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From: mlawless@ncrwic.Wichita.NCR.COM (Mike Lawless)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Power Supply Calculation:
Message-ID: <5381@ncrwic.Wichita.NCR.COM>
Date: 19 Jun 89 16:46:49 GMT
References: <6566@cs.Buffalo.EDU> <7988@saturn.ucsc.edu> <7993@saturn.ucsc.edu
Reply-To: mlawless@ncrwic.UUCP (Mike Lawless)
Organization: NCR Corporation, Wichita, KS
Lines: 28

Some things to keep in mind when discussing power consumption:

1) as previous posters have mentioned, actual power delivered by the supply
is likely to be less than its rated capacity.

2) power consumed by the supply will be greater than the power it delivers,
due to the less than 100% efficiency of the supply.

3) AC power consumed by the supply will NOT by AC RMS volts x RMS amps, 
because the power supply is an inductive AC load; also, current is generally
not sinusoidal in a switching power supply, which is what most PC supplies
are (the current waveform is more like a series of spikes than a sine wave).
Therefore, the power factor is somewhat less than 1.  I read a recent article
about power factor correction techniques in switching power supplies, which
is required in order to get higher capacities without causing excessive
current draw on a 15 or 20 amp circuit.

4) One piece of PC peripheral equipment which is a real power hog is a laser
printer, because of the fuser heater.  I just moved into a house with older,
non-grounder, 14 gauge wiring, and whenever I turn on the printer, it dims
the lights in the entire house (I believe it draws about 8-10 amps).  Moral:
don't leave a laser printer on for long idle periods.

-- 
Mike Lawless, NCR E&M Wichita, Box 20     (316) 636-8666   (NCR: 654-8666)
3718 N. Rock Road, Wichita, KS  67226     Mike.Lawless@Wichita.NCR.COM
{ece-csc,hubcap,gould,rtech}!ncrcae!ncrwic!Mike.Lawless
{sdcsvax,cbatt,dcdwest,nosc.ARPA}!ncr-sd!ncrwic!Mike.Lawless