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From: rambler@pnet51.orb.mn.org (Dan Meyer)
Newsgroups: sci.electronics
Subject: Re: 12V car --> 3V cd player
Message-ID: <3710@orbit.cts.com>
Date: 28 Dec 90 04:05:02 GMT
Sender: root@orbit.cts.com
Organization: People-Net [pnet51], Minneapolis, MN.
Lines: 65

In article <1990Dec27.141809.2856@phri.nyu.edu> roy@phri.nyu.edu (Roy Smith)
writes:
>elrond@ccwf.cc.utexas.edu (hlista) writes:
>> Does anyone have a schematic for a 12V DC to 3V DC 600 mA?
>> Thanks for any help.
>
>       Seems like a perfect application for a 7803 three-terminal
>regulator.  Can't get much simplier than that.  Should run you a couple of
>bucks at Radio Schlock.  If they don't have the 7803 (it's an unusual
>value), they will probably have the variable version, which just requires
>adding a couple of passive components to set the output voltage.
>       The only question is whether a 7803 is hardy enough to handle all
>the horribleness that goes on in a automobile electrical system (I'm mostly
>worried about voltage spikes).  You might want to protect it with something
>like a 20V zeiner across its inputs, and a 1-ohm power resister in series
>with the input, after the zeiner.

Even Simpler - And output overvoltage protection is included!
              
The diodesin series with the output of the 7805 drop 0.7v per diode: 
5v-1.4v=3.6 v. add another one if you want run off of 2.9 v.

The Diodes paralell with the output (which are installed the wrong way in the 
schematic!) form a crowbar circut. if the output rises above 0.7 * 6 = 4.2 v
you pop the 1 amp fuse (be sure to use diodes of 1a or greater capacity!)
                        



        1a          |---------------|
+12V O--####--------| Generic       |->|- >|-o------o----------O  output 
        fuse        |     LM7805    |        |      .     
                    |_______________|        -      .
                            |                ^      .
                            |                |    ----- .01 uF capacitor (can
                            |                -    ----- you get away without 
                            |                ^      .   it? Probably!)
                            |                |      .
                            |                |      .
                            |                -      .
                            |                ^      .
                            |                |      .
                            |                -      .
                            |                ^      .
                            |                |      .
                            |                -      .
                            |                ^      .
                            |                |      .
                            |                -      .
                            |                ^      .
                            |                |      .
ground O--------------------o----------------o------o---O ground



Come to think of it, simplify the circut a little, use a 4.1 ( or so ) zener
instead of several standard diodes for the crowbar. Simpler and still almost
all junk box parts!

-- Dan Meyer

Remember: " Buffalo never Oink " Seen on a South Dakota travel brocure.
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