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- Newsgroups: sci.electronics
- Path: sparky!uunet!infonode!ingr!b30news!catbyte!medin
- From: medin@catbyte.b30.ingr.com (Dave Medin)
- Subject: Re: 12 V -> 3V, 0.6 A
- Message-ID: <1992Nov11.164649.13864@b30.ingr.com>
- Sender: medin@catbyte (Dave Medin)
- Reply-To: medin@catbyte.b30.ingr.com
- Organization: Intergraph Corporation, Huntsville AL
- References: <1992Nov7.141550.538@ludd.luth.se> <5150062@pollux.svale.hp.com>
- Date: Wed, 11 Nov 1992 16:46:49 GMT
- Lines: 49
-
- In article <5150062@pollux.svale.hp.com>, ryoung@pollux.svale.hp.com (Roderick Young) writes:
- |> Warning on using LM317 to drop 12V to 3V at 600mA...
- |>
- |> You car battery could put out as much as 14-15V sometimes. Let's say 13V.
- |> That means a (13-3 = 10V) drop across this linear regulator. At 600 mA,
- |> that's 6 WATTS. It might work, if you use the TO-3 version, and a very
- |> good heat sink. You might consider putting a 8-ohm, 10-watt resistor
- |> in series with the LM317 input to disspate some of the power.
- |>
- |> I actually used a (surface mount) LM317 to drop 12V to 5V, as less current
- |> than you specify. The chip got so hot, it melted its own solder!
-
- The common TO-220 cased LM317 is good for 10+ watts on a reasonable
- heat sink within the human comfort ambient temperature zone...
- The 317 MUST BE HEATSINKED IN THIS APPLICATION. A 10 sq in piece
- of metal mounted where there is some air movement or convection
- can be set up should be a good start. Or, use the project case you're
- building it in. An very CONSERVATIVE approach is to say that if the
- heatsink is too hot to hold comfortably, then it needs to be increased
- in size, have some air moved over it, etc. Take care that the metal tab
- on the 317 is insulated from the car's frame (a mica washer, shoulder
- washer, and heatsink grease from RS do this nicely). I've
- used 317's before in this type of automotive application with
- good results.
-
- The resistor is a great idea, but don't mount it close to the heatsinked
- part or you're back where you started (as you raise the ambient temp
- close to the heat sink and decrease its effectiveness). Heat removal
- still needs to be considered. I've had several power resistors
- unsolder themselves without so much as smoking first.
-
- For the poster that suggested buying commercial from Radio Shack, I
- say great! You'll find in some revisions of their product they have
- a transistor/zener regulator and in some others, ta-da, an LM317
- (grossly under-heatsinked). I think your point on tinkering was
- correct, though. Make it yourself, endure some hassle, and maybe
- learn something. Or, simply pay out.
-
- --
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- Dave Medin Phone: (205) 730-3169 (w)
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