How to use your old useless low power atx PSU's for powering extra GPU's

Sub-Ether

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Mar 31, 2014
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Ok guys, if you're mining on any sort of scale, no doubt you have had a PSU go bang and replacing it with an (unnecessary) 900 watt version for your planed GPU upgrade does not come cheap- and after all the money is better spent on more GPU's isnt it :cool:. So heres a short explanation on how to re use that old power supply that you were going to bin due to the fact it won't power your mobo anymore.
On the typical GPU there are two 6 pin molex connectors (sometimes an 8 pin, but is same 12 volts only pinout), now these heavy duty connectors carry all the power(high wire melting currents) for your mining enterprises.
Now the important point is that the two 6 pin molex 12 volts supply rail is separate on most GPU's to the motherboard supplied 3.3 volts and 12 volt rails and PCI-e signal stuff.
All you need to do, is to introduce a second power supply that is manually fired up and tap the 12 volts output rail and connect it to the 6 pin molex connectors making sure you have black as earth and yellow as 12 volts (check with a cheap voltage meter to make sure). Make sure you connect all 3 blacks together in 1 and all yellows together in 1.
In order to get the 2nd PSU to fire up, you will need to short circuit the green wire on pin 16 of the main 24 pin atx connector to the adjacent black earth wire on pin 17 or 15. stripping the wire just before the big 24 pin connector and putting a small bridge between green and black is how I do it, but you can just put a small loop into the connector across pin's 16 and 17. Make sure you do not short any other pins out as it may destroy the PSU as it fires up into a total short and melts!
Do not fear, you only want the 12 volt line so after the 2nd psu has fired up using the big 24 pin connector shorted green pin 16 and black 17 technique, check your pinout and yellow wire with a $5 meter just to make sure the polarity is right and the voltage is around 12V (it will drop to about 11.5V under heavy load and poor wiring at the load end)
All yellow wires coming out of the atx supply are 12 volts and I suggest finding the thickest looking(cutting off a 4 pin molex for example) and using 1 yellow wire/1 black wire per 6 pin molex as it is good practice to spread the load out.So you will be cutting off two old molex flat 4 pin types to generate to 2 new 6 pin molex's.
you should be able to run at least 2 GPU's off an old power supply.For example if the GPU draws 10 amps each and the PSU is a 350 watt atx it should be rated around 22 amps for the 12 volt rail so you would just be able to do it-switching to scrypt in this case would probably fry it though. Check the back of case for detailed spec and add the numbers in amps.

Important: Remember to switch on the 2nd PSU to power the 2nd GPU before you turn the computer mobo main PSU on otherwise the 2nd GPU will not be detected on start up (and perhaps transient induced power spikes across rails)
And when shutting down, turn off the main PSU mobo first as normal and then after the computer is completely off, turn off the 2nd PSU.
You can do this for 4 or 5 GPU's in one computer and so need never buy another expensive high power PSU again ! :)

(warning, there may be cross voltage rail exceptions to this, I have only tested on the radeon 6900 and 5000 series, but I tend to think its a standard on the radeon devices if not across the board but don't hold me to it I may be wrong but it works for me)

http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress
http://en.wikipedia.org/wiki/ATX

happy mining!

Sub-Ether
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tip 1 : cut old 6 pin molex connectors off any broken PSU's and soldier (for good connection) onto old (but still working) 2nd PSU yellow and black wires and try to avoid plugs and sockets were possible as they have high resistances and tend to melt easy!

tip 2 : try to use thick multi stranded copper wire were possible as some PSU's use cheap steel and they get hot and have a higher voltage drop.
 

Sub-Ether

Well-known Member
Mar 31, 2014
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Btw I just got hold of an old dell AA23290 power supply, now this fairly quiet and compact ex server PSU can put out 75 amps at 12 volts so it can power 5-7 extra GPU's depending on coin and load. I have shorted out a couple of the pins to earth and fired it up fine. Am at the point of trying to get the motherboard to recognize the extra GPU in the next PCI-e slot . I have seen similar on ebay for only $30, am willing to bet they will last a lot longer than your cheap and chearfull $90 750 watt version from China !