question about power supply requirements for gpu's

rob martin · Apr 11, 2013

I am thinking of buying the low profile version of the NVidia GeForce 640 GT by Zotac. It says that it requires a minimum 350W power supply. The card I am using right now, is the Sapphire Radeon HD 5450, the 1GB version, and it requires a 400W power supply. Here is the fun part, currently my desktop, (if relevant it is a desktop and not a tower computer) which is using the Radeon card with no issues what so ever, has a DVD burner, TV tuner card, 8 GB of memory, nothing on it is OC; has a 245W power supply! I think that because the Zotac card requires a smaller power supply, it will work with my computer. Am I right, or is there some vital bit of information, that I didn't include, or haven't taken into account that would let the Radeon card work, but not the GeForce card. This is the only card I am interested in, so please don't recommend a different card. As for the power supply, it has a funky design to it, so I don't think I could just take it out and put in a better one. Also, with the way the slots and I/O portion of the motherboard is placed, (when you look at the back, it is the slots and then the on board components. Most cases I have seen have it the other way around) a case transfer is out of the question. If it helps at all here are the specs I know about it:

socket 775 LGA motherboard
8 GB of DDR PC2-6400 memory
149 GB SATA II hard drive
Intel core 2 quad Q9400 2.66 GHZ CPU
DVD rewritable burner with lightscribe technology.

Not sure about the amps on the 12V rail for the psu, but I can find out if really important.

prototype18 · Apr 11, 2013

The 5450 consumes ~15W on peak .While the GT 630 ~45W.

And yes find the Amp on the 12V rail so we can give a secure OK

Onus · Apr 11, 2013

Graphics card manufacturers overstate PSU wattage requirements in the (often vain) attempt to account for all the liar-labeled junk out there (Logisys, Diablotek, Apevia, etc).
In any case, the GT640 requires more power than the HD5450. Based on my assumptions about fans, and allowing for 10% capacitor aging, the PSU sizing calculator at http://www.extreme.outervision.com/PSUEngine suggests that a 280W PSU will be needed for your system with the GT640 (253W was suggested for the HD5450). You should probably upgrade your PSU before replacing your graphics card.

RetiredChief · Apr 11, 2013

The first thing you have to realize is the the recommendations for a PSU (By GPU Manuf) are based on NOT knowing if you have a quality or crappy PSU. They tend to base the recommended value on the fact hat many do NOT have a quality PSU.
The "grappy" PSUs only provide about 60 % of what they advertize!!!!

Your System, with 640 GT will be around 160 to 200 Watts (the 200 watts is with the GPU working at FULL throttle). Which may be slightly higher than your 5450. The TDP for 5450 is <20W while the 640 is around 75 W TDP.
NOTE @ Idle and when Not tasking the GPU heavly, probably arround 125 Watts

Flip a coin, heads probably will work, Tails May work, On edge - will fry your system instanly.
Really A lot depends on the QUALITY of your PSU and the +12V power
- The two power hungry components on a computer are the GPU and the CPU as they both use the +12V.

To apply the below Reference, LOOK at their complete system vs yours, mostly you will find your CPU is less than theirs, therefore your power might be less.
Ref: http://www.anandtech.com/show/5969/zotac-geforce-gt-640-review-
http://www.guru3d.com/articles_pages/radeon_hd_5450_review,6.html

rob martin · Apr 11, 2013

Onus :

I don't know if I can, the power supply is basically on a hinge, meaning it isn't screwed in. Also its shaped in such a way that when you rotate the psu up, there is a 5-1/4 bay that you can put a hard drive in. It really is a funny design.

rob martin · Apr 11, 2013

prototype18 :

it said 7.5 if I read it right

RetiredChief · Apr 11, 2013

7.5 A x 12 V is ONLY 90 Watts
That 20 -> 30 watt increase MAY put you 9in the danger mark.

Onus · Apr 11, 2013

90W on +12V would not even be enough to run your CPU at high loads. That can't be right.

prototype18 · Apr 11, 2013

Well take a pic of the PSU and the Case. Maybe we can help. And no 90W on 12 V its a big NO

rob martin · Apr 12, 2013

prototype18 :

I couldn't get my camera back from my sister, but I wrote down the exact sticker. I checked it twice, so it should be correct. Here it is:

240W max
model DPS-240MB-3A
part number 46074-001
input 100-240V 15A 50-60Hz
+3.3v 15A
+5.08V 17A
+12v 7.5A
-12v .15A
+5.08vAUX 3A
+12VCPU 11.5A

That says to me, that this power supply has two 12V rails, the first one, the 12V @7.5A, is for the graphics Card, the second one, 12VCPU@11.5A, is for the CPU! According to the site:

http://www.geforce.com/hardware/desktop-gpus/geforce-gt640/specifications

The 640 has a Maximum Graphics Card Power of 65W. So put all this information together, I think the 640 will work in my computer!
12V*7.5A=90W for the graphics card
12V*11.5A=138W for the CPU

Is my logic flawed? I do realize that there is a chance that this power supply isn't giving the proper info, but I have had this system for 6 months, I only turn it off if I 'm not home for more than three days, and it hasn't given me any trouble.

I think I will try it, and if it doesn't work, than no big deal. I can give the card to my brother or nephew.

Onus · Apr 12, 2013

Going by that label, that PSU should indeed be able to power a GT640. Of course, as you realize, the label could be an exaggeration, but I don't think that's as prevalent on low-wattage units as on much bigger ones. If that is indeed a HP power supply, it is probably good enough.

rob martin · Apr 12, 2013

Onus :

Thank you for your quick reply. I am going to buy the card, along with the 3D vision glasses at the end of the month, and hopefully this time next month, I will be watching 3D movies and playing Rusty Hearts in 3D with my NVidia 3D Vision ready monitor.

RetiredChief · Apr 12, 2013

Just a little on PSU's
For your 245 W PSU
+3.3 V @ 15 A = 49 W
+5 V1 @ 17 A = 85 W
+5 V2 @ 3 A = 15 W
+12 V1 @ 7.5 A = 90 W
+12 V2 @ 11.5 A = 138 W
--------------------------------
Total = 379 Watts - 135W >245 listed as Max
Bottom Line is that it can NOT provide the listed wattage if all were at the rated current.
Your 12V1 Plus 12V2 is 90 + 138 Watts which is equal to 228 Watts. This means that if the power for the +3.3V and +5 V were only 20 Watts, you could NOT provide 238 Watts to the +12V

NOTE 1: forget the -12V
NOTE 2: This is only for Info as your sytem should be less than 200 W.
If PSU is a quality PSU - should work. The unknown is quality. HP probably did NOT make that PSU, it was most likely a contract based on their specs. My best quess is it is not a "crappy" PSU, nor a "quality" PSU, but probably a middle of the road PSU.

prototype18 · Apr 12, 2013

If you are upgradin the budget i say better get the 6670

rob martin · Apr 12, 2013

prototype18 :

I could, but the site I looked at said the 640 I was looking to buy, is 17% better, and besides that according to the research I have done, my monitor requires NVidia's flavor of 3D. Besides that, is the 6670 low profile?

prototype18 · Apr 12, 2013

There are some with lowe profile. But if you want 3D get 640

Sasa_4 · Dec 30, 2016

Hi! I know its an old thread, but I got the same PSU, its maybe even the same pc, I am looking to revive it just for fun, I have got a radeon hd 6450 gpu and was wondering if this psu will be enough? How was your experience in the end?

Onus · Dec 30, 2016

Closing necrotic thread.

question about power supply requirements for gpu's

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