🚀 go-pugleaf

elrond@ccwf.cc.utexas.edu (hlista) writes:
> Does anyone have a schematic for a 12V DC to 3V DC 600 mA?

In <1990Dec27.141809.2856@phri.nyu.edu> I suggested a 7803 and added:
> You might want to protect it with something like a 20V zeiner across its in-
> puts, and a 1-ohm power resister in series with the input, after the zeiner.

	David Lesher <wb8foz@mthvax.cs.miami.edu> pointed out to me that I'd
be asking the zeiner to dump enough current to de-spike the entire car's
electrical system, something which it is not going to be happy doing.  One
very likely mode of expressing its unhappiness is to simply open, but quick,
and end up doing nothing useful at all.  I agree with David, and so ammend my
proposal to be:

                   2W-1ohm           2W-1ohm     +------+
         +12 O------/\/\/----+-------/\/\/\------| 7803 |------O +3
                     R1      |         R2        +------+
                           __|__/                    |
                          / /_\  20V zeiner          |
                             |                       |
                             |                       |
         Gnd O---------------+-----------------------+---------O Gnd

	If your +3 load is drawing 600 mA, the I^2*R loss in R2 should be
360 mW so 2W seems like a reasonable safety factor.  If you like, put a 1A
slow-blow fuse in series with R2 to protect both it and the 7803 against an
overload.  I'm assuming the zeiner will only be called upon to zap low duty
cycle spikes, so while R1 may have to handle big spike currents, the average
power shouldn't be significantly higher than R2, so 2W should be OK for it
too.  At 600 mA, you'll see a 1.2V drop between the car's nominal +12 and
the input terminal of the 7803, but since you'll only need something like 5V
in to regulate properly, that's no big problem.
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
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