using external power supply for graphics card

[theorw]

I did this by connecting the VX450 to a separate MB without a CPU. I would first power up my MB powered by the VX450W feeding the HD5770s.
Then I would immediatly power up my CPU MB powered by the TX650W powering CPU and other related hardware listed above.

Now why whould u do taht when its known that u shortsircuit the green mobo pin of the psu in need with any ground and then u power it on and of with the power switch!
I had done this and i had also connected the main and the secondary PSUs green cables and the opened at the same time!

 

[Sir_Real]

First time i've notice this thread but i've been running 2 psu's on this pc for months. I just did the wire mod ( green to black ) to have a second atx psu powering my hd4850.
I was running the HD4850 from a 12volt / 5 Amp Lcd monitor mains adapter for a while worked ok but the brick mains adapter was getting to hot when gaming. Hense it got swopped for a atx psu.



.

 

[Geofrancis]

Never mind all this PSU talk, your morex case is WAY TOO SMALL to handle the thermal exit requirements for a 4850, esp. with that RAID rack.

You have a fabulous rig there... dont spoil it trying to get a hot number in a tight space

i have been thinking of the heat from it as it is going to be a tight fit. but i have reversed the fan on the raid rack so that its blowing air out of the front of the case so none of the hard drive heat is drawn in. and if the 4850 does run too hot in the case i dont mind taking a dremel to it and cutting some vents in the right hand side of the case and adding an extra fan if need be.....come to think of it i have got a watercooling setup sitting in a drawer but i cant see anywhere to fit the pump and radiator.

First time i've notice this thread but i've been running 2 psu's on this pc for months. I just did the wire mod ( green to black ) to have a second atx psu powering my hd4850.
I was running the HD4850 from a 12volt / 5 Amp Lcd monitor mains adapter for a while worked ok but the brick mains adapter was getting to hot when gaming. Hense it got swopped for a atx psu.
.

the 12v mains adapter i am planning on using does 10A so if you ran a 4850 on a 5a psu then it should be ok i think.


this is going to happen i dont care how much cutting, soldering, modifying i will get the 4850 in this computer lol

 

[Lazzer408]

 

[Geofrancis]

ok well i have ordered this 12v 40a relay
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=270464779173&ssPageName=STRK:MEWNX:IT
this 2.5mm dc power socket for the back of my case
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=390098309116&ssPageName=STRK:MEWNX:IT
and this pci-e ribbon riser so i can move the 4850 a slot over
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350204890259&ssPageName=STRK:MEWNX:IT


so i think thats all i will need plus the 120w psu i already got.

 

[Lazzer408]

With a relay that big you might want to go with a diode. 1N4001 should do fine.

 

[Geofrancis]

With a relay that big you might want to go with a diode. 1N4001 should do fine.

what exactly is the diode for. i know what they do but why do you need one across the relay?

 

[pantherx12]

Hey man maybe just buy a 5.25 psu?

there's 2 for sale in this thread

http://forums.techpowerup.com/showthread.php?t=105973

 

[Geofrancis]

have you seen the photos of the case? there is barely space for a 3.5" hard drive never mind a 5.25" psu. plus i would then also have to deal with the heat of the hd4850 and a 300w psu dumping heat into the case.


the relay/pci-e cable/power socket are already ordered and i already have a 120w psu.

i might invest in a couple of 12v car ammeters and instal them on the case somewhere so i can see the power consumption in real time.

 

[pantherx12]

Well I thought seeing as you were having it external anyway, and the PSUs I mentioned are cheap ha ha

 

[Geofrancis]

Well I thought seeing as you were having it external anyway, and the PSUs I mentioned are cheap ha ha

those vga power supplies are just a polished version of what im building. when they get power from a molex plug on the main psu they come on and when it turns off they go off. i am going to do the same thing with a relay that is just a electromagnetic switch that turns on when it gets power form the molex it pushes a switch that connects the slave 120w power supply to the graphics card.

how much of a dely can there be between the main psu coming on and the slave power supply kicking in? i imagine for these external psu's to work there cant be a delay of more than a few milliseconds of the main psu coming on.

 

[mikek75]

Its a bit more involved than that with my Thermaltake, it has a 24pin passthrough connector. As far as I've been able to tell it "backs up" the main PSU (or else why the need for the 24pin)? There is certainly no delay in start up, it just works.

 

[Geofrancis]

Its a bit more involved than that with my Thermaltake, it has a 24pin passthrough connector. As far as I've been able to tell it "backs up" the main PSU (or else why the need for the 24pin)? There is certainly no delay in start up, it just works.

how many wires are there connecting the thermaltake psu to the main power supply? i have seen people connecting the psu-on and a ground pin from the atx plug on the slave power supply into the motherboard along with the main power supply so that they both get the on signal togther and power off together.

 

[mikek75]

Oh dear, I've made a boo boo. Its been a while since I looked at it. The 24 Pin connection is basically a riser, with a cable from the unit with 1 green and 2 black wires going into it. Apologies for the mis-information.

 

[Lazzer408]

what exactly is the diode for. i know what they do but why do you need one across the relay?

When you remove power from a coil the magnetic field collapses and induces a voltage into the coil. It's called flyback or back/reverse emf. The voltage can be 100s or 1000s times more then the voltage applied to the coil and can have damaging effects if left uncontrolled. Adding a diode gives the voltage a path back into the coil to prevent voltage spikes.

