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From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: Power supply for IBM PC interfacing projects
Message-ID: <29469@netnews.upenn.edu>
Date: 12 Sep 90 19:28:55 GMT
References: <15286@shlump.nac.dec.com> <1990Sep12.021220.11551@swbatl.sbc.com> <1990Sep12.142550.9853@wsrcc>
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Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo)
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In article <1990Sep12.142550.9853@wsrcc> wolfgang@wsrcc (Wolfgang S. Rupprecht) writes:
>adams@swbatl.sbc.com (Tom Adams - 235-7459) writes:

>I have just such a gem here.  The trick is that on cheap imports, they
>just assume that the 5v has some moby load on it.  The Taiwan clone
>"200w" KPI supply I have needs a 1 ohm (25w)  load on the 5v to work
>well.  Well defined as the 12v supply actually being within 10% of its
>rated voltaage. ;-(
>
>With no load the thing goes into over-voltage trip mode.  With a small
>load of 1/2A, it does power up. It just has poor regulation on all
>supplies.

Many I've seen do this in one way or another.  AT&T supplies manufacturered
by Altec have a heat-sinked resistor across the 5v supply.  However, they
will -sometimes- power up without any load, though I haven't looked at
the output on a scope.

Most IBM supplies, particularly from AT's work fine without a load.  However,
the old AT's had a heat sinked resistor between 12V+ and ground that
was used if there was no hard drive installed.  I've used an AT power
supply to power 12 volt devices (HT's, test equipment, etc.) and have
never had a problem even though I had no load on the 5V side.

By the way, BEWARE OF OLD AT SUPPLIES!  There are quie a number of them
still out there.  If you turn them off and then turn them back on quickly,
they seriously lose regulation.  I learned this the hard way, but
not before  doing it twice.  First time it cost me a hard drive controller
and an EGA card, the second time the system board  :-(  I've heard many
other horror stories to the same tune.

								--- Jeff
--
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Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104