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PC Power Supply Mod - Quick question for someone with an EE etc.
PC Power Supply Mod - Quick question for someone with an EE etc.
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Old 5th May 2009, 06:30 AM   #1
calee4nyaboy is offline calee4nyaboy  United States
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Default PC Power Supply Mod - Quick question for someone with an EE etc.

I have a funky project I'm working on that will require 12V DC to power a car audio amp inside a home. Part of the project guidelines is to use existing parts I have lying around the house so that the project wont really cost anything. Cheap is good, right? : )

Since PC power supplies offer 12V DC I thought that would be a natural choice. The car amp, as it will be used, should not draw more than 20A. The PC power supply I have is a NEW unit that has DUAL RAILS (which is what this thread focuses on). Each rail is rated at 22A (but I think that spec is like a max rating and is taken with other rails not being used) In other words, a more realistic number is probably more like 20A per 12V rail, all rails driven. Inside the PC power supply, there are two separate areas on the PC board where the 12V rails come out (one for each 12V rail).

My Question: Can I simply COMBINE the rails where they come of the PC board to have a single 12V rail that can handle more current? Or will that totally blow it up?

Granted I could probably get by with just the single rail but IF you can combine the rails, you might as well because the second rail is doing absolutely nothing if you dont use it.

Thanks for any input!

John
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Old 5th May 2009, 10:53 AM   #2
Mr Evil is offline Mr Evil  United Kingdom
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Most ATX PSUs that are described as "dual rail" are not. Instead they have a single 12V rail but with two separate current limits. Thus it is safe to simply parallel them.

Note that there will almost certainly be a minimum load required on the 12V rail. If you find that the output voltages are not what you expect, stick a power resistor across there to provide that load.
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Old 5th May 2009, 04:36 PM   #3
calee4nyaboy is offline calee4nyaboy  United States
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Quote:
Originally posted by Mr Evil
Most ATX PSUs that are described as "dual rail" are not. Instead they have a single 12V rail but with two separate current limits. Thus it is safe to simply parallel them.

Note that there will almost certainly be a minimum load required on the 12V rail. If you find that the output voltages are not what you expect, stick a power resistor across there to provide that load.

Yes, I've heard that some are not dual rail even when advertised. But I don't know how to verify if mine is true dual or not. Is there an easy way to verify?

When you say minimum load required, do you mean the PSU wont put out 12V unless something, for example, has a load of lets say 1A or more? At idle, I'm pretty sure most car amps draw at least 1A. The amp I'm thinking of using is one of these new full range, class D, Alpine amps so who know what it draws at idle. I wanted to use this amp in particular because it should be more efficient than a regular class AB car amp so the PSU wont have to work as hard.

If I do connect the rails, and it messes up the power supply, what should I expect?...lol. Will it smoke up immediately when the PSU is powered on (with no load attached)? Will it need a load? Or will it take a while to blow up or?

I'm just a little concerned because I dont want to damage the amp in any way. I dont care a whole lot about the PSU, though, which is why I'm willing to attempt to mod it.

John
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Old 5th May 2009, 05:47 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by calee4nyaboy
Yes, I've heard that some are not dual rail even when advertised. But I don't know how to verify if mine is true dual or not. Is there an easy way to verify?..
Measure the voltage difference between the two rails. It should be 0 or very close to it. If it is, then connect them together with progressively lower value power resistors and check again. If at any point you get significant current flowing through the resistor then you can't parallel them directly.

Is it a stereo amp you're making? You could use one rail per channel, then there would be no worries at all.


Quote:
Originally posted by calee4nyaboy
...When you say minimum load required, do you mean the PSU wont put out 12V unless something, for example, has a load of lets say 1A or more?...
Yes. I'm not sure what a typical minimum load is.
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Old 5th May 2009, 08:06 PM   #5
calee4nyaboy is offline calee4nyaboy  United States
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Quote:
Originally posted by Mr Evil

Measure the voltage difference between the two rails. It should be 0 or very close to it. If it is, then connect them together with progressively lower value power resistors and check again. If at any point you get significant current flowing through the resistor then you can't parallel them directly.

