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M Philbrook

In article , says...

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.

You may have two problems..

One, the timer is pulsing and thus causing a load variation.

two, the 317 is a monolithic device and is sensitive to RF, meaning
it can actually act as a detector in a sense..

of course, oscillatiion is common with these things, hence the need
for caps.

You should have a good storage cap on the output to handle riples from
the load and also use some low inductive small caps across the
electrolytic, input side and ref pin.

~50 uf for the load cap and maybe .1uf ceramic caps or the like for the
RF snub. make sure you put a .1uf in parallel with the 50 or more uf
cap.

Jamie