Dual PSU problems

[OVerLoRDI]

So I got my hands on a Kill-A-Watt to figure out my total electricity usage, and as you can guess from my sig its pretty high.

One of my rigs is a 3x 5870 machine, so I thought it would be wise to use my extra 500 watt psu to assist my 650 watt psu in powering 3 video cards. I jumpered the pins and added it in. However after checking the 650 watt, it is pulling nearly 700 watts from the wall, and the 500 watt psu is pulling 0. So 700 watts, ~ 80% efficiency, is ~560 watts, and well within the limits of the psu. So the 650 watt is powering the entire machine and has been successfully for about 3 weeks now.

That being said, why was the 500 watt psu contributing nothing? It has 2 pci-e 6 pin plugs that were plugged into 2 different video cards.

 

[Zap]

Interesting. What PSUs are they?

A PSU in theory can put out what it says on the label. Some can put out a bit more. Also, it DRAWS more than it puts out. For instance an 80Plus PSU puts out 80% of what it draws. The Kill-A-Watt tells you what it draws. Multiply it by 80% (or whatever your PSU is rated at) to find DC output.

Also, the card may not have needed much more than what one PCIe plug could put out. I'm also not sure the card will draw even amounts from all the sources.

 

[OVerLoRDI]

Interesting. What PSUs are they?

A PSU in theory can put out what it says on the label. Some can put out a bit more. Also, it DRAWS more than it puts out. For instance an 80Plus PSU puts out 80% of what it draws. The Kill-A-Watt tells you what it draws. Multiply it by 80% (or whatever your PSU is rated at) to find DC output.

Also, the card may not have needed much more than what one PCIe plug could put out. I'm also not sure the card will draw even amounts from all the sources.

One PSU is an XFX 650 watt, single 12v rail good for 52A.
Other PSU is a Corsair builder series 500 watt.

It's possible, but the cards are overclocked, all running at 975mhz. The cards are also not throttling or anything either. I would think that they would draw some power from the 2nd pci-e plug.

 

[jtzou]

I think it also needs a minimum load on the 5V rail. I've seen some things mention throwing a 10 ohm, 10 watt resistor on the 5V rail. Some power supplies don't provide power on the 12V rail if there isn't also a load on the 5V rail.