Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!rex!uflorida!mailer.cc.fsu.edu!sun13!murray From: murray@sun13.scri.fsu.edu (John Murray) Newsgroups: sci.electronics Subject: Re: 12V car --> 3V cd player Message-ID: <1739@sun13.scri.fsu.edu> Date: 27 Dec 90 18:52:45 GMT References: <41842@ut-emx.uucp> <1990Dec27.141809.2856@phri.nyu.edu> Organization: SCRI, Florida State University Lines: 59 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. I'm pretty sure the Schlock only carries 2-4 78xx or 79xx. '05's and '12's and such. > 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. Anybody happen to know what is likely to occur when a 7{8,9}xx or a 317 melts down? Specifically, I am wondering if it is reasonably safe (for the load) to go with the quick-and-dirty circuit below in a less than benign environment: |---------------| +12V O--------------| Radio Schlock |------o----------O adjustable, approx. | LM317 | | . +1.5 to +12 |_______________| | . |------| | | . | V | | ----- .01 uF capacitor (can o--\/\/\/\/\/\---o---\/\/\/\/\--| ----- you get away without it?) | 10K variable 5K 1/4W resistor . | . ground O---o------------------------------------------O ground About $3.00 in parts at Radio Schlock, or about $4.00 if you get a 15-turn variable resistor (I recommend it). If the LM317 blows up, buy another one for $1.79 :-). If it blows up the load along with it, buy another CD player for $100 :-| Educational notes: a 78xx is a 3-terminal voltage regulator which takes a somewhat higher positive DC voltage and regulates it down to xx volts. A 79xx does the same thing for negative voltages. An LM317 does the same thing, but the output voltage is adjustable. An LM337 does the same thing as an LM317, but for negative voltages. 78xx's and 79xx's in TO-220 cases (the flat kind with the metal tab sticking out the top, like you see at RS) are rated to 1 amp, properly heatsinked (heatsunk?) LM317's and LM337's in TO-220 cases are rated to 1.5 amps. 78xx's and 79xx's have the advantage of needing no external components. >-- >Roy Smith, Public Health Research Institute -- Disclaimer: Yeah, right, like you really believe I run this place. John R. Murray | "Never code anything murray@vsjrm.scri.fsu.edu | bigger than your head.." Supercomputer Research Inst.| - Me