LM317 Votage Regulator Instability?

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

Too small an output cap, or too low an ESR? Three terminal regulators can oscillate. Try hanging 100 uF aluminum electro on the output and see if it starts behaving better.

Cheers

Phil Hobbs

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

Cheers

Phil Hobbs

The datasheet says output caps are "optional" although I do always use 'em. The one I'd fitted was 1nF which seemed a bit small, so I subbed it for a 100n but it's made no difference. 100uF seems HUGE! I might try it, though.

Tried it before posting but no joy, I'm afraid.

Can you measure what the board is actually drawing?

The data sheet at: says that 37V is the maximum output voltage with an input-output voltage difference of 40V absolute max. With 37v in and 12v out, you're within spec, but seems to me too close for comfort. Maybe you blew up the LM317? Try another LM317 and watch your input voltage carefully. Or maybe add a big dropping resistor between your 37v supply and the LM317 input.

JOOI What version of the 555? If it's a bipolar version they can exhibit a "crowbar effect", and the 100uf cap is usually suggested to ameliorate it.

The ones I'm using here are all NE555CN.

When connected to an external bench power supply at 12V., the board draws

When connected to PSU that uses the LM317, however, it draws 3mA and the voltage across it drops to around 6V.

Don't see how that's possible, Jeff, since it's still happy to supply passive loads that are much heavier, and at a rock solid stable output voltage.

Cheers

Phil Hobbs

I'll certainly give it a try first thing tomorrow, Phil. But I'm not optimistic given this confident pronouncement which I quote from the datasheet:

"Typically, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors, in which case an input bypass is needed. An optional output capacitor can be added to improve transient response. The adjustment terminal can be bypassed to achieve very high ripple rejection ratios that are difficult to achieve with standard 3-terminal regulators."

So the manufacturers don't seem to think it's of any great importance at all. BTW, on the input side, the filter capacitor is less than one inch from the the reg's Vin terminal.

It might be interesting to see if you have the same problem with an ICM7555, but it's much easier just to add the cap, as suggested :) :)

Quite! And I shall certainly do so first thing tomorrow. IF it works it will highlight an astonishing and appalling omission from the datasheet IMHO.

IMO, The problem isn't so much the omission from the LM317 datasheet, it's more an omission from the NE555 one.

Interestingly the ICM7555 datasheet says: "The ICM7555 and ICM7556 devices are, in most instances, direct replacements for the SE/NE 555/556 devices. However, it is possible to effect economies in the external component count using the ICM7555 and ICM7556. Because the bipolar SE/NE 555/556 devices produce large crowbar currents in the output driver, it is necessary to decouple the power supply lines with a good capacitor close to the device. The ICM7555 and ICM7556 devices produce no such transients"

Jeff's point is you are at the maximum recommended input voltage, anything changing there and the device will go south,( east, west, or north - ) which may prove catastrophic.

John :-#(#

I think Phil is suggesting uF's on the output. (I always put 4.7- 10 uF there.) I'm going with Jeff L's idea, that you've blown it up. Try another. They also make a HV version of the lm317.. which is the only version we stock.

George H.

Noted, Lee, and many thanks for the info. It's late here now so I won't be posting again until morning whereupon I'll report back with my findings!