Lets see if any of this information is helpful.I'm using a spare cordless drill motor to turn a threaded rod to move a project of mine. No heavy lifting here just raising and lowering a 15a mitre saw motor on a link frame I built. I got a working fine 380w psu out of a pc, new style atx. The psu is rated at 16a for 12v and 36a for 5v. The drill motor was originally powered by a 19.2 volt battery. In the application I'm using, the motor will rarely run and only for short periods. I tried to use the 12v rail to power it, and it shuts the psu off immediately after opening circuit. (I'm still using the trigger mechanism to control power to the motor, so voltage is regulated by the trigger) Doesn't matter how little or far I crack it open, the psu shuts off. So I tried the 5v rail and it works fine under all conditions. It satisfies me because it works but a bit more speed would be nice, i.e. 12v or higher.
I'm curious as to why the 12v doesn't work and the 5v works fine. Seems to me if you do the math as far as voltsxamps the wattages are similar and even favor the 12v rail. What am i not understanding here?
Anyways this will work fine for me until I find a microwave in the trash and can get higher voltage that will handle more current.
Thanks for any and all replies..
The 12V rail can output far less current than the 5V rail.
12V @ 16A
Lets pretend the motor's resistance is 0.5Ω
@ 12V it would draw 24Amps... there goes the power supply!
@ 5V is would draw 10Amps... which is well below the limit.
Additionally, some power supplies don't operate to the fullest extent when 'unloaded' and will occasionally require a 'dummy load' on the 5V rail to trick it into working at it's full capacity.
This would only help if the drill pulls less than 24Amps and is to be determined.
Also, keep in mind that the 'wattage' of a PSU does not directly relate into the current output of the 12V or 5V rail... there is no standard, and the wattage is a form of combined SUM of all the rails on the PSU. If in doubt read the actual current outputs on the power supply.
As Colin bluntly 'tried' to suggest is adding a large resistor in-line with the motor, this may be enough to limit the amount of current drawn to less than the limit imposed by the 12V rail. Make sure that the power rating is high... or you will find that you will burn it out quite quickly.
Best of luck! o/