How do I test this PSU?

Factual Questions
1

Paging the electronics gurus.

I have a Shuttle SS31T to which I’m trying to add a Nvidia 8600 GTS. The power requirements exceed the capability of the existing PSU and the machine can’t take a standard-sized PSU - height and width are fine, but max depth is 120mm and the standard depth is 140mm. So I’ve purchased a 300W Booster PSU.

Without the 8600 GTS, it all works fine, but when I add the graphics card, the PSU start to complain (the VRMs make a loud noise), even with the booster PSU and the graphics card connected directly to the booster PSU. It’s as if the booster PSU isn’t providing any power to the graphics card, yet the front LEDs light up.

I’ve tried another graphics card too, with the same result, so it’s not the graphics card.

If I switch the main PSU off, the back fan, the main PSU fan, and the CPU fan keep running, so the booster PSU must be supplying some power.

I don’t have a suitably powerful full-size PSU that I can plug in outside the system that can do all the functions. I have tried another small-size PSU with the same result.

How can I test if the booster PSU is actually providing power to the graphics card?

2

I’m not clear on how the aux PSU is powering the fans with the computer turned off. Through the video card?

The aux PSU has two 12V rails of 150W each (assuming that’s accurate), with two PCI-E video card connectors. If you only have one of the connectors plugged into the card that 150W may not be enough. I doubt there are any two PCI-E female into one PCI-E male connector adapters available, you’d have to make one up to see if that would help. Or maybe just splice the two set of wires together going to one connector.

The card will also draw power through the PCI-E slot in addition to the aux connector. That may be too much for the original supply to power as well, in which case it’s just not going to work.

Do you know what the power requirements for your card are through the connector and the slot?

3

There’s a 12V (and ground) cable between the two PSUs.

It should be more than enough: it only needs 71W.

How can I test that power is coming out of the graphics card connectors?

4

A voltmeter? If you don’t have one, maybe a friend or relative does. A cheapo one can be had for like $15 if you’re really desperate.

5

I still don’t understand the fan thing. Is the 12V cable used to turn the aux supply on or what? The aux supply is on when the main supply is off and feeds back through it to the fans? The two PSs both outputting to the same 12V bus is strange.

The card draws 71 watts in toto, or just through the connector? Any idea what it will draw from the slot when the connector is powered? The slot provides both 12V and 3.3V, perhaps the 3.3V is maxed out.