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I have a 500W PSU that has both 12v and -12v supply rails with the dual +12v rails rated for 360W (20A each) and a single -12v rail rated for 9.6W (0.8A).

My question is; if I was to power something using a +/-12V rail voltage what would the maximum ratings of the supply equate to? If the -12v rail can only sink 0.8A does the entire +/- rail supply then limit to 0.8A and become a 9.6W dual supply (or exceed this and damage the unit)?

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Since you're re-purposing a computer PSU, beware that some of them won't regulate properly, or may misbehave in other ways, without a load on +5V too.

My question is; if I was to power something using a +/-12V rail voltage what would the maximum ratings of the supply equate to?

That depends on whether your load is equal on the +12V rail as well as the -12V rail.

If the -12v rail can only sink 0.8A does the entire +/- rail supply then limit to 0.8A and become a 9.6W dual supply (or exceed this and damage the unit)?

If your load is equal on the +12V rail and the -12V rail, then yes, the -12V rail limits what you can draw in total.

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    \$\begingroup\$ I will use a breakout board that ensures all the preconditions for the supply to function but only use those two rails. Many thanks for your help. \$\endgroup\$ – NBoss Nov 11 '18 at 9:07
  • \$\begingroup\$ @NBoss - You're very welcome. In case it's important to you, I'll also mention that at such a light load, the PSU will probably be quite inefficient e.g. I wouldn't be surprised to see a 20W input power, for that <10W output. Good luck! \$\endgroup\$ – SamGibson Nov 11 '18 at 9:11
  • \$\begingroup\$ @NBoss Regarding the 0.8 A and your -12 V rail. Your PSU will, as always, supply whatever it can. And the thing that foremost determines how much current you will get, is the resistance of your load. So just use ohm's law and don't fool yourself. It's oh so time consuming if you fool yourself, for you, and for us. \$\endgroup\$ – Harry Svensson Nov 11 '18 at 9:31
  • \$\begingroup\$ @SamGibson Sorry to revive this again but I just had a thought after reading this question. My PSU has two +12v rail outputs, could I connect the two rails and their grounds respectively and get +/- 12V without explicitly using the -12v rail? Having 12v GND -> 12V GND(now -12v)? That way I could use 20A max instead of 0.8A? \$\endgroup\$ – NBoss Nov 14 '18 at 0:02
  • \$\begingroup\$ @NBoss - Hi. Unfortunately that approach isn't possible with a typical computer PSU, as there is only 1 internal "return" rail (typically called 0V or GND) for all of the supply voltages. The PSU in that linked question is a lab PSU, which (a) has multiple independent rails & (b) the return rails are isolated from incoming mains GND. Your PSU's 12V rails are not independent & both "12V GND" rails connect to mains GND. Therefore your planned connections would short one of the 12V rails to GND with bad (or very bad) results. Is it worth buying a more suitable PSU (Digikey, Mouser etc.)? \$\endgroup\$ – SamGibson Nov 14 '18 at 0:28

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