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gfx&PSU: How much (A) is needed on 12V pin?

Tags:
  • Be Quiet!
  • Pure
  • Power
  • HD
  • Graphics
Last response: in Graphics & Displays
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December 29, 2013 12:00:56 PM

I currently use a 530W PSU (be quiet! pure power) for my single HD 7790 and its 12V connector runs at 22(A).

I stumbeld upon some threads that said 22(A) is very low for modern gfx-cards, then again I can't seem to find out how much (A) HD 7790 requires on the 12V connector.

Can someone give me a rough brakedown how much (A) modern cards need and how to find out, because I never saw (A) of 12V connector in any gfx-cards requirements.

More about : gfx psu needed 12v pin

December 29, 2013 12:23:54 PM

I think your 7790 uses 23A on the 12V rail, could be wrong as it's been a while where I read that...

I do not know of a place to find this info either. But if you know the Wattage of the card, divide that by 12 should give you a good idea of the Amperage needed. W=V*A.......
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December 29, 2013 1:14:10 PM

endeavour37a said:
I think your 7790 uses 23A on the 12V rail, could be wrong as it's been a while where I read that...

I do not know of a place to find this info either. But if you know the Wattage of the card, divide that by 12 should give you a good idea of the Amperage needed. W=V*A.......


thx for your quick help

According to this site http://www.tomshardware.com/reviews/radeon-hd-7790-bona... HD7790 utilises 85W. Even with 100W it would only consume 8 (A) for 100/12=8.3.

This seems extremely low and a little bit strange. Have I done everything correct?
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December 29, 2013 1:55:58 PM

Yes it does seem low, perhaps my methodology is wrong? But one thing for sure is wattage=voltage * amperage.
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December 29, 2013 3:33:00 PM

endeavour37a said:
Yes it does seem low, perhaps my methodology is wrong? But one thing for sure is wattage=voltage * amperage.


As long as the number under the category TDP stands for the maximum Watt this card draws, your methodology seems correct.

If there is some mistake we could solve it, if you know the (A) needs of a specific card. Then we could just find out the TDP of this card and check if we could use this TDP in your calculation.

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December 29, 2013 3:41:13 PM

Yeah that's correct
But once you factor in the other components, it adds up.

22 Amps is REALLY low for a "530w" PSU though
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Best solution

December 29, 2013 3:53:53 PM

this is so utterly simple, and yet so many as so misinformed

quick-n-dirty: 1 amp per 10 watts of power (either cpu or gpu) when talking about 12V lines

(technically, 0.8A per 10W but the 1A is easier, faster, and inherently includes a buffer)

that's it.

cpu is 85W to 125W, which means 8A to 12A
gpu like 7790 is 85W, which means 8A

basically, 20A in total for a cpu+gpu combination such as yours. add some more for hard drives, fans, etc, and 25A might be a reasonable starting point.

now, SOME power supplies make a 12v rail, and then get the 5v and 3.3v from that, in which case the 12v power will be nearly the entire supply rated power.

but SOME OTHER supplies might make each rail separately, in which case you'll see the 12v power be a fair bit less than the entire supply rated power. to me, this seems like a silly way of doing things since it's extra circuitry for rarely used voltages.

anyhoo.....

1A per 10W when talking about your common 12V system. cpu and gpu are the main users.
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December 29, 2013 4:29:29 PM

giantbucket said:
this is so utterly simple, and yet so many as so misinformed

quick-n-dirty: 1 amp per 10 watts of power (either cpu or gpu) when talking about 12V lines

(technically, 0.8A per 10W but the 1A is easier, faster, and inherently includes a buffer)

that's it.

cpu is 85W to 125W, which means 8A to 12A
gpu like 7790 is 85W, which means 8A

basically, 20A in total for a cpu+gpu combination such as yours. add some more for hard drives, fans, etc, and 25A might be a reasonable starting point.

now, SOME power supplies make a 12v rail, and then get the 5v and 3.3v from that, in which case the 12v power will be nearly the entire supply rated power.

but SOME OTHER supplies might make each rail separately, in which case you'll see the 12v power be a fair bit less than the entire supply rated power. to me, this seems like a silly way of doing things since it's extra circuitry for rarely used voltages.

anyhoo.....

1A per 10W when talking about your common 12V system. cpu and gpu are the main users.


thx for this detailed answer

It seems like my PSU uses four seperated 12V rails each at 18 (A) (I was mixing up my PSU with a very similar model until now) and has a seperated (A) value for 3.3V and 5V rails. So it seems like those individual 12V rails aren't shared by CPU & GPU, which means they both get 18 (A) each, which should be more than enough. right?
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December 29, 2013 4:40:33 PM

typically if a PSU has 2 or more separate 12V rails, one rail is dedicated to the ATX/mobo connector, one rail is for the PCIe power connector & various molex/sata power connectors, and if there's more rails another PCIe power connector.

in your case, the 7790 is not a very power-hungry card so most power supplies should handle it without issue, whether single 12v rail or multiple rails.
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December 29, 2013 4:41:38 PM

endeavour37a said:
I think your 7790 uses 23A on the 12V rail, could be wrong as it's been a while where I read that...

I do not know of a place to find this info either. But if you know the Wattage of the card, divide that by 12 should give you a good idea of the Amperage needed. W=V*A.......


A 7790 should use about 7.08A.
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December 29, 2013 10:12:22 PM

Mousemonkey said:
endeavour37a said:
I think your 7790 uses 23A on the 12V rail, could be wrong as it's been a while where I read that...

I do not know of a place to find this info either. But if you know the Wattage of the card, divide that by 12 should give you a good idea of the Amperage needed. W=V*A.......


A 7790 should use about 7.08A.


Ya, I think that 23A number I came up with is a total recommended 12V capacity of the PS. Giantbucket sort of hit the same number in his calculations also with 20-25A, funny how that worked out.

I think we are all using the same equation of W=a*v, pretty basic electronics stuff, giantbucket also uses this to come up with his results that look right to me anyway.

The HD 7790 is rated at 85W, we all pretty much agree with that, and that would indeed work out to 7.08A as you say. But looking into this further I found that AMD only recommends 75W on the 6-pin PCIe power connecter, it must be drawing 10W from the bus itself for the supporting components.

I think the original question was how many amps were needed on the 12V connecter, with the above in mind then 6.25A would be the correct answer it seems.

But thanks for bringing this to my attention and getting me straight on this. :) 
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