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 Post subject: Powering a video card with an external power supply?
PostPosted: Wed Aug 20, 2008 3:42 pm 
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So, I really like my low power system, but I'd like to have a little more headroom without sacrificing the small space footprint that the PW-200-M currently gives me. Currently, I have a Dell 220W 12v DC passive power brick sitting on my desk, doing nothing because it refuses to boot with my motherboard. It works just fine if I use the PicoPSU 120W instead of the PW-200-M. So, it works, and it's 12v DC, exactly what my HD3870 needs from the 6-pin power plug.

Can I use the power brick, instead of the PW-200-M to supply the 12v DC? Do I need to make sure the brick shuts off with the system, or would it not matter? I'd like to try this, but I'd like some feedback before I risk hardware failure.

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PostPosted: Wed Aug 20, 2008 5:28 pm 
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I looked around for those dedicated video card power supplies, and found this review for the Thermaltake Power Express 250W. The unit uses the green "on" wire for sensing when the system is on. I could easily run a jumper wire from the 24-pin to the Dell power brick so that it's only turned on when the system is. So, in theory this should work.

Edit: Maybe that wiring wouldn't work. The Dell has two pins that need to be shorted to turn the unit on, so, would the "power on" line in a 24-pin connector and a ground line work?

edit # 2: Found something that changes a few things, the FSP X3 Booster. It doesn't have any "power on" adapter, just seems to get power straight from the wall, and is always on. Makes sense, when the system is off, the video card needs no power, so none would be drawn from the unit. Almost makes me want to try it....

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 Post subject:
PostPosted: Thu Aug 21, 2008 12:56 am 
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Wouldn't the different power sources need to have a common ground somehow? Would they have that automaticly?

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 Post subject:
PostPosted: Thu Aug 21, 2008 4:48 am 
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Vicotnik wrote:
Wouldn't the different power sources need to have a common ground somehow? Would they have that automaticly?


Well, if you look at the FSP unit, it doesn't take any wires from the ATX power supply for the main system. It only gets it's power from the wall, suggesting it's always on. Assuming you plug them into the same power strip, wouldn't they then be sharing a common ground?

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 Post subject:
PostPosted: Thu Aug 21, 2008 6:35 am 
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ryboto wrote:
Assuming you plug them into the same power strip, wouldn't they then be sharing a common ground?

If the brick has a three pin plug, yes, it should be on a common ground. I don't think most bricks have three pin plugs. I think you would still be okay. 3Dfx was going this route with their Voodoo 5 6000. The common ground would still be achieved through the video card connection to the motherboard. You could always splice a ground wire from the main PS to the PCIe power connector to make sure.


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 Post subject:
PostPosted: Thu Aug 21, 2008 6:42 am 
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QuietOC wrote:
If the brick has a three pin plug, yes, it should be on a common ground. I don't think most bricks have three pin plugs. I think you would still be okay. 3Dfx was going this route with their Voodoo 5 6000. The common ground would still be achieved through the video card connection to the motherboard. You could always splice a ground wire from the main PS to the PCIe power connector to make sure.


The brick uses a 3-pin power cable, same that you'd plug into a standard PSU. The pin configuration is exactly the same as a 6-pin PCIE connector. In theory, all I'd need is to jumper the unit into the on position, and just patch a 6-pin pcie extension cable between it and the card. I could also run a ground between the two power supplies, but, if these FSP units aren't using them, do you really think it would be necessary?

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 Post subject:
PostPosted: Thu Aug 21, 2008 6:59 am 
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ryboto wrote:
do you really think it would be necessary?

You're going to have to try it. I once watched a Radeon 9500 burst into flames back when it was a $140 video card. I lived through it.


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 Post subject:
PostPosted: Thu Aug 21, 2008 7:57 am 
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QuietOC wrote:
You're going to have to try it. I once watched a Radeon 9500 burst into flames back when it was a $140 video card. I lived through it.


Well, now you're scaring me. At the same time, I don't see how my power brick is any different than what FSP is selling. If I have time tonight, and I can figure out how to connect the unit without buying a pcie power cable extension, I'll try it.

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 Post subject:
PostPosted: Thu Aug 21, 2008 8:38 pm 
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So, I had to dig around the european FSP site, but I found a really poorly scanned copy of the FSP X3 Booster manual. I think the 4-pin molex is for power on-sensing. I wonder how I could trip the Dell brick on easily. Anyone have any suggestions? There's some info on the brick here: http://www.silentpcreview.com/forums/viewtopic.php?t=38787&highlight=nsk1300.
Could I just splice it to the green "on" wire? it gets shorted to ground when the power-on switch is pressed, and all the dell needs is for one of the pins to be shorted to ground.

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 Post subject:
PostPosted: Sun Aug 24, 2008 2:25 pm 
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So, I did some testing with my multimeter. It seems the standby line on the dell brick puts out ~7V DC when not grounded. This is the line that needs to be grounded to turn the brick on. The green pwr-on line on my power supply only outputs ~2V DC when in standby. Could I still splice these lines together, or would this difference in potentials cause some issues? Is 2V DC a standard for all ATX units?

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 Post subject:
PostPosted: Sun Aug 31, 2008 1:16 pm 
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well, I did some testing, it wasnt much since the system didn't turn on, but anyway.. I plugged the main system into the Dell DA-2, and plugged the PCIE connector from my FSP Zen into the video card. When they were both in standby, I saw no ill-effects, nothing smoked/popped. I hit the power switch and of course, the DA-2 shuts off! Just like it always has with my previous Abit boards. So, it seems I'm going to need to get 2 things, a new power supply, and a PCIE extension cable so I can plug the DA-2 into the video cards PCIE connector.

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 Post subject:
PostPosted: Fri Sep 05, 2008 11:10 am 
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So, I did further testing. I've recently made some changes to my PC, swapped the X2 4000+ for a Q9400, power draw hasn't increased much, only ~15W under load with the CPU at stock speed.

I decided to try the external power supply idea once again. I fashioned a plug to go from my Dell power brick to the 6-pin PCIE power plug on the video card. Both the main system PSU and the external brick are plugged into the same outlet. The system works fine, and even under load. The external brick reduces the main system draw by ~20-30W, which makes sense, considering the HD3870 isn't a high powered card.

My next test is to swap the PW-200 for a PicoPSU. I know, the Pico is only rated at 120W, but in an email from Short-Circuit, and from a phone call with the makers of the unit, the PicoPSU is a 200W design with thermal limitations. The 20-24 pin adapter in my case basically acts as a riser, putting the PW-200 directly in front of the CPU fan. Since the Q9400 is a very low heat producer, the air isn't very warm. The PicoPSU would be getting a large amount of airflow.

Full load power draw at the wall is 155-160W(GPU+CPU), though that's ~20-30W less than it would be if the entire system was being powered by one PSU. At 80% overall PSU efficiency, that's ~130W DC required from the main system, which is above the Pico's rating, but not it's true capacity. The only reason I want to try this, is because the PW-200 will not work with the power brick, requiring me to leave the FSP Zen on my desk, which is a bit of an eye-sore. The Pico does work with the brick, so it would free up a lot of desk space.

I'll update my system post with this info.

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