I removed all unnessesary cables from some old 200W PSU and added connectors, so I can use it as Power supply.

But there is problem :D When I turn PSU on (this is one without green start wire) I have only 4v/9v on output. I know old PSU must have some load to work properly, but is there way how to disable this? I dont want to drain 2A just to stay on.

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    \$\begingroup\$ You need load on the +5V rail before you have regulated output. Add a resistor, fan, lightbulb or something as load. No real way around it without pretty much rebuilding the entire thing. \$\endgroup\$ – winny May 26 '18 at 11:21
  • \$\begingroup\$ @winny I know but I dont want to drain power as waste. Is therevother way? Some way to disable this \$\endgroup\$ – Baterka May 26 '18 at 11:22
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    \$\begingroup\$ The other choice is to use a completely different PSU. \$\endgroup\$ – Ignacio Vazquez-Abrams May 26 '18 at 11:31

Your power supply may be one that requires a minimum load. OEM supplies intended to be embedded into equipment where the load is somewhat known would often be like this. Typically the minimum load is 10% of the maximum load, but of course only the datasheet can tell you for sure.

Since you don't seem to know anything about this power supply, try putting a 5 Ω or so power resistor across the 5 V output and see what happens. If that seems to help but still doesn't quite get you there, try 2 Ω or even 1 Ω resistors. Be mindful of the power these resistors need to dissipate. The 5 Ω will dissipate 5 W, the 2 Ω 12.5 W, and the 1 Ω 25 W if the supply output is actually held at 5 V.

In 5 V and 12 V supplies, especially those intended for computer use, the 5 V output was often considered as the "main" one. It was tightly regulated, as long as the load conditions were met. The 12 V output then is what it is, but usually not more than 500 mV or so off from nominal. You probably don't need a minimum load on the 12 V, but you'll just have to experiment unless you can find the datasheet.

  • \$\begingroup\$ I loaded 5v rail with 5Amps and 12v rail have 13V with 1A current draw :D \$\endgroup\$ – Baterka May 27 '18 at 11:20

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