Computer PSU to Lab Power Supply Conversion





Computer Power Supply Units, or PSUs, are excellent power supplies that can be recycled once the computer is obsolete. In fact, even when the computer is long out of date, or its components have failed, it is more than likely that the PSU is still in good working order. Sadly, many of these perfectly working units will end up as landfill. In this instructable I will show you how to rebuild a PSU as a lab power supply.

SAFETY WARNING! I this instructable I will be opening the case, exposing potentally dangerous voltages. Do not attempt this if you do not understand the dangers of electricity.

LEGALEZE: I do not accept any responsibility for any injuries caused as a result of attempting this instructable.

Ok, several people have done projects like this. Just search Instructables for PSU and you'll see how many there are. So I thought I'd put my two cents worth in with this instructable, to demonstrate my "remote switch" idea, and just to show how I went about my version of this popular rebuild.


Most PSUs follow the same output configuration: +3.3V +5V, +12V, -5V, -12V and probably a +5V standby. This gives you a number of different voltages up to 24V; across the +12V and -12V there is 24V, +5V to -5V gives you 10V and so on.

The standby is also useful. When used in a PC, the standby is always on even when the PSU is otherwise powered down. This means you can still have a device running while the PSU is on minimal power.

This particular PSU has a master switch on the back, in addition to the "soft switch" that all PSUs employ. With that in mind, I decided to implement a "remote switch" idea that uses a switched 3.5mm jack. This works such that the PSU is powered up when nothing is plugged into the 3.5mm jack, but when you plug a switch into this jack, you can control the PSU from it. This switch can be on a lead so that you can control the PSU from a distance. It also means that you could have some device running off the 5V standby whilst the PSU is in standby, then that device could power up the PSU using the remote switch. At the end of this instructable there is a short video that demonstrates how the remote switch works.

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Step 1: Disassembly

The first step was to remove the cover, and cut off all the connectors. I left some length of cable to work with.

During this step, I observed closely the cables and how they are connected. Most of the wiring will be standard across most PSUs:

Yellow: +12V
Red: +5V
Orange: +3.3V
Blue: -12V
White: -5V
Purple: +5V standby
Black: Common

Pink and brown: I was unable to determine what these are for. As you can see from the photos the pink wire connects to the red wire in the ATX connector, and the brown wire connects to the orange. At first, I thought these wires might serve as a detector to indicate that the connector is still connected and is good, but the unit still functioned as expected once I had cut the connector off, so I still don't know what these do. Elsewhere I read that the brown line is a "3.3V feedback line", but since it doesn't appear to function that leaves me none the wiser. I left these two disconnected.

Green: This is the "switch". Connecting it to common (a black wire) will cause the unit to power up. The normal arrangement is to have a switch on this line to turn the unit on and off. When it is off, the only power available is the +5V standby.

Grey: Apparently, this line is supposed to have a small load on it, like an LED and a resistor, but the PSU functions fine without it. I left this one disconnected also.

Step 2: Front Panel

The next thing I did was to drill a 3x3 matrix of holes in one of the sides that was unpopulated with vents, fans etc. This side actually has the large hole where the bundle of wires came through, but since I'm not using them in this way I decided this would be the place for the 5mm jack for the remote switch. I blocked this hole using a small piece of PVC with a 5mm hole in it.

The matrix of holes is for the binding posts. You can see them in place in the last image above. As you can see, I have added a little blob of epoxy resin to each post to hold them firmly in place, and to prevent them coming unscrewed with use. The binding posts actually have flat sides, which should key into a hole with a flat edge on two sides, i.e. something like an oval hole. However, I don't have the necessary skills, or the right tool to drill such a hole. So I used Araldite instead.

I tried to match the colours of the posts to the corresponding wire colours, however, I couldn't source an orange or a purple post, so I used yellow and blue respectively.

The label that covers the whole panel is just a print I printed out on my inkjet printer. I used Inkscape to create the image, printed it on photo paper, then covered the print with self adhesive vinyl book covering. I then stuck it to the panel with double sided tape.

Step 3: Wiring

The binding posts accept ring connectors on the back, so that is what I used. Rather than just connecting one wire of each colour to each, I decided I may as well use them all, or as many as would fit in each connector, since the PSU can deliver up to 30A. In addition, I doubled up some of the connectors to give up to eight wires onto one post. This is probably overkill, as it is probably providing more current capacity than is required, and made the area behind the panel overcrowded and cramped. With the benefit of hindsight, I should have cut most of these off, and provided maybe only two to each post.

For the 3.5mm jack, I simply soldered the green wire and a black wire to it in the appropriate configuration so that it behaves like a switch. Not all jacks have this feature. If you want to go down this path, make sure the jack works in that manner. It should have a kind of "hook" arrangement at the back. This appears as a flat metal plate on one side of the jack, as seen in the last photo above.
For more information about these jacks, and how they work as switched connectors, read this Wikipedia article.

