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Trying to understand my PSU requirements

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  • Graphics Cards
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April 29, 2011 7:02:02 PM

Hey all,

I was unsure whether I should post this question under video cards or PSUs, but I guess it's really more of a power supply question.

I'm still kind of a novice builder and, admittedly, the differences between PSUs and, indeed, the whole concept of power requirements has been rather foreign to me. For the past four years or so, I've been running a 550w Rosewill PSU (Newegg link). As far as I could tell, it's been doing just fine. I run a reasonably modest system, only dabbled in overclocking a little, 3.0 Intel Core 2 Duo, 4 gigs memory, and a Geforce 9600 GT. I have a handful of case fans and such, but nothing out of the ordinary. Never seen a BSOD or anything with this one.

But, recently, I tried to do a video card upgrade with rather... catastrophic results. I got myself a Radeon HD 6850 and, within five minutes of installation, my entire system instantly powered down and refused to come back to life until I removed the card. I assumed it was a problem with the card since, after that, no matter what, the system wouldn't even attempt to power up with the thing in (I can't imagine why it would power up the first time if it wasn't a problem with the card directly). So, I returned the card and (after seeing how much it would cost me to ship it a second time) I decided to get my money back and go a different route.

But, it got me thinking... Maybe it did have something to do with the PSU? I'd certainly like to avoid the same situation with another card. So, I did a little sleuthing. My first confusion came in the form of amp requirements. I found this handy-dandy list: here, and realized that my old 9600 GT card had a minimum requirement of 26A on the +12V rail. My PSU has two +12V rails listed @ 18A each. For about an hour of internet study, I had no idea what this meant, but I gather now that it basically means that I can handle a minimum requirement up to 36A (even if that's not a particularly good idea), which is why it had no problem with the 9600 GT. Interestingly, the same list suggests that the HD 6850 only requires 25A and the same wattage. That seems to support the theory that I just had a faulty card.

So, while my PSU may be a little on the whimpy/old side, all the data I've seen so far seems to suggest that I'd be fine with most mid-range cards, as long as I keep clock speeds normal and don't try any SLI/Crossfire tricks. But, I'm still skeptical. I'm thinking of going back towards nvidia cards after that experience, so I have my eyes set on a GTX 460. From that earlier list, the minimum power requirements are supposedly exactly the same as my 9600 GT. However, I did some studying on some charts here at Tom's and it looks like the GTX 460 can demand a full 70 more watts (when going full-steam) compared to the old card. Hence, my hesitation.

So, any opinions? Does it sound like I'll be okay? Is there a factor here that I'm not looking at? Am I just being paranoid? I'm sure there are plenty of other reasons I should go ahead and upgrade the PSU, but for this situation, I just want to know if I can pull it off with what I have before I start throwing money every which way.

Thanks in advance!

Edit: Just noticed this right after posting, and feel a little silly :p  Still, any opinions on this particular situation would be appreciated, particularly if I'm wrong about 2 +12V rails being added together. From the looks of it, the card I have my eye on (the 460) and the card that failed (the 6850) both have a minimum recommendation of 36 amp on this list. I have the feeling I'm playing with fire at that kind of requirement... Does anyone know what the amp recommendations for the 9600 GT were?

More about : understand psu requirements

a c 104 ) Power supply
a b U Graphics card
April 29, 2011 7:07:23 PM

http://www.thermaltake.outervision.com/

'build' your system into this, it will let you know how much power your rig needs roughly, bear in mind the age of comonents can massively affect their ability,

did you update/clean drivers for the card/s?
Moto
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April 29, 2011 7:08:18 PM

Have you checked out a site like this one? http://www.extreme.outervision.com/psucalculator.jsp

You can add all your components and it'll give you the estimated power supply size you need to run it all. I would think your 550w would have been enough to handle one 6850. Did you check the support section for the card to see if the card is faulty?
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April 29, 2011 7:21:51 PM

Thank you both for the quick replies!