Is it a stereo amp you're making? You could use one rail per channel, then there would be no worries at all.



Yes. I'm not sure what a typical minimum load is.
The application is a weird one: this is coming off of a Logitech Squeezebox BOOM unit. The unit has actively crossed over woofers and actively crossed over tweeters. I'm going to take those signals and boost them with the amp and real speakers. I'll get more power this way and real stereo separation because I dont have to use the speakers in the boom box that are only inches apart.

So I needed 4 discrete power channels. The amp is a 4 channel Alpine, class D. I thought about using two 2 channel amps, one on each rail (if I couldn't separate the rails) but my only spare 2 channels amps are class AB, not class D.

When testing each rails voltage, even if they are truely separate rails, shouldn't they still measure very close in voltage? PC power supplies have very tight tolerances and arent supposed to vary.

Maybe I should take it step by step: I think I'll just hook the rails together and watch it for 10 minutes with no load. Then I'll hook a spare old test amp up to it at idle. Then I'll hook speakers up to the amp and drive them. The whole time I'll keep an eye on the 12V line to see if it acts funny (fluctuates) and I'll keep a clamp meter to watch the current.

One of the reasons I wanted to use the car equipment is because I dont have to leave the equipment on all the time (or turn it on and off manually). When I turn the BOOM on, a signal sensor (the only part I will have to buy, and they are cheap) will throw 12V to an automotive relay which will turn on the PC power supply (grounding out the green wire) which will turn on the amp, all automatically. And then it will shut it all off as well.

Yes, there are some home amps (very, very few) that have signal sensing but I dont own any and remember, part of the guidelines is to use parts I already have. : )

I guess I'm just nervous about connecting the rails and turning on the PSU in fear smoke will come out...lol
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Old 6th May 2009, 06:58 PM   #6
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by calee4nyaboy
...When testing each rails voltage, even if they are truely separate rails, shouldn't they still measure very close in voltage? PC power supplies have very tight tolerances and arent supposed to vary...
Look inside the case and I bet you will see lots of 5% resistors and a few 1%, so you can expect a similar variation in output voltage between units. This is ok because the absolute voltage doesn't matter much. The ATX spec says 12V should be +/-5%.


Quote:
Originally posted by calee4nyaboy
...I guess I'm just nervous about connecting the rails and turning on the PSU in fear smoke will come out...lol
Don't worry, it would be a very rare PSU that didn't have both 12V rails from the same source. They have current limits too, so at worst the PSU should simply turn itself off.
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Old 6th May 2009, 10:39 PM   #7
calee4nyaboy is offline calee4nyaboy  United States
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Quote:
Originally posted by Mr Evil

Look inside the case and I bet you will see lots of 5% resistors and a few 1%, so you can expect a similar variation in output voltage between units. This is ok because the absolute voltage doesn't matter much. The ATX spec says 12V should be +/-5%.



Don't worry, it would be a very rare PSU that didn't have both 12V rails from the same source. They have current limits too, so at worst the PSU should simply turn itself off.

Thanks for your replies. I feel more confident this will all work as expected. And if most PSUs do in fact supply all the 12v rails from the same source, that is great news because I'm sure there are many more projects that I would want to do in the future where combining the rails would be helpful.

John
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Old 31st December 2009, 05:10 PM   #8
rick465 is offline rick465
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I know this is an older thread but its about my question. I have the same issue. However, my info is a little different. It is my understanding that a car amplifier takes 21 amps not 20 or less. My PSU is from a server and has 4-12 volt rails all at 20 amps each. When I hook it up the amplifier powers on fie. When I put a load on it by turning on the stereo the PSU shuts off. Since my PSU is from a server it has 2 lines with 4 pin connectors and 2 SATA lines. Instead or taking the PSU apart and combining the rails (which I could probably never do) can I connect the 12v wires from 2 different lines coming from the PSU for the same effect? This may be WAY to simple and blow up the PSU and the amplifier.
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