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Hi I am trying to modify an old pc psu 350w in order to run some 12v and 9.6v cordless tools. I put 2 10Ω 5w resistors in parallel between 5v and ground and I sorted the green wire with ground. I can run easily a 5v screwdriver directly from the molex. I can also run a 12v motor with no load direct from the molex. But when I try to run my 9.6v drill from the 12v rail the psu turns off. I can run this drill from a 12v wall adapter and runs fine but the adapter doesn't provide enough amps so the drill is weak. What could be the problem with the power supply? Thanks a lot

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  • \$\begingroup\$ Have you stuck an ammeter in there to check the current your drill is trying to use? \$\endgroup\$ – MCG Nov 3 '17 at 8:53
  • \$\begingroup\$ No I haven't because my multimeter can measure up to 10 A \$\endgroup\$ – Mike T Nov 3 '17 at 9:01
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    \$\begingroup\$ Buy a 100A shunt and measure the voltage across that then. With an oscilloscope so you can see the starting current spike, which is probably about 5x the steady state current. \$\endgroup\$ – Brian Drummond Nov 3 '17 at 9:50
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It probably detects an overcurrent and shuts off the 12V Output.

Edit: The ATX Standard states (i am guessing ist an ATX power supply) that Outputs shall be switched off if short circuit/overcurrent occures at the Output.

That means for you: You could measure the current the drill draws, to be sure you are on the right track for solving this Problem. If there really is an OC condiction you could ramp-up the drill's Motor using PWM. If your Multimeter does not support more than 10A just add a small current shunt resistor in series and measure the voltage over that resistor. But a Multimeter will most likely be too slow for that kind of measurement so it would be helpfull if you have access to an oscilloscope.

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  • \$\begingroup\$ It says that can provide 15A on the 12v rail and the drill I believe that draws less than that. Can I do something to deactivate overcurrent protection? \$\endgroup\$ – Mike T Nov 3 '17 at 8:22
  • \$\begingroup\$ "and the drill I believe that draws less than that" - As my teacher used to say: "To believe is something you can do in church. In here, you need to know." The road to the junk bin is paved with assumptions. \$\endgroup\$ – Dampmaskin Nov 3 '17 at 9:26
  • \$\begingroup\$ It's 150w and its battery is 9.6V so if I am correct it should draw 15.62A from battery. But now I have 12V so I suppose that it draws 12.5A. Do I calculate something wrong? \$\endgroup\$ – Mike T Nov 3 '17 at 9:33
  • \$\begingroup\$ For a DC/Universal motor, increasing the voltage will increase the current. That, and there is probably a significant starting current requirement. Batteries can deliver exceedingly high currents for brief periods, your ATX power supply cannot. \$\endgroup\$ – R Drast Nov 3 '17 at 9:38
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    \$\begingroup\$ Power your tool from a 12v car battery : measure the current and voltage at load and no load then source a power supply : this will save a lot of messing around - unless you just want to keep away from the washing up :) \$\endgroup\$ – Solar Mike Nov 3 '17 at 10:24
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A 350W ATX PSU can usually supply around 25-30Amps I think. Given the power draw of your drill the PSU can definitely handle the currents at NO LOAD. But while trying to drill it will draw more current which the PSU might not be able to handle.

How many wires from the +12V rail are you using to run the drill? Have you bunched it together? Usually the 12V current capability is split over the entire +12V wires.

If bunching all the +12V doesn't fix your problem, then you are out of luck.The ATX can't handle full load current. You need a higher wattage one like 475W or 500W ATX at least(Can't say for sure without knowing the full load current of the drill @ 12V).

Also please do not attempt to open and disable the over-current protection, it was added there for a reason.

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  • \$\begingroup\$ I use only one yellow wire for test. I will try to use all of the yellow wires bunched together. But it falls as soon as I press the trigger \$\endgroup\$ – Mike T Nov 3 '17 at 10:07
  • \$\begingroup\$ did you mean it shuts off at no-load even after bunching? \$\endgroup\$ – The_Vintage_Collector Nov 3 '17 at 10:09
  • \$\begingroup\$ No. I haven't try the bunching yet. But I think that all of the wires terminate in the same contact on the circuit board. So I believe that the overcurrent protection will trigger again \$\endgroup\$ – Mike T Nov 3 '17 at 10:13
  • \$\begingroup\$ Well Mike if this method fails then you are out of luck, it will be better if you could get a 500W or above PSU that has +40A on its +12V rail. \$\endgroup\$ – The_Vintage_Collector Nov 3 '17 at 10:16

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