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electrical engineering community.

I would like to ask for your help in this matter - my computer PSU does not start. I got an old PSU which I'd like to make something very useful of it: transforming it in a lab bench power supply like the one in this video. I started by cutting the heads of the wires and sleeve them. Next, I tried to start the PSU by wiring the green cable (power-on) to a black cable (ground), but I had no success... I wonder if it has some protection activated or if I can't have the other wires disconnected.

Thanks in advance.

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closed as unclear what you're asking by PeterJ, Dmitry Grigoryev, uint128_t, Voltage Spike, Dave Tweed Sep 15 '17 at 11:07

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Did it work before you tried to modify it? Or was it already broken? (it should start connecting the green and black wires. Provided that the switch on the back is ON!). If it was already broken, then really I would not bother of doing anything... too dangerous, too much work, too unreliable. \$\endgroup\$ – next-hack Sep 5 '17 at 22:28
  • \$\begingroup\$ It has no back switch. \$\endgroup\$ – rtrigo Sep 6 '17 at 10:18
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Before turning it with green wire to ground, any ATX12 PSU must have the +5VSB (standby) voltage on as soon as it is connected to AC mains. This +5VSB comes on pin8 of the 20/24 pin connector. If there is no +5VSB, the PSU is broken, and won't start by green-to-ground short.

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If you mean the Power En. (green) to nearest gnd (blk) with a jumper and no power out. Check to see if fan starts to rotate then stops. This means it is a very old design that detects no load often needed to regulate in vintage PSU's

If so, add 5 watt bulb or suitable dummy load to 5V such as a disk drive on Molex cable.

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  • \$\begingroup\$ I connected them with no jumper. I sleeved them and merged them for testing. The inner 12v fan does not start. \$\endgroup\$ – rtrigo Sep 6 '17 at 10:20
  • \$\begingroup\$ Try another supply with just a jumper. on correct pin #. Was it a known working PSU or unknown? is it old? is it preloaded with dummy load. Verify you have correct jumper positions. Fan start might only be a 1/4 turn before shutdown \$\endgroup\$ – Sunnyskyguy EE75 Sep 6 '17 at 14:30
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When you cut off the connectors, did you connect together all wires of each color? You'll notice that on at least some of the output voltages (usually +5V red and +3.3V orange) and ground (black), there is one wire that is a smaller gauge than the rest. These are sense wires used to overcome voltage drop in the wiring harness. These must be connected to at least one other wire of that voltage. Otherwise, the PSU may fault regulating the voltages and shut down.

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I wanna thank everyone who gave useful answers. I finally did it. As odd as it may be, the white wire in the picture is the only one which starts the psu (when connected to GND, of course). Now the fan is running and I can resume my project.

Odd color combinations...

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    \$\begingroup\$ Then they did not follow the standard color-code... The white pin is actually in the /PS_ON position, that's why it works. Your green wire is -12V, and should be blue. Brown ones should be orange (3.3V)... \$\endgroup\$ – next-hack Sep 6 '17 at 10:56
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    \$\begingroup\$ The lesson here is: don't trust color-coded wires; rely on pin numbers instead. \$\endgroup\$ – Ale..chenski Sep 6 '17 at 18:41

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