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Read PSU101, tell me if I've got this right.

Last response: in Components
October 31, 2006 9:19:54 PM

Very informative forum here, btw!

OK, my current PSU is a several years old Antec True Power 480W, with a single +12V rail that can supply 22A max output, probably less on a continuous basis with other devices drawing power. This PSU has served me well up to now, but I'm soon adding a GeForce 7800GS AGP card that is going to need 20A just for itself. As I understand it now, that is perilously close to the limits of what my PSU can comfortably supply it. On the other hand, a newer PSU with say, 430W power but two +12V rails supplying 17A each for a total of 34A is more likely to supply all the juice I'll need. Even though the rails are separate, the combined amps are available to all devices hooked up to the system. Correct?


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October 31, 2006 10:05:35 PM

Usually, the combined output of multi rails are significantly less than simply the combined total.
The TT Toughpower W106 700 watt PSU has 4x 12v rails of 18A, but can only deliver 54A over the 4 rails...

Regardless, I am running an overclocked Opteron 175 at 2.64GHz, 2x Raptors plus 2x 250GB drives, an overclocked X1800XT, water cooling pump, 5 HO fans, a DVD Writer...

And using a Kill-a-Watt meter at the wall plug, I draw less than 350watts under stress at full load..
This means that with my 85% effieciency PSU, my PSU is running at a 300watt load... So I could get by with a quality 350watt PSU.

So your Antec TruePower 480watt (which is a quality unit) even taken into account aging capacitors, should still be able to sustain 325 watts of load... but if voltages vary out of spec, might consider switching then...
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October 31, 2006 11:45:43 PM

Ah ha, I made a big mistake reading the specs for the card. I thought it said the card needed 20A, but that was just the recommended amps for the 12V supply, which I'm over. Thanks for the link to the graphics card power requireds, very helpful!

I used that PSU Calc first thing, came out at around 275W with my current video card (ATI Radeon 9800 Pro) and 282W with the 7800GS. I wasn't quite sure how related to my exact amperage requirements (282W / 12V = 24A?). Anyway, thanks again for all the help, I will see how this PSU does with the new card!
a b ) Power supply
October 31, 2006 11:53:04 PM

You didn't include any 3.3V and 5V draw with you calculation.

But yeah W = A * V , but it's not all on the 12V...
November 1, 2006 9:02:33 PM

Thanks for taking time to help me understand all this!
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