Power Supply Requirements

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Power Supply Requirements

Postby socalmikey » Sun Jun 06, 2004 10:46 am

Im trying to make this a sticky so we can limit the amount of questions asked about the subject.Feel free to add to this information so we can have a close to complete guide :D

Heres a handy guide that I pulled out of a Maximum PC:

Component Power Requirement Lines Used

Older CPU 20-40 watts +3.3v
Athlon XP,64,or P4 60-90 watts +12v
Motherboard 20-30 watts +3.3v,+5v, and +12v
RAM 20 watts per 256MB +3.3v
PCI Card 5-10 watts +5v
AGP videocard 20-50 watts +5v and/or+12v
(+3.3v from motherboard)
CPU Fan 2-4 watts +12v
5,400 RPM Hard Drive 5-10 watts +5v and +12v
7,200 RPM Hard Drive 5-15 watts +5v and +12v
10,000 RPM Drive 5-20 watts +5v and +12v
Floppy drive 5 watts +5v and +12v
CD/DVD drive 10-20 watts +5v and +12v
Cold Cathode Lights 3 watts +12v
LED Lights <1 watt +12v or +5v[/list]
So figure out how much wattage youre going to eat up on each rail(line used) and how much amperage(current) is available per rail.

Once you know this its simple Ohms Law


Example: if you require 150 watts on the +5v rail, then that +5v rail needs to supply at least 30 amps(which should be indicated on the side of your power supply)if it does not then you need a larger power supply.

Keep in mind that while I say +5v rail or +12v rail all power supplies are not created equal and you should stick to name brands, the power supply is not where you want to skimp out on your system.

Good Luck
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Last edited by socalmikey on Wed Mar 09, 2011 3:49 pm, edited 1 time in total.
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Postby Simps_ » Sun Jun 06, 2004 11:56 am

Good idea, and great post. (With a little help from Maximum PC magazine though! ;) )
Here's a site that gives you a general idea of how much wattage you should be putting out:

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Postby Bill_Bright » Sun Jun 27, 2004 2:09 am

Hey Socalmikey - Need to rethink your Ohm's Law application.

Ohms's Law states E = IR

therefore, Resistance (R), not power, is calculated by dividing Volts (E) by Current (I).

Power (in watts) = Voltage TIMES Current.

(Note: E = Electromotive Force = Volts)
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Postby socalmikey » Sun Jun 27, 2004 4:03 am

good job bill youre absolutely right E= IxR, or P=IxE or many other formulas are out there.
Im an electrician, I should have caught that
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Last edited by socalmikey on Wed Mar 09, 2011 3:50 pm, edited 1 time in total.
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Postby Bill_Bright » Sun Jun 27, 2004 6:03 am

No problem - been involved with electronics for some time and it is easy to twist it up. It just caught my attention as Ohm's Law came up recently in a discussion with a friend about UPS (battery backups) ratings causing confusion.

To ramble a bit...

UPS manufacturers use the VA or "volt-amps" to indicate the power capability of their products - actually a fine DC spec for the batteries themselves. But the enduser needs to be concerned with the AC output at the power outlets, UNDER LOAD! Watts is the more appropriate unit of measure.

The UPS manufacturers pitch the VA value because it is a much bigger number. Bigger is better, right? You often have to hunt the fine print to find out its true power listed in watts. I have an 800VA rated UPS but it really provides 450Watts of just plain old (but clean, fault-tolerant, regulated) 115VAC - Plenty for my PC, LCD monitor, and network gear.

You all DO have at least your PC on an UPS, right? If not, wake up!
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Postby Twisty » Sun Jun 27, 2004 6:07 am

I have now left the Building :tongue8:
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Much better system

Postby Toasterman101 » Mon Jun 28, 2004 9:39 am

wow this is a much better system then what i was usin (add stuff until ur rig locks up or restarts) i will definatly see about this since im using only a 350 watt offbrand with 2 hard drives and a p4
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Postby kltsin » Thu Jul 08, 2004 4:30 am

I havent had time to check all the links you guys gave as of yet.

I just wanted everyone to know for right now I do have a power supply worksheet and info on power supplys on my web site.

Yes the page needs much refinement, I also am desperatly need to find the draw of video cards at any given time, peek vs average.
Since nvidia has changed from using just the agp to power the fan to using the 4 pin psu cable then to not using it in the fx 6500, then to using 2 psu connections in their latest card that uses about 112 watts and recommends a 480 watt power supply!!!!!!

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Postby Ultim4 » Sat Aug 21, 2004 6:49 am

What rail do system fans use?

Sorry, just found out:
High Performance Fan - 4w - +12v Rail
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