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From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: 12 to 9 volts dc
Message-ID: <1991Feb11.051608.2280@athena.cs.uga.edu>
Date: 11 Feb 91 05:16:08 GMT
References: <1991Feb10.220719.17078@cbnewsl.att.com> <1991Feb11.012255.3022@ariel.unm.edu>
Distribution: usa
Organization: University of Georgia, Athens
Lines: 27

In article <1991Feb11.012255.3022@ariel.unm.edu> ee5391aa@triton.unm.edu (Duke McMullan n5gax) writes:
>In article <1991Feb10.220719.17078@cbnewsl.att.com> wally@cbnewsl.att.com
>(Brian S Walden) writes:
>>I want to take my old Sony Dicsman D-5 in the car with me,
>>the problem is that it requires 9 volts and 1/2 amp.
>
>OK:  First, let's think 13.6 volts, not 12.  That's the actual (as opposed to
>nominal) voltage when you're running the engine.
>
>Second, you _can_ use a resistor/zener_diode/buffer_transistor circuit such as
>the one you mention, but there are easier ways.
>
>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:
>
>+ 12V o----------------|<-----------o + 9.1V
>			4.5V
>			zener
>			diode
>
   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.