On 07/14/2016 11:49 AM, Ralph Mowery wrote:
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.

Try loading it down some. The data sheet calls from about 3 to 10 ma as
the minimum load.

Too light a load causes the output voltage to go too high, though.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net