Mobo manual recommends minimum of 350W PSU -- Needed?

Ken5d · Post by **Ken5d** » Wed Aug 25, 2004 8:29 pm

Do I really need to heed the recommendation in the motherboard manual? I'm still in the process of spec'ing my system components, so I haven't bought the mobo yet. But the manual is online, and here is what it says:

If you intend to use a PSU with 20-pin and 4-pin power plugs, make sure that the 20-pin power plug can provide at least 15A on +12V and that the PSU has a minimum power rating of 350W. The system may become unstable or may not boot up if the power is inadequate.

They prefer an ATX12V v2.0-compliant PSU, but I was wondering what would happen if I used a 300W PSU instead. The system is going to have only 2 HDs (Seagate 7200.7s) and a DVD writer. That's it. (It's a 915G mobo, so I don't even need a separate graphics card.) Well, and the CPU uses a little power, too!

(Most likely it will be an E0 stepping of the 540 -- a 3.2 GHz P4. North of 100W max power dissipation.)

I've got a 300W PSU in the case I just bought. Can I get into trouble trying it out (once I get the mobo)? It does claim to have 15A on +12V. I'll probably still buy a new PSU anyway (the SS Super Tornado 400W looks good), but if it won't hurt, I'd like to try this PSU first.

Thoughts?

Thanks,

Ken

silvervarg · Post by **silvervarg** » Wed Aug 25, 2004 11:24 pm

Have you read the thread How much will a 300W power supply run?

A very quick summary of what I think is most important in your case:
Your motherboard does not need a 350W PSU. This is an easy way in the manual to write that it might not be a good idea to use any crappy PSU marked 300W. PSU's marked 350W is at least a good bit better than the lousiest 300W PSU's. A good 300W PSU will work fine for you.

You have to make sure you have enought amps on the rails (does not look like a problem with your intended hardware).
You must make sure the PSU has the 4-pin-square plug if the motherboard has this connector.

Ken5d · Post by **Ken5d** » Thu Aug 26, 2004 10:29 am

Thanks for the reply, silverarg.

I had quickly looked at the thread you mentioned, but didn't read it far enough. Now I found, on page 3, the link to takaman's Power Supply Calculator, and that was interesting.

It calculated that I needed only a 248W PSU ... but 16.2A or above on the +12V line. Ouch. And, worse yet, it didn't take into account that this particular motherboard probably draws more power than the one I selected. (This mb has built-in graphics, as mentioned, and also gigabit LAN, sound, etc.)

I'll have to verify that my HDDs actually use as much power as the HDDs I selected. If I can get them down a bit, maybe it'll fall under 15A. But then again, I'm still not accounting for the mb.

I think I can at least try the PSU I've got temporarily (it does have the required 4-pin, square plug - thanks!), just to see what it's like. But I probably need to count on purchasing that SS I've got my eyes on.

silvervarg · Post by **silvervarg** » Fri Aug 27, 2004 5:49 am

Looking at that calculator I would say that they are counting quite far on the safe side, especially on the 12V line.
Your worst problem from the harddrives is that they pull a lot of power when they just start to spin. Fortunately your CPU is very unlikely to be even near high load while your drives are spinning up.
Once drives are spinning I would expect them to draw max 1A on 12V.

You can probably disregard any optical drive, since they will typically not be spinning up during HDD spinup or when CPU is on top load.

My guess is that you will draw max 200W power and probably quite a bit under 15A on the 12V rail.

Just for the record, the name is silvervarg, but you are probably the 100:th person to miss that v. Perhaps I should have a simpler name? Nah...

Ken5d · Post by **Ken5d** » Fri Aug 27, 2004 6:22 am

Hmmm. I see the 'v' very clearly; not sure how I managed to miss it when typing. Sorry about that.

Thanks, that makes me feel better about using a lower-powered PSU. Last night, I swapped the PSU in my wife's computer (which had a VERY noisy, very old Seasonic) with the 300W freebie that came in my new case. It is so much quieter, it's amazing. Unfortunately, it also must be far less efficient than the old SS, because the air it exhausts is much hotter. Great in winter, but not so comfy during the summer.

So, thank you silvervarg for your replies, but it looks like I'm not going to be using this 300W PSU in my new system, or my wife's old one. But at least I now have a much better idea of what I should be looking for in the way of new PSUs. Thanks!

Ken