will an external power brick damage my graphics card?

Ant6n · Post by **Ant6n** » Sun Jun 22, 2008 2:59 pm

Hio,

I have a pw200m power supply which can deliver only up to 200W. The system I have draws less than 100 without graphics card, but I am thinking about using a card that might use more than 100 Watts.
I have a couple of 12v 120W bricks sitting around. So naturally it comes to mind to just wire these 120W@12V to the external connector of the graphics card. Since the external connector only needs 75W everything should be fine. The system would then use ~90W (sys) + 75W (gfx) = ~165 Watts, and up to 75 Watt on the secondary brick.
But, the power will be connected to the graphics card even when the pc is off.
Is this a bad/dangerous idea? Can this extra power at all times damage the graphics card?
thx

Anton

tehcrazybob · Post by **tehcrazybob** » Sun Jun 22, 2008 7:44 pm

I wouldn't recommend it, but I doubt it'll be necessary. What graphics card are you looking to use? Most mainstream cards are well below 100w, though if you're looking at a completely ridiculous high-end card, all bets are off.

Ant6n · Post by **Ant6n** » Sun Jun 22, 2008 7:46 pm

i was looking at something like a 4850. It takes too much to be powered with 200 Watts alone (even if the cpu is a mobile intel core duo).

Matija · Post by **Matija** » Sun Jun 22, 2008 8:58 pm

Your entire PC will go up in flames if you do that without grounding the two PSUs together.

Ant6n · Post by **Ant6n** » Sun Jun 22, 2008 9:01 pm

eh? Why is that?
I guess I can always connect the 12v and the grounds of both external power supplies.

bexx · Post by **bexx** » Sun Jun 22, 2008 10:12 pm

Hmm, why would it go down in flames? Doesn't Thermaltake have VGA PSUs? Never actually seen one but how would they share common ground?

bexx · Post by **bexx** » Sun Jun 22, 2008 10:19 pm

http://www.thermaltake.com/product/Powe ... /w0099.asp#

ROFL I love Thermaltake... that PSU is only 21db! at 2000rpm! Yet then they show graph of rpm vs load and at no point on the graph could the PSU be at 2000rpm... it doesn't go below 3000rpm and goes all the way up to 6k.

I guess maybe it'll share the ground with main PSU but it mostly looks like that connector is used to sync power on.

Matija · Post by **Matija** » Sun Jun 22, 2008 11:30 pm

Well, you can use two power supplies without problems, but you can also have everything go to hell immediately.

Remember that voltage is a difference between potentials.

If the two PSUs don't share a common ground level, you'll get a ground loop, which can be very harmful. I don't know how to explain it best, but I'll try.

PSU1: "0V" = 0V, "12V" = 12V
PSU2: "0V" = 3V, "12V" = 15V

Even though both PSUs do supply 12V to their respective equipment, in reality they are at different absolute potentials, with a difference of 3V between them. That's why you want a common ground level.

I guess maybe it'll share the ground with main PSU but it mostly looks like that connector is used to sync power on.

"24-pin adapter: to Connect main PSU & MB & Power Express". One of the wires (black) is ground.

Ant6n · Post by **Ant6n** » Mon Jun 23, 2008 2:09 pm

that makes sense. Would it be sufficient to just connect the two 12v rails and the two ground rails?

Code: Select all

gnd1  --+--- system      12v  --+--- system
        |                       |
gnd2' --+--- gfx-aux     12v' --+--- gfx-aux

The other problem would that there is constantly a current supplied to the gfx card. My gut feeling tells me that doesnt matter (i.e. the card doesnt use power if its not asking for it); but then again, I might be wrong.

Matija · Post by **Matija** » Mon Jun 23, 2008 9:58 pm

Just the ground should be connected. If you connect +12V together, then you are really asking for trouble