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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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Old 12-12-2012
misha256 misha256 is offline
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Question Am I asking for trouble? A tiny PSU for my new PC

Hi there everyone,

I am about to assemble a small PC and, having read lots of reviews/blogs/forums about PSUs, I am not sure if the PSU I have is sufficient. Any advice would be much appreciated.

Here are the PC components, to be installed into a microATX desktop case that only accepts a TFX (slim) power supply.

SeaSonic 250W PSU (SS-250TFX Active PFC 80 PLUS Bronze)
Intel DB75EN Motherboard
Intel i5-3570 3.4 GHz CPU (Ivy Bridge)
16GB RAM (4 x 4GB DDR3-1600 1.65V)
240GB SSD Hard Drive (Intel SSD 520)
On-board Video 1 x DVI, 1 x VGA (2 monitors)
On-board Audio, LAN, and eSATA for externally powered normal HDD
Samsung SATA DVD Writer
No PCI/PCIe cards, no overcloking

I found the "eXtreme Power Supply Calculator Pro" website, here's what the calculator says I need vs what the SeaSonic PSU says it can deliver (the calculator was set to 100% CPU utilization TDP, 100% system utilization, 10% capacitor aging):

+3.3V 5.0A required (PSU can do 13.0A)
+5.0V 11.6A required (PSU can do 16.0A)
+12.0V 14.6A required (PSU can do 20.0A)

The PSU is limited to 80W total on +3.3V and +5.0V combined. Based on above numbers, 75W is required so that's just under the 80W available.

But 14.6A @ 12V = 175W, and 175W + 75W = 250W which is bang on the PSUs maximum.

OK so perhaps the above numbers are for some kind of super "peak" usage that never happens in real life. But then I'm not sure whether I should trust the eXtreme Power Supply Calculator I hope it's not underestimating?

The thing is Intel's own specifications for the motherboard call for 20A on both +3.3V and +5.0A (hah, that's way more than this PSU can do). Then a friend of mine suggests that you should only use a PSU to around 50% of its capacity just to be on the safe side. And now I read that there are many PSUs that can't even deliver the power they claim to be capable of. Also seems that when a PSU gets hot it can't deliver as much power as when it's cold potentially an issue in a small case like mine with an i5 CPU/16GB RAM generating lots of heat?

Anyway, sorry for the long-winded question, and thanks in advance for any advice.

Michal
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Old 12-12-2012
rafal_iB_PL rafal_iB_PL is offline
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You should be fine, Ivy Bridge i5's are pretty power efficient. I doubt you'll be hitting even 100W at the wall with such setup.
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Old 12-12-2012
allikat allikat is offline
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I agree. The main things that draw a lot of power are things you haven't fitted (like big graphics cards) and won't be doing (like overclocking). That 250w unit is plenty.
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Old 12-12-2012
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Way more than enough.
That thing might eat 120w on a (really) bad day.

I did some testing with a stock 3570k+SSD. Full load on both iGPU and CPU pulled 88w from the wall. This was with a PSU advertised as 92%. (Antec ITX-VESA case's internal / power brick)
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Old 12-12-2012
misha256 misha256 is offline
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Hah, what was I worried about?! Thanks for your help guys, puts my mind at ease. I shall happily go ahead and build this thing now :-)

BTW awesome forum, and great PSU reviews learning a lot!

M
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Old 12-13-2012
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Be sure to report back.
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Old 12-30-2012
misha256 misha256 is offline
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OK here's the report back. So the new PC has been up and running for the last two weeks. Tortured it with Memtest86+ and Prime95 (24 hours each), no problems. PC is running rock solid even in heavy use (concurrent virtual machines, video rendering, Visual Studio dev, etc.)

I don't have the means by which to measure the system's power draw but I have observed that the PSU fan stays at low RPMs and is cool to the touch even during the heaviest use.
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Old 12-30-2012
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Correctomundo!
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Old 12-30-2012
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Quote:
Originally Posted by misha256 View Post
I don't have the means by which to measure the system's power draw
Keep an eye out for hot deals on a Kill-A-Watt for measuring power draw. Basic ones have been available as low as around $16 shipped, though usually around $20. Well worth the money!
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Old 12-30-2012
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Quote:
Originally Posted by Zap View Post
Keep an eye out for hot deals on a Kill-A-Watt for measuring power draw. Basic ones have been available as low as around $16 shipped, though usually around $20. Well worth the money!
Amen to that.
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