LM317 Votage Regulator Instability?

Cursitor Doom · 2016-07-14T15:40:04+00:00

Hi all, I'm having serious difficulty trying to get this issue resolved and was wondering if anyone here has come across the same problem. I've got a linear PSU that uses a LM317 (TO-220 case) to drop 37V down to 12V for a small 555 timer board that's set up as a pulse generator (at about 0.1Hz). The thing is, as soon as I connect the board, the 12V from the reg drops to between 6V and 7V and wanders around this level (like as if I'd put a really heavy load on it). But the 555 board's resistance is about 22k ohms so it only needs a very small current. When I swap the board for a 10W 50 ohm power resistor drawing 0.25A, the reg output is totally fine, staying rock steady at 12V and the resistor and reg get warm as you would expect. I can only think there may be something about the timer board that 'upsets' the LM317 and causes it to incorrectly partially shutdown, because nothing on the board is getting warm and trying spare identical timer boards causes the voltage to drop by the same amount also. I can't recall this ever happening before! Any suggestions? TIA.

Jul 14, 2016 40 Replies

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Cursitor Doom 10 years ago

Tell me about it! Having a natural gift for such activities makes a massive difference. BTW and FWIW, there's a very interesting discourse on the correct approach to troubleshooting in Robert Persig's Zen and the Art of Motorcycle Maintenance. And it's not restricted merely to motorcycles, either:

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LM317 Votage Regulator Instability?

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