ATX DC Power Supply

Hi, I am working on a project that requires a 24V DC voltage source and a reasonably high supply current (3-4 amps). I was hoping to use an ATX power supply I have here. I have read the specifications for it, and it says it will provide 17A on the +12V rail, but only 0.8A on the -12V rail. If I source the 24 volts I need for my project using the the voltage across the -12 and +12 rails, will I be limited to 0.8A? Or something more like an average of the 0.8A and 17A?

Thanks in advance!

You will be limited to 800 mA for what you have. If you combine the +12 and -12 volt rails and exceed the current capabilities of the -12 volt rail bad things will happen.

Ron

Oh ok no worries, thanks for the quick reply!

I don't know about bad things, it should just go into over current shut down, I've done it a time or two.

Sceadwian said:

I don't know about bad things, it should just go into over current shut down, I've done it a time or two.

Click to expand...

Yes, it should but it will not deliver the 3 to 4 amps the OP wanted. Then too, if the over current on the -12 volt rail (seldom used rail) fails on a cheap PSU "bad things will happen".

The bottom line is that combining the plus and minus rails of 12 volts for 24 volts in this configuration won't work for 3 to 4 amps. Either the PSU will shut down or fail. Now if the OP wants to try it he is more than welcome to give it a try, I just won't be the one to say go for it.

Ron

Some friends have traced out their PC power supply. About half of those I've seen used a 7912 to regulate the -12. If 7912 shuts down, the +12V will be coming in and force the 7912 output positive. It probably will be too late before overcurrent kicks in.

Others just had 1A diodes in reverse from the same winding as +12. If you don't burn these diodes it might work for a little more than rated current. If you had a schematic you could find these diodes and replace them.

This is a link to an older 200 watt ATX form factor PSU schematic. Note that it has not only a -12 volt rail but also a -5 volt rail (long gone from the design guide). Note per Mneary's post:

Others just had 1A diodes in reverse from the same winding as +12.

Click to expand...

Which is how it was done in the above link. Most of the newer supplies use the same method of operation, with a few added features. Most ATX form factor PSUs I have seen only have two transformers, one for the 5 volt SB (Stand By) power and one for everything else.

Ron