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PSU 12v voltage drop on boot

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July 24, 2012 3:59:20 AM

Ok, so here's the deal. I'm working on a pretty major case mod. I decided to use an exceptionally small psu and modify it into what will basically become a laptop power brick. The 12v out from the psu will power a dc/dc itx power supply to power the cpu and drives.

So, I wired all the yellow 12v wires together and all of the black - wires together so that the two 12v rails would be in parallel adding their max amperages. 16+16=32amps 12v current which should be more than enough for what I'm using. So, since the dc/dc power supply doesn't put out enough power for the gpu, I wired the gpu in parallel from the 12v source before reaching the dc/dc psu and since the external psu has 32 amps, this shouldn't be an issue.

The problem is that when I start the external psu by shorting the green wire to ground, my multimeter reads 12v as it should, but when I start the computer (dc/dc psu and the itx-mobo and drives) the 12v from the external psu drops to 10v. This results in the computer repeatedly restarting. Gets through the bios and when it attempts to boot from cd or hard drive, it just restarts the mobo. Why would this happen, and how can I fix the voltage drop?
a c 286 ) Power supply
July 24, 2012 4:09:49 AM

Where are you measuring and how did you attempt to tie them together?

The attempt to merge all the 12 V lines may have resulted in a high resistance junction that is causing you a significant voltage drop, also try to minimize the number of connectors you pass through, each connector has a significant amount of resistance associated with it since the pins don't meet perfectly.

Check out the write up below to see just how much extra connections can hurt your voltage level.
http://www.motherboards.org/articles/guides/1488_9.html
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July 24, 2012 4:15:59 AM

I'm measuring it straight out of the external ac/dc psu. before load 11.9v after computer starts, 10.8v.

Ok, I'll try that tomorrow.
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a c 76 ) Power supply
July 24, 2012 4:16:04 AM

+1 to what hunter said, and to add to it that just splicing rails together like that wouldn't linearly add up to the amperage available from that spliced "single" rail, as far as I'm aware.
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July 24, 2012 4:19:26 AM

DJDeCiBeL said:
+1 to what hunter said, and to add to it that just splicing rails together like that wouldn't linearly add up to the amperage available from that spliced "single" rail, as far as I'm aware.


I don't see why it wouldn't. That's how parallel circuits work don't they? Voltage is constant, but amperages are additive. The reverse of series.
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a c 76 ) Power supply
July 24, 2012 4:24:04 AM

ultrastatic said:
I don't see why it wouldn't. That's how parallel circuits work don't they? Voltage is constant, but amperages are additive. The reverse of series.


Well, just basing it off of the fact that multiple rails never add up to the actual max amperage available in total. The rated max per rail is just that, the max that one rail on its own can produce.

Two 16A rails would REALLY be around a 28A or a little less max from the PSU (when compared to a similar wattage single rail unit).
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a c 286 ) Power supply
July 24, 2012 4:25:53 AM

Except you just made a false assumption about how rails on a power supply work.

The PSU has a single 12 V source, from this it splits into two large traces on the main board and passes through a pair of shunt resistors that monitor the current, if it exceeds 16 A through either shunt the PSU will trip and turn off; however, this does not mean that you can draw 16 A through each rail for a total of 32 A because the 12 V source likely doesn't support it, most dual rail units rated for 2 16 A rails would likely have a combined capacity around 28 A unless otherwise stated.



If you are seeing that large of a drop right at the PSU's main connector that implies that it just isn't able to generate the necessary current on it's end, is it an older unit? It may have finally failed, did you test it before you attempted to use it in this configuration to confirm it was functional or is it still in a state that you could hook it up to a simple system to confirm that it is functional?
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July 24, 2012 4:33:27 AM

Well, that's great to know, and 28A is still enough.

It's currently only drawing about 10amps. So, not even the max for one rail. And I'm not having issues with current, it's the voltage that is dropping.

It's brand new. I purchased it specifically for this build.
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July 25, 2012 2:24:09 AM

Got it fixed! The wire from the psu the computer was too long and not large enough gauge. And at 10amps, the .13 ohms from the wire dropped the voltage about 1.3-1.5 volts.
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