# 12v rail vs 5v rail computer psu

#### Gryd3

Jun 25, 2014
4,098
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..
Lets see if any of this information is helpful.

The 12V rail can output far less current than the 5V rail.
12V @ 16A
5V @36A
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/

#### Martaine2005

May 12, 2015
4,888
Thanks Gryd3,, you made that quite clear.
I apologize for opening my (keyboard).
No excuse for a wanting forum user.

#### Gryd3

Jun 25, 2014
4,098
Thanks Gryd3,, you made that quite clear.
I apologize for opening my (keyboard).
No excuse for a wanting forum user.
No worries Martaine, I have taken the 'safety' off my keyboard a few times. It has never accomplished anything. As blunt as Colin is, if he took the time to explain his post to help the OP understand he could be an amazing asset. Communication is key, and it does not matter from whom. Everyone needs to pitch in.

#### Martaine2005

May 12, 2015
4,888
Thanks Gryd3. I can assure all the Admins that you will not need 'safety button' . Your all nice.
Martin

#### Minder

Apr 24, 2015
3,475
IMO, the P.C. power supply is the last thing I would use for a DC motor, a transformer and a bridge is all that is needed.
M.

#### kellygoose

Jun 16, 2015
5
Ok everyone thanks for all the help. I will put some sort of resistance on the 5v rail and see if it runs on 12v.

IMO, the P.C. power supply is the last thing I would use for a DC motor, a transformer and a bridge is all that is needed.
M.

If I had those parts laying around that's what I'd be using, the project I'm building is entirely from free parts I've collected, and I haven't come across a microwave yet. The DC motor will rarely be used so I'm ok with using the psu.

#### Arouse1973

Dec 18, 2013
5,178
No worries Martaine, I have taken the 'safety' off my keyboard a few times. It has never accomplished anything. As blunt as Colin is, if he took the time to explain his post to help the OP understand he could be an amazing asset. Communication is key, and it does not matter from whom. Everyone needs to pitch in.

I don't think MARTIN's comments were un-just. Colin is yes an experienced guy. But if he is going to carry on with his blunt replies then it is not worth him carrying on in my opinion he just irritates the newcomers. There is nothing wrong in telling him that what he said could have been useful but wasn't. He should actually know better having helped thousands of people in his time with "Talking Electronics". I am surprised at the lack of explanation to be honest. Maybe he doesn't have the time to reply at home, he might be busy doing house work, like cleaning carpets or something.

#### Gryd3

Jun 25, 2014
4,098
Ok everyone thanks for all the help. I will put some sort of resistance on the 5v rail and see if it runs on 12v.
Best of luck with this. It 'may' help, but there is a slim chance. This entirely depends on the motor not actually drawing in excess of the 12V rail AND that the particular PSU model behaves differently with an unloaded 5V rail. The PSU is hit and miss, some units don't care, others do. I can't say a thing regarding the motor though. (Which is the most likely culprit.

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