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
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.
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.