way radio. I used the two five volt output windings in series and readjusted the voltage regulator to regulate the output to 13.8V. I changed a couple of resistors to set the OVP to 14.5V and the overcurrent to 30A. It’s still working. Cost me a couple of afternoons of trying to match up what I saw on the PCB to the schematics I had available, and the datasheet for the regulator chip had a sample application schematic, which helped a lot.

  4. Yeah, this is a really sloppy way to make a bench supply. His metering system is ridiculous. Couldn’t he just get meters that don’t require separate power supplies? Or use, as everyone else said, an extra rail on the PS…The whole thing is kind of massive for no reason as well.

    To make a good variable power supply from an ATX PS, you really need to hack into the PWM control circuit and feedback loop system that regulates the voltage output. There should be a place where you can add a variable resistor so the voltage output can be adjusted AT the max current of the PS. You’ll need the datasheet of the PWM IC. You should end up with a variable voltage power supply that can swing +- 10 or so volts, if not more, around the 12v rail while providing high current. Forget the LM317 that everyone seems to use with these power supplies…

    1. You cant just tweek the pwm like that, the control chips runs off the resulting power from it.
      There usually is a pot to adjust the ouput up/down about 1V. The ENTIRE regulation is around the 5 or 3.3V rail, Other rails are monitored for power good and overvotlage protection.
      Don’t mod an AT[x] supply for 24V to charge batteries and then connect it to a battery backwards or it’ll end up on the repair pile like mine.

  5. This old-timer just wanted to say that the minus 5 volt supply goes back to 1982 when the IBM PC first came out. Back then the dynamic RAMs required bipolar 5 volts. Within a few short years, it was no longer needed. However, since it was brought out to the bus for the same reason, there was no way of knowing what aftermarket boards might be using it. Thirty years of legacy.

    BTW, the minus 12 volt was for the RS-232 serial interfaces.

  6. Just curious, what are the advantages of building a DIY bench supply out of an ATX unit like this over something out of one of the supplies like the Mean Well ones?

    1. agreed. there are far fewer gremlins in a transformer/stepdown circuit than there are in any SMPS. There are plenty of great places for SMPS’s, don’t get me wrong, but a bench supply just isn’t one of them.

  7. I was in need of some higher ampage 12v PSUs and looking at the prices of even the cheap bricks they cost more than the LED strips I wanted to power with them, and if I wanted to get something a bit more trustworthy than the cheap Chinese imports it would cost even more.

    Then i remembered reading something about old Xbox360 power supplies, and sure enough they fit the bill perfectly – genuine Microsoft ones can be had 2nd hand off eBay for less than £10 including postage, they deliver 5v 1A all the time and have a trigger line you connect to the 5v line to turn on the 12v circuitry (the trigger line draws so little power you can safely connect a microcontroller pin to it), of which you get up to 12.1A from the 150watt PSU and up to 16.5A with the 203watt PSU.

    And there’s no need to cut the plug off the PSU (unless you intend to use the full amperage it gives out) as you can chop up an Xbox to Xbox Slim power adapter cable, meaning if the PSU does die you can just plug in another, or if you want to use the PSU on different projects just put an adapter cable on each of them.

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