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Electronics => Beginners => Topic started by: kr15_uk on November 18, 2016, 03:28:18 pm

Title: -12V A - ATX PSU Bench PSU
Post by: kr15_uk on November 18, 2016, 03:28:18 pm

Ladies and gents,

I've built ATX Bench PSU some time ago, all good, but never had a chance to play with -12V or so
Now I'm working on one small project where I'll finally need my 24V out of +12V and -12V

But... I'm a bit lost with AMP rating for this combination.
+12V1 is rated for 15A max but -12V 0.5A
Soooo.... My 24V in theory will be ??A... And I'm stuck here...

Thanks.


Kris M

Title: Re: -12V A - ATX PSU Bench PSU
Post by: TheAmmoniacal on November 18, 2016, 04:12:28 pm

CORRECTION: you're limited to 0.5 A because the current has to go through both rails (and thus you will be limited to the rail with the lowest rating).

Title: Re: -12V A - ATX PSU Bench PSU
Post by: kr15_uk on November 18, 2016, 04:30:06 pm

Thanks for the reply.
Do you mind to share how you've calculated this so I don't bug people next time when I get something similar?!
Like next possible option +5V 16A and -12V 0.5A

I've tried to figure it out myself but none of the options were even close to 967mA...

Thanks.


Kris

Title: Re: -12V A - ATX PSU Bench PSU
Post by: MarkF on November 18, 2016, 04:51:05 pm

I assume you intend to connect your projects +V to the supply +12V and -V to the supply -12V. The current out of the +12V must equal the current into the -12V (i.e. You're limited to 0.5A).

I don't recommend doing this as your final solution. Your project will be referenced to -12V. Therefore, your project MUST be isolated from ground to prevent a short of your power supply.

Title: Re: -12V A - ATX PSU Bench PSU
Post by: kr15_uk on November 18, 2016, 05:02:13 pm

Thanks Mark.
0.5A makes more sense to me.

24V is only for testing. If I'll find that 24V is way to go then I'll get proper bench PSU.

Thanks.

Title: Re: -12V A - ATX PSU Bench PSU
Post by: raspberrypi on November 18, 2016, 11:19:11 pm

The only way I know of is to read the label on the side of it. To generate the -12 volts requires dedicated circuitry that they don't beef up like the +12 because computer mother boards don't use alot of - voltage. I built one ofthose and was all excited to play with the negative voltage until I realized it was such little current. A crude way is to count how many wires come out for that voltage, lots of wires are to carry lots of current. 

Title: Re: -12V A - ATX PSU Bench PSU
Post by: MarkF on November 18, 2016, 11:19:49 pm

Quote from: kr15_uk on November 18, 2016, 05:02:13 pm

Thanks Mark.
0.5A makes more sense to me.

24V is only for testing. If I'll find that 24V is way to go then I'll get proper bench PSU.

Thanks.

Careful with scope probes during testing. That ground clip on the probe WILL short your ATX supply.