I punched my data into those calculators (they seemed to be the same calculator, actually, heh) and it looks like with my current system, I'm using well under 300W. The 6850 kicked it up to 309W and the GTX kicked it to about 340W... Not what I would call the "danger zone", I would think. Of course, this doesn't tackle that whole amperage thing, which I guess is the stuff I don't quite get yet.

To Motopsychojdn: I did update my drivers for that failed card right before it failed. I would have tried to roll them back if I could have, but the system was literally dead with the card in there; couldn't even start up a fan (though the mobo still clearly had power, even reaching USB devices)

To Poisonpanik: I did a lot of homework on just what happened with that darn card, but nowhere did I hear any situations quite like mine. Blue screens, sure, maybe some problems with clock speeds, but no one reported the whole system shorting out. That's why I started questioning the PSU.

Another edit (gee, the internet has a lot of information...): So, somewhere in that power guide I found on Tom's it suggested taking the required system wattage and dividing it by 12 to find the amperage. My result on that calculator gave me a requirement of 337W which roughly equals 28A. Cool? Cool. Now the big question: +12V1@18A, +12V2@18A... Does this equal greater than 28A, at least in theory?
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a b U Graphics card
April 29, 2011 7:32:25 PM

You can't just add 12V rails. Tables shows maximum amperage for each rail, not the total combined (some do, but that a separate number for total amperage combined). On the other hand, many tables shows total wattage for 12V rails. You can calculate combined amperage from that by dividing 12V rail total wattage (not the total PSU wattage) by by Volts (12 that is for 12V rails).
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April 29, 2011 7:41:59 PM

yyk71200 said:
You can't just add 12V rails. Tables shows maximum amperage for each rail, not the total combined (some do, but that a separate number for total amperage combined). On the other hand, many tables shows total wattage for 12V rails. You can calculate combined amperage from that by dividing 12V rail total wattage (not the total PSU wattage) by by Volts (12 that is for 12V rails).


Ah-ha! Thank you for that! I took another look at the sticker on the side of the PSU and, sure enough, (now that I know what the heck I'm looking at) "Combined power, 12V = 420W (35A)". It even did the division for me :p 

My conclusion, there's a lot to worry about when dropping a couple hundred bucks on a shiny new computer part, but for now, at least, my power isn't one of them.

Thank you to all the answers! I'm glad I asked here! Learned a lot in a really short time! :D 
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April 29, 2011 7:42:42 PM

Best answer selected by EvanBGood.
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a b U Graphics card
April 29, 2011 8:01:00 PM

Actually, your power may be one of the problems. I'll explain why. Rosewill is not exactly known for quality PSUs. I heard that they become better lately, but can't confirm that. The problem with low quality PSUs that they may not deliver the advertised wattage. Even when they do, output may still may be dirty. That is, they may drop voltages too much, produce unstable direct current. For quality PSUs you may look for Corsair, Seasonic, XFX, Antec (I run Earthwatts,500W D Green, pretty nice). Many manufacturers outsource PSUs, so you'll have to look where they came from. Some of brands are not that well known, but they still may come from a decent or good source, so they may be pretty good too. At the moment, I don't have sources on top of my head, so I'll have to look those up. Three brands I mentioned tend to be overall good.
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April 29, 2011 8:14:08 PM

yyk71200 said:
Actually, your power may be one of the problems. I'll explain why. Rosewill is not exactly known for quality PSUs. I heard that they become better lately, but can't confirm that. The problem with low quality PSUs that they may not deliver the advertised wattage. Even when they do, output may still may be dirty. That is, they may drop voltages too much, produce unstable direct current. For quality PSUs you may look for Corsair, Seasonic, XFX, Antec (I run Earthwatts,500W D Green, pretty nice). Many manufacturers outsource PSUs, so you'll have to look where they came from. Some of brands are not that well known, but they still may come from a decent or good source, so they may be pretty good too. At the moment, I don't have sources on top of my head, so I'll have to look those up. Three brands I mentioned tend to be overall good.


Of course, I jumped the gun a little there. I know there's always that "x factor" of brand quality that comes into play with these kinds of things. Right now, I'm just trying to get a grasp of the basics of what "should" work, so I can tell the difference between a faulty part and me trying to do something that isn't possible.
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