Thread: Modifying a regular power supply

Originally Posted by LittleDragon

I'm curious, is it possible to modify a regular power supply to take 12 volts. Can't you just remove the transformer and feed 12v right into the voltage rectifier? I haven't cracked a PS open to check yet but it doesn't seem too difficult, all you would need after is a shut down controller.

http://acs.comcen.com.au/atxps.html

I don't think this has been updated for a while. I was going to try this but haven't got around to it. I think there is one or two people on the board that have sucessfully converted an ATX supply.

the transformer wouldnt give out 12v.. threre will be some power loss along the way and to output a -12v line there is a 30% Vloss in doing so..
so it would out put something higher than 15v.. i would imange

I think it's not quite as simple as ditching the transformer. I imagine that the typical ATX supply uses multiple windings on the tranformer to produce an array of different voltages that are then fed into rectifiers, caps, and finally the various voltage regulator ICs.

Assuming you have an ATX supply designed to work with 12v on the output windings, I'd guess you might be able to make it work. IIRC, the guys that mod'd the ATX supplies, rewound their tranformers to account for the differences in input voltages, and also used some sort of oscillator circuit (kind of a simple DC to AC conversion) to pulse the transformer primary.

Depening on how you value your own time (figure your current hourly wage), it could be MUCH cheaper to just break down and buy an OPUS.

Originally Posted by rando

I think it's not quite as simple as ditching the transformer. I imagine that the typical ATX supply uses multiple windings on the tranformer to produce an array of different voltages that are then fed into rectifiers, caps, and finally the various voltage regulator ICs.

Close.

A conventional switcher rectifies the mains as the first thing it does. It then switches this into the primary of the main transformer (under the control of some feedback circuitary from the low voltage section).

There are usually multiple secondaries as you mention, but they are switched at high frequency and the multiple regulator ICs are replaced by a feedback loop that controls the primary switching.

To modify the power supply into a forward converter (and that's a search term if you wanna find stuff in this forum), you have to rewind the primary of the transformer and uprate the primary switches (you'll be switching more current at a lower voltage). There are significant efficiency issues.

Most atx power supplies are switch mode, getting thier power direct from the 240ac, then thru a down step inverter, to all the different voltages needed. There is no main transformer. The closest thing that resembles a transformer in them is the inductors. Some are multiple windings, so they could be classed as a transformer, but there is no transformer employed to step down the mains voltage to a lower voltage. This is why new atx supplies are so light.
Some people have tried connecting the cars 12v directly to the storage caps in the atx supplies but its not high enough and you end up getting very low voltages out.

This is why people have gone to the trouble of biulding their own from scratch.