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From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: 12 to 9 volts dc
Message-ID: <37352@netnews.upenn.edu>
Date: 11 Feb 91 20:53:02 GMT
References: <1991Feb10.220719.17078@cbnewsl.att.com> <1991Feb11.012255.3022@ariel.unm.edu> <1991Feb11.051608.2280@athena.cs.uga.edu> <1991Feb11.084349.13358@ariel.unm.edu>
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In article <1991Feb11.051608.2280@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:>
>>We want to carve about 4.6 volts off that 13.6.  If we call it 4.5, and you
>>can come up with a 4.5 volt zener, then just put the zener in series with the
>>load:
>   NO! You'll fry a Discman.
>   The voltage in the car is not a constant 13.6V;
>   it varies from 11 to 17 volts!!!!!  (Measure it if you don't believe me.)
>
>   You don't want a constant voltage DROP, you want a constant
>   voltage OUTPUT.  A 7809 regulator chip will do the trick.

Amen to that.  At idle speed, my car shows right around 14 volts, slightly
higher on cold days.  At 1000+ RPM's, it's right around 14.5 volts.  With
the engine off, it's in the 12.5 volt range.  I have seen voltages
above 15 in some cars, especially when cold as some of the modern 
systems boost up the altnerator voltage to quickly charge the battery
(the premise here is that when a battery is at less than full charge,
it should be brought back up ASAP in case the trip is going to be too
short to fully charge it at a normal voltage, according to a mechanic).

The 7809 should handle your 1/2 amp, but may need a heat sink.  A better
way to go is to use a 78H09 (H for high-current) and mount it to 
some sort of a metal object, preferably using a heatsink.  The cost of
a 78H09 is about $2.00.  You can try a 7809 (probably available at
*gasp* Radio Shack) first, but if it gets too hot, it will (or should
I say, it is supposed to) shut itself off.  It's definately going to be
warm, so don't touch it if you've been using it for a while.


I wouldn't use a zener unless I had a crowber with it as well (requires
another zener, an SCR, and a few resistors).

								--- 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