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Combining PSU rails for lab power supply? Answered

I recently received 2 old computers from friends for tinkering purposes. I'd like to take their PSUs and create a power supply for projects. However, they only have max outputs of 110W.

So, I have a few questions:

1.) Is it possible to combine rails?
2.) Is it safe?
3.) What kind of power increase would I be seeing?

Please try to answer all 3 individually.

15 Replies

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frollardBest Answer (author)2013-04-07

1. It depends on the psu, some will work fine, others will START ON FIRE. Some computers even use multiple ATX power supplies on one motherboard, but its becoming much more rare.
2. It depends on what you call safe - for most people, being on fire is not safe.
3. if it works, a 1:1 increase - as paralleled psus will double the current. Do not combine them in series because ground is usually literally ground, and your first +12v rail would connect directly to ground of the 2nd, which is unironically the same ground...so it's just a short circuit.

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General Eggs (author)iceng2013-04-07

Excellent. The experimentation will begin in the morning.

+1 to you both!

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General Eggs (author)frollard2013-04-07

So, I run the risk of frying them and burning the house down. Is there any way to tell whether or not they'll do this other than trial and error? I'd rather not ruin 2 perfectly good PSUs.

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General Eggs (author)2013-04-08

Actually, is it even worth it to combine these? If I'm using it for small projects at home, will 110W be enough, or am I going to probably need more power?

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iceng (author)General Eggs2013-04-08

Every non and then there will be a project that needs more the you can deliver.

Then the Scout handbook will have to be your Eagle guide.

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General Eggs (author)iceng2013-04-09

Ok, now I have a problem. I shorted the power wire to ground and tested out all the rails just to make sure theyre working in the first place. However, the +12V rail is only running at about 9-10 volts. Is this something I should be worried about?

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iceng (author)General Eggs2013-04-09
Hey you can use a voltmeter !
  • Is the supply set for 220VAC ?
  • Are the 5VDC outputs correct ?
  • How old is the PSU ?
  • Shorting output ain't good
  •  

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General Eggs (author)iceng2013-04-10
Yes I can!
  • I'm in America, so I have it set for 115V.
  • The 5VDC output is correct.
  • The Psu was made in 2001, so it's fairly old.
  • I've tried it on a few different ground wires, I get the same result each time.

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iceng (author)General Eggs2013-04-10

Its nice when someone can and does use a voltmeter Kudos 4 U :)

Are you saying that the PSU on its own is 9-10 VDC ?

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General Eggs (author)iceng2013-04-10

I'm saying that the 5v and 3.3v rails are running fine. The 12v rail is just low.

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iceng (author)General Eggs2013-04-10

And this was always the case or something you did to it ?

Anyway if you don't smell smoke the only fear is it might go lower at any time.

A

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General Eggs (author)iceng2013-04-10

I havent modified it yet, so I doubt it was me. Other than that, it runs smoothly, quietly, and smoke-free :D I guess its just something I'll have to live with.

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iceng (author)General Eggs2013-04-11

Pragmatism is a good philosophy in electronics :)

A

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steveastrouk (author)2013-04-08

Careful: the tracks are designed for one current, not 2x 1 current

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