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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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Old 03-01-2007
Aalelan Aalelan is offline
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Default Possible to merge rails?

This may be a dumb question but is it possible to merge two +12v rails in a PSU? Specifcaly I have FSP550-60PLN http://www.fsp-group.com.tw/english/...id=54&proid=33

One rail is deticated to the 8pin motherboard power connector and the other rail is everything else.. the 24pin header and all the I/O device ports. I'm loading up the I/O side I think too much.

Second question.. If I put my Kill-A-Watt meter on just my PC measure the watts used, then take the average effiency of that unit (which at full load is 62% ) so say 68% at mid load?, and multiply it by that.. Would that give me the combined wattage of my machine (all rails and voltages considerd)?

Hope that is understandable as work is pulling me away now
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Old 03-01-2007
ianm2 ianm2 is offline
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you mean connect 2 12volt rails together?

There are 2 ways, in parallel, and in series.

series wise, the volts double

parallel, you double the current, but!!!

I wouldn't do it, you will kill your psu, basically, its hard to tell you how it happens, its technical, but, instead of the current going into your desired part, ie. your load, which is your computer circuits, it will go into the other part of the psu and fry it.

its to do with output impedance of your psu, which should be pretty much zero, or very low, thus lots of current drawn according to ohm.

so don't!!

by all means try it on a cheap psu, but I am assuming a smps is a constant voltage source electrically, and I may not be correct, it may well double the current capacity, but there are transistors and diodes in their, which will likely blow as I have described.

but even if I am wrong, safety wise, it does no harm to discourage.

yes I did do it foolishly myself on a constant voltage source, I connected 2 audio amps in parallel, before I understood a bit of audio design, and no it doesn't make it louder, it goes very quiet, with some blue magic smoke that you can't get back in the box, if a smps is similar which I strongly suspect, its bye bye psu

Last edited by ianm2; 03-01-2007 at 10:47 AM.
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Old 03-01-2007
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Quote:
Originally Posted by Aalelan View Post

One rail is deticated to the 8pin motherboard power connector and the other rail is everything else.. the 24pin header and all the I/O device ports. I'm loading up the I/O side I think too much.
Why do you think that?
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Old 03-01-2007
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In a quick word...the rails are already merged in a way. The 12V1 and 12V2 rails start out as a large 12V rail. The PSU maker builds in 2 separate over current protection circuits and runs the "rails" from the 12V rail through them. What this means is that if you aren't suffering from shut downs from tripping the OCP on one of the virtual rails (they're virtual in that the power is coming from the same source) then you have nothing to worry about.

Let me break this down in a manner you should be able to understand better. Say you have a 120V lead in your house. It's good for 40A before it trips the breaker. You "Y" that off to two 25A breakers but make the circuit such that if you trip one of the 25A breakers the 40A breaker cuts off too. That's basically what's happening with your PSU. Yes they rate the rails as say 20A per rail and 30A combined (strictly an example, I dunno the rating of your unit) but they'll often give some headroom to the OCP so that a slight surge on the line (HDD spinning up for instance) won't trip the OCP as it passes the rail's continuous rating.

as to the Kill-A-Watt just multiply the reading you're getting by .68 or whatever you figure the efficiency for your unit is and that's the actual power being consumed by the PC DC parts.
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Old 03-01-2007
Aalelan Aalelan is offline
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I have had a few instances where during heavy video usage (i.e. in a game), the PC will just shutdown, my thinking was the video card was taking that rail over the top and hitting the OCP and turning itself off.

I'm getting ready to add two more Drives to my array in that machine so I'm thinking the problem is just going to get worse..

I just want to get as much life out of this PSU before I buy a new, better one. btw love reading your reviews Jonny.
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Old 03-01-2007
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Quote:
Originally Posted by Aalelan View Post
I have had a few instances where during heavy video usage (i.e. in a game), the PC will just shutdown.
Ok. Yeah. If an OCP is overloaded, it'll trip the PSU.

Just get a new PSU.
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Old 03-01-2007
Aalelan Aalelan is offline
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A new one is on my wish list. but so are a lot of other parts.. But I will tell you my next PSU will be a single rail one, I kinda wish I had got the Sparkle version of the one I have now, it was a SR

Thanks for the input guys..
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Old 03-01-2007
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And in a few years.....they will all be talk of how much better multi rails are....wheel of time turns and all that....
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