24V from an atx psu

Hi
I would like to convert an old atx power supply to provide +24V. the max current that I need is about 3amps.
is it possible to modify it and if so, how it's done? what are the alterations needed?
Thanx.

Much simpler to use 2 old ATX units and in Series. technically feasible, with some precautions.

Or use the +12 Volts and -12 Volts leads to get 24 volts floating...........; though 3A may be not be possible.

Have you checked ebay? 24V 3A supplies can be had for around US$10.

Mike.

Thanx for the replies.
using two psu's is too cumbersome and as far as I know the -12v rail can't privide 3amps.
of course I know that I can order from Ebay or Ali but it will take some time and I need it promptly.

Maybe the -12v rail could take it if you beef up the rectifier and filtration. What is your load?

Rail of led's.

A typical atx psu has a big dual diode in one direction for the +12V and a small single or dual one for the -12V, plus some additional output inductor and capacitor. I think if you add a dual diode for the negative side, then you should have 24V without much of an issue.

You would need to mess with the feedback within the smps, the control ic has a reference voltage fedback from the output, you can change the voltage by relplacing resistors.
However there are drawbacks, the output capacitors would need to be replaced as the voltage rating wont be high enough, and you might need to do the same with the rectifier diodes, also the controller chip will have over voltage protection which you'd need to modify, and also over current should be modded too to protect the supply as the thing wont be able to give as much current at twice the voltage, worse than that datasheets for pc supply control ic's are often hard or impossible to get.

-12V is generally provided by a linear regulator (LM7912 or similar). That's probably the limiting factor in the current rating... if the output voltage and regulation is not critical, you might be able to remove this - although 3A might still be pushing if from the -12V.

Dr Pepper raises some good points on modification of the main supply. I did once modify a PC supply to give +/-50V, but that was a fairly extensive rebuild (rewound transformer etc), and it was an old, simple, AT style supply to start with.