Step 4: Reassembly

Time to reassemble the box. It was a bit scungy around the edges, since it wasn't designed to be sitting out on a workbench for all the world to see, instead hidden away inside a computer case. So I finished the edges off with black insulating tape (curiously, in this part of the world referred to as "duct tape"). I have designed it such that the fan is underneath, and I have extra large rubber feet on the unit so as to allow for a good airflow. It blows out from underneath, so the unit doesn't pick up dust etc. off the surface it's sitting on.

In usage, I've discovered that the 3.3V line is very sensitive; only being able to drive small light globe before the unit cuts out. The 3.3V line should be able to deliver 30A. I have yet to resolve this issue. Either the unit is faulty, or this behaviour has something to do with the mysterious pink and brown wires. The brown was connected to the orange 3.3V wire. This is apparently the "feedback line". I'll have to do more research.

This video demonstrates the remote switch idea.

Incidentally, you may have noticed in the photos throughout this instructable that the PSU's case is actually gold coloured. It doesn't really show too well in the photos, but it looks quite good in real life. That's why I took the trouble of lining it with the black tape.

I hope you've found this instructable of benefit. I don't expect anyone to build this exact unit, but I put this up because I haven't seen anyone else use the remote switch idea, and thought someone might find that useful.

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    11 Discussions


    4 years ago on Introduction

    Just a note to let you know I have added this instructable to the collection:
    Encyclopedia of ATX to Bench Power Supply Conversion
    Take a look at about 70 different approaches to this project.


    5 years ago on Introduction

    Some points to note regarding your wiring.

    The brown wire is the +3.3v feedback line and needs to be connected. It helps the PSU maintain +3.3v in case of excessive voltage drop. It must remain connected to the orange wires.

    The pink wire is the +5v feedback line and functions like the brown wire, and must remain connected to the red wires.

    A computer PSU is a switched PSU and thus needs a minimal load in order to main consistant voltage levels. This can be achieved by connecting a 10 ohm 10 w resistor between the +5v line and ground inside the power supply. make sure it is properly heatsinked against the case.


    7 years ago on Introduction

    Because they are such special purpose devices PC PSUs make lousy bench supplies. Just because you can do some things doesn't mean that you should. Someday when I get around to it I will build a bench supply out of parts I scavenge out of PC PSUs. My design will address some of the limitations, and remove all of the oddities PC PSUs possess.

    Until then enjoy some other bench supplies I've uploaded to this site:

    I put both on this site to combat the ATX conversion craze which is my opinion is of such limited utility it should only be done for very specific applications, general bench supply tasks not being one of them either.

    2 replies

    Reply 7 years ago on Introduction

    Well, doing this stuff is a great way to learn. And of course there's no better way to learn than by making mistakes.


    6 years ago on Introduction

    Yea I just have the PSU in the case and the 2 fans. Also kept the cables for - the HDD LED, POWER SW, RESET SW, the 2 USB ports, a firewire port, MIC and Headphone socket. Plus there are some lights behind the power and reset buttons that have a black and red cable. Was thinkin, I could still use them, apart from the reset sw and firewire port. on another instructable, a guy has used the headphone socket as a remote switch. Might not use the mic and headphone socket either. Was just a bit confused about the different combos of the - and + wires. How does that work? Like getting 1.7v from combining the -5v and the 3.3v. Sorry if i'm i'm being a bit thick lol. Is there a full list of the combinations?


    6 years ago on Introduction

    The "switch"... I've noticed the power button on my pc case has a green wire and white wire, can i just connect those up accordingly with the white and green wire i have coming from the supply, and use that as the switch? It's only a push and release switch, as in I tested it as a circuit with a battery and fan, pressed it but the fan didn't stay on, only when i held it in did it run. Plus I was thinking I could use the HDD led as the standby indicator, so do I just connect it to the purple wire and a black wire?

    1 reply

    Reply 6 years ago on Introduction

    If you're using the PSU while it's still in place in the PC, i.e. everything's still connected up with the motherboard etc., then that should work. The "push and release" switch, called a momentary switch, only turns the PSU on whilst pressed, during which time the motherboard turns on its own internal switch to keep the PSU on. So you'd need to replace that switch with an on/off switch if you're using the PSU stand-alone.
    With the purple and black wires, you may need to add a resistor in series with the HDD indicator as the voltage is 5V and may be too high for the LED.


    7 years ago on Introduction

    one safety features of at/atx power supplies, is if no load is present on its 5Volts/12Volts hi amps output, it will go to standby mode, usually you have to connect a 100ohms 10watts resistor on the 5Volts 30 amperes line for the power supply to stay on full on.
    also if short is present on its 5V or 12Volts output, it will shut down.

    btw, great casing mode!

    1 reply

    Reply 7 years ago on Introduction

    Thanks. I must get on to fiddling around with this again, and see where I went wrong.