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ATX PSU with two 12V rails

Posted by Eriksen 
ATX PSU with two 12V rails
March 28, 2013 05:06AM
Hi there.

I am about to wire up a set of Sanguinololu electronics for my old Prusa. I have got a heated bed Mk. 2a ready, but I have a question about the PSU.

I have an ATX PSU with two 12V rails, rated for 14A and 10A, respectively. Combined, this should be adequate for Sanguinololu plus heated bed, but I am a bit unsure how this works. The 14A rails should be able to cover the heated bed and the 10A rail should be sufficient for the electronics+steppers. Right?

The Sanguinololu is set to be powered through the 4-pin ATX plug. Which 12V rail is used here? The plug has two yellow wires and two black. Does this mean that both 12V rails are used by the 4-pin plug, so that I get the full 24A to 'play' with?

Thanks for any help smiling smiley
Re: ATX PSU with two 12V rails
March 28, 2013 06:28AM
All the PSUs I opened so far had all 12V wires soldered to the exactly same spot on the internal PCB.

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Re: ATX PSU with two 12V rails
March 28, 2013 06:50AM
Its not always the case though. I have a relatively cheap atx supply for mine. I use the motherboard connector and the P4 connector as the atx spec says they should be on different rails. They both have slightly different voltages and if one is loaded the voltage drops but the other says the same.

But you are correct most are connected together.

So your best bet is just give it a try. It will either work or the over current protection with shut down the power supply.
Re: ATX PSU with two 12V rails
March 28, 2013 10:22AM
They are supposed to have separate current limits to protect the wiring. However, the cheap ones ignore safety and connect them all together, but that would fail a EN60950 safety approval. Funny how they come CE marked!

If they have a single 12V supply rail and then split it with two current sensors it would be OK to common them at the Sanguinololu. If they really are two separate 12V rails then simply connect one directly to the bed and the other to the Sanguinololu. That is a better scheme anyway with Sanguinololu and connect the other bed wire direct to the MOSFET tab to avoid using the underrated bed connector.

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