 

[Geofrancis]

When you remove power from a coil the magnetic field collapses and induces a voltage into the coil. It's called flyback or back/reverse emf. The voltage can be 100s or 1000s times more then the voltage applied to the coil and can have damaging effects if left uncontrolled. Adding a diode gives the voltage a path back into the coil to prevent voltage spikes.

ok thank you i have ordered 10 in4001 diodes from ebay. i got the 10 for £1 inc delivery
http://cgi.ebay.co.uk/10-x-1N4001-1...s_ET?hash=item439a5fed25&_trksid=p3286.c0.m14


so now i have the 120w 12v dc power supply
the dc power socket so i can plug the powersupply into the back of the case
12v relay to power on the 120w power supply
1N4001 diode for the signal side of the relay

now to connect the graphics card pci-e plug and cable to the relay i was just going to run the pci-e cable into a heavy duty terminal block and connect it to some heavy duty silicone wire i have from some rc projects into the relay then to the power connection

like this

 

[angelkiller]

your math for the 120W is overrated...you are doing your math based on a 100% efficient PSU. More realistically you are getting 6-8 amps durring opperation. I would still be concerned that it isnt enough power, then you risk burning the spare PSU and possibly the card itself.

Hm. I think I gotta call you out on this one () The output is measured in volts DC. Just like any PSU. This adapter turns 120v AC into 12v DC. That is where the inefficiency lies. The power brick is rated at 120W @ 12v. That means it can deliver 120W @ 12v. The inefficiency lies in the AC-->DC conversion, so when the something is drawing 120W DC, the AC adapter is drawing ~134W from the wall (assuming ~90% efficiency) because the AC-->DC conversion is not 100% efficient.

120W @ 12v means 10A. There's no other way to give 120W @ 12v without giving 10A. And one assumption I've made during this post is that the brick can give its full power. Which is a quite safe assumption, as that's what the rating means. That brick can give 10A at 12v maximum.

As for the situation, I say go for it! I think it's totally doable. Under Furmark the 4850 draws ~150W. That means that the PCI-E slot has to give 30W minimum. But in that situation would be out of spec because PCI-E says 75W max per 6pin connector. So to come within spec, the PCI-E slot will have to give its full 75W. That creates a question. What determines where the power comes from? How do we know that the card won't draw 150W from its 6pin connector? Because under Furmark, the card must max out both its PCI-E connection and its 6pin connector. What's regulating that? Why not just draw 60W from the slot and 90 from the 6pin? IMO, the best option would be to draw ~100W from the 6pin and 50W from the slot.

But also realize that that's furmark. If you deal with a realistic load, the 4850 only peaks at 85W, which is much easier to power. Worst case here is that the slot gives a full 75W, which leaves only 125W for the rest of the system. But that may/may not be an issue depending on the rest of the system. And for every watt under 75 that the PCI-E slot doesn't give will give that much more power dedicated to the system. You could in theory run the 4850 completely on the 12v adapter, assuming you don't run Furmark.

The way I see it, the worst case scenarios are all manageable and safe.

 

[Lazzer408]

Geofrancis: Your drawing is correct. You could have salvaged any rectifier diode from unwanted electronics. It didn't specifically need to be the 4001s. Their just very common and cheap.

For what it's worth, this runs off a cheap-o 300w m-atx psu with a single 16a +12v rail.

Zotac GE9300 m-itx motherboard
Intel e6750 2.66ghz overclocked @3.2ghz (1600fsb)
G-skill DDR2 4gb (2x2gb) 4-4-4-12@800mhz
WD 750gb RE3 7200rpm 3.5" hdd
ATI HD3870 512mb

I also tried a 12v 6a adaptor to run the 3870 and it worked fine. The card shown in the pic is a 1950.

 

[Geofrancis]

well i have ordered the diodes anyway they were only £1 and i have to wait for the pci-e riser from china first before i do anything.

i will just glue the relay to the side of the internal power supply and run the wires from the dc plug that will be mounted on the back of the case to the relay and the internal psu 12v for the signal.

 

[Geofrancis]

i have took a couple of photos of my computer with a different hd4850 that i have at the moment i am swapping it for a friends single slot 4850 cos this one doesnt fit.

it dwarfs the case and the reference single slot version is longer.

and a shot of the back

the black cable sticking out the back of the case is the esata-sata cable. the reason i got the j&w board was it was the only itx board i could find that had the capability of running a quadcore, 5 sata hard drives and a graphics card

 

[Lazzer408]

isnt that a microatx psu in there?

 

[Geofrancis]

isnt that a microatx psu in there?

no its much flatter than a micro atx psu and the power cable is in the wrong place.

 

[Lazzer408]

looks easier to mod a matx psu.

 

[Geofrancis]

i could have modded a micro atx psu as you can get 500w ones now but to get it in to the case i would have had to either open the power supplies up and try and swap the guts of them around or trying to get the micro atx psu to fit into the case but then the power cable would be in the wrong place etc with the risk that if something fucked up could cause serious damage.

 

[Lazzer408]

I did a few tests on my HD3870 to see what it was drawing off the 12v connector.

DC amps / activity

1.90 / Post and booting.
0.25 / Windows desktop.
2.6-3.2 / 3dMark06 with the highest current draw being Firefly Forrest.

I read this card is 80w. I also read the standard for 6-pin is 75w and 8-pin is 150w. Divided by 12v = 6.25a and 12.5a respectively.