# Thread: atx psu turning on for less than 1 second then off ...

1. Hi all, I have an atx power supply that I salvaged from an old computer (P3) and am looking to keep it for a project but when I bridge the green cable (on) or reconnect the momentary action switch the psu simply turns on for a second and then suddenly off. The 12v rail only ever reaches about 5V.

Had a look inside and all looks ok on visual inspection...any ideas what could be wrong?

2.

3. Some power supplies need a minimum load to function properly; you might try to put a resistor that will draw an amp or so across the output before you power it up. Some power supplies also have "sense" inputs" that have to be parallelled across the supply output in order to properly regulate the output. Hope this helps.

4. Originally Posted by thinkatron
Some power supplies need a minimum load to function properly; you might try to put a resistor that will draw an amp or so across the output before you power it up. Some power supplies also have "sense" inputs" that have to be parallelled across the supply output in order to properly regulate the output. Hope this helps.
Thanks, I had a single 12v fan connected at the time which runs at 0.3A...would this be enough?

5. Well then stop pressing the power button off.

6. To be sure what value resistor would I need for a 12v system to consume 1 amp?

7. Originally Posted by fatman57
To be sure what value resistor would I need for a 12v system to consume 1 amp?
To draw 1A, you'd need (from Ohm's law) a resistor R= V/I = 12V/1A = 12 ohms. It's also important to note the power such a resistor would have to dissipate: P = VI = 12W. Message: You can't use a standard low-power resistor, unless you want to generate really bad smells.

8. Originally Posted by tk421
Originally Posted by fatman57
To be sure what value resistor would I need for a 12v system to consume 1 amp?
To draw 1A, you'd need (from Ohm's law) a resistor R= V/I = 12V/1A = 12 ohms. It's also important to note the power such a resistor would have to dissipate: P = VI = 12W. Message: You can't use a standard low-power resistor, unless you want to generate really bad smells.
Thats what I thought...i'll see if I can find a 1+ amp load...

9. SOLVED!

It needed a load on the 5V rail (it was an old PSU with a white -5V cable), modern ones need a load on the +12V rail (of which there will be many). I used a 5A load by the way, couldn't find anything less!

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