Maximum current from ATX PSU as benchtop power supply

Hi,

I am slowly gathering the bits together for this as a side project - and a few of the online instructions for using a donor ATX PSU as a benchtop switching power supply talk of using -12V and +12V to create a 24V source, from which you can supply a variable regulator and dial up different voltages. I want to make a somewhat more considered choice on which of these PSUs will be the one for the job.

The question is, if the +12V rail can provide 10A (say) and the -12V rail is rated at a max of 0.3A would the combination of the two be capable of 10.3A, or is it the lesser of the two, or some different calculation. Happy to do some reading if you know of a decent explanation somewhere and can provide a link. My google-fu has failed me on this one.

Thanks ! Geoff

Hi Geoff

There are 2 problems here,

1) The maximum current you'll be able to draw with the +12V & -12V in series will be 0.3A.

2) I suspect that in most PC PSUs the outputs aren't floating, ie, the 0 Volts for +5V and +12V/-12V are tied together. This means that unfortunately you won't be able to configure a +24V and +5V supply scheme as you would like.

John

Hi John, and thanks for your quick reply.

Eriba: 1) The maximum current you'll be able to draw with the +12V & -12V in series will be 0.3A.

I thought this might be the case, thus the question. So if I'm going to make this work I'm going to have to select the PSU with the highest amperage on the low value. That rules out the 0.3A one I'm looking at here...

Eriba: 2) In any PC PSU the outputs aren't floating, ie, the 0 Volts for +5V and +12V/-12V are tied together. This means that unfortunately you won't be able to configure a +24V and +5V supply scheme as you would like.

I was hoping to have a fixed +5V from both a pair of binding posts and a couple of USB sockets, binding posts for a fixed 12V, and binding posts for a variable output to provide 2-22V or thereabouts. Is that not going to be possible after all?

Again, thanks for your advice, Geoff

I was hoping to have a fixed +5V from both a pair of binding posts and a couple of USB sockets, binding posts for a fixed 12V

No problem with that, Geoff - although you might need to strap a load resistor across the +5V to get it over its minimum load requirement (otherwise it would struggle to regulate itself and the +12V output)

variable output to provide 2-22V

I think you'd need to buy or construct this - I doubt that the topology of a PC PSU would lend itself to be modified to your requirement (well, not easily anyway).

Eriba:

I was hoping to have a fixed +5V from both a pair of binding posts and a couple of USB sockets, binding posts for a fixed 12V

No problem with that, Geoff - although you might need to strap a load resistor across the +5V to get it over its minimum load requirement (otherwise it would struggle to regulate itself and the +12V output)

variable output to provide 2-22V

I think you'd need to buy or construct this - I doubt that the topology of a PC PSU would lend itself to be modified to your requirement (well, not easily anyway).

Excellent news on both fronts then. I do have a variable switching reg chosen, and a small display to show the current drawn and voltage selected on the variable channel...with atmega to bring it all together.

Just need to find a PSU with more to give on the -12V rail...

Cheers ! Geoff

Given how few devices use -12, I wouldn't expect to find a supply with much more current capability, than the one you have.