HV caps in computer supplies.

[WSHU ENGINEERING]

----- Original Message -----
From: Avery Comarow <acomarow at USNEWS.COM>
To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
Sent: Wednesday, August 08, 2001 8:53 AM
Subject: Re: restoring the caps...


you wrote:>
> How come PC power supplies have HV caps? I thought everything coming out
> was 12 or 5 volts. Not that I've torn one apart--but I sure will start
> doing that if they're good sources of HV electrolytics!


Computer power supplies are switchmode, the 120v AC mains are rectified with
a doubler to produce 220vdc, or with a full wave bridge if used on 220 ac,
then switched at up to 200khz square wave to feed a step down transformer,
which can be very small at this frequency. The secondary of this transformer
is then rectified and filtered to produce the 5 and 12 volts. Regulation is
achieved by varying the duty cycle of the square wave. Switchmode rated caps
are of a very high quality with low internal resistance and inductance, and
made to handle ripple currents much higher than conventional electrolytics.
The primary caps are typically 100 to 1000uf at 250 VDC.

Computer supplies are also a good source of 12 volt fans and RFI line
filters. You can also use them for 12 volt power supplies at 4 to 10 amps
(depending on the supply) but most will require a minimum load of 1 to 2
amps on the 5 volt output for proper regulation.


(former switchmode design engineer, before I caught the RF bug.)
Paul A. Litwinovich
Chief Engineer/ Webmaster
WSHU AM&FM, WSUF-FM
N1MUC
"RF IS GOOD FOR YOU"

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