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Computer PSU to lab power supply conversion

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INTRODUCTION

Computer Power Supply Units, or PSUs, are excellent power supplies that can be recycled once the computer is obsolete. In fact, even when the computer is long out of date, or its components have failed, it is more than likely that the PSU is still in good working order.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.


BACKGROUND INFO

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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DraakUSA5 days ago

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.

pfred21 year ago
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:

http://www.instructables.com/id/300-Watt-Linear-Power-Supply/

http://www.instructables.com/id/Dual-POS-NEG-Power-Supply/

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.


Cambenora (author)  pfred21 year ago
Well, doing this stuff is a great way to learn. And of course there's no better way to learn than by making mistakes.

I find learning through others mistakes can be less painful.

Skarz881 year ago
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?
Skarz881 year ago
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?
Cambenora (author)  Skarz881 year ago
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.
profpat1 year ago
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!
Cambenora (author)  profpat1 year ago
Thanks. I must get on to fiddling around with this again, and see where I went wrong.
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