LM317 dropping voltage with load

[Rerouter]

i have an odd problem for you guys, an lm317 that with the bare minimum load (red led ~5mA) will regulate its output correctly over its full span, however when any additional loading is added, say a 1K resistor, the output will fall dramatically, in this case its span of 2-32 is cut down to 2-6V i have checked and believe my pinout is correct, however i would like to know if any of you have seen a similar failure mode,

the schematic to it is effectivly this
http://cdn.head-fi.org/e/e4/e431d7c1_317_337Schematic.png
with only a few small differences, the capacitors are rated for 75V volt, and the outputs have a red led and a 2K resistor (i feel that could be the cause)

[alm]

Is the input voltage stable under this load? What is the voltage drop across the LM317 under load? Do you have access to a scope? What do the input and output voltages look like?

[Rerouter]

the input is a fairly stable 36V (with no load) and generally would fall to as low as 30V on its maximum load, as measured by my multimeter, i will have access to a scope tommorow to check the ripple.

[digsys]

Hiya,
Most likely you have a simple case of feedback instability due to incorrect filtering.
First off, place a 0.1uf monolithic and 4u7-10uf Tantalum BOTH on the Input and Output of the LM317, close as possible to the device.
Make sure the GND wire of the LM317 is separate from the main GND wire from the Input Filter caps, and joins at the output Terminal.
You really should NOT need any other output caps, if so, MAX 220uF. Also reduce C1 etc to 1-4u7 max or even a 0.1mono
As is, on an oscilloscope you'll see a 1MHz + square wave on the OP, due to the LM317 taking off. Very common problem.

[Zero999]

Yes, the output capacitors appear to be ridiculously oversized - a 1500uF aluminium electrolytic will have a poor high frequency characteristics and a low self resonant frequency (typically under 20kHz) so won't reject transients very well. A smaller ceramic or tantalum capacitor will fair much better.

[IanB]

Putting a giant capacitor on the output of a voltage regulator has just generally got  to be a bad idea. The capacitor is going to dominate the voltage seen by the regulator.

I am not an electronician, but consider this:

A voltage regulator is a device that tries to change the voltage on its output terminals by varying the amount of current it delivers to the load.

A capacitor is a device that absorbs current variations and tries to resist changes in voltage.

Do you see a potential problem here?

[Zero999]

A capacitor is a device that absorbs current variations and tries to resist changes in voltage.

Do you see a potential problem here?

Yes, see my previous post. Large capacitors are poor at dealing with short, high frequency transients because they are non-ideal.

[free_electron]

i have an odd problem for you guys, an lm317 that with the bare minimum load (red led ~5mA) will regulate its output correctly over its full span, however when any additional loading is added, say a 1K resistor, the output will fall dramatically, in this case its span of 2-32 is cut down to 2-6V i have checked and believe my pinout is correct, however i would like to know if any of you have seen a similar failure mode,

if you see this kind of behavior it means the lm317 is working as current source... you change the load at the output but the lm317 gives you a fixed current... this changes the max voltage ...

Check the connection from ADJ to ground... there is something wrong with the resistor there .... bad potmeter ?
you didn't connect the led and that 1K resistor to the ADJ pin did you ? make sure you connect to the output pin .

[whonline]

Your schematic doesn't show your ac source.  I assume it is a center tapped transformer.  Check the center tap ground connection.

[Torrentula]

free_electron has got a point there. Happened to me the first time I was working with the LM317 and fell victim to the TO-220 package. Beware of the pinout!

[IanB]

A capacitor is a device that absorbs current variations and tries to resist changes in voltage.

Do you see a potential problem here?

Yes, see my previous post. Large capacitors are poor at dealing with short, high frequency transients because they are non-ideal.

I hope you did not think I was responding to your post--I was providing additional insight to Rerouter.

However, I don't see a connection between non-ideal behaviour of capacitors and the interaction between a capacitor and a voltage regulator? Ideal or non-ideal, a capacitor is going to be a source of problems in that location.

[Rerouter]

finally got aroound to looking on the oscilloscope, under 26V there is some harsh level of oscillation on the output about 8Vp-p , and above that it functions just as one would expect, even to its maximum rated load , the input pin is a nearly dead flat 36-34V, (loading), and the adj pin is stable until the ocillation begins,

also on the diagram looks like i missed the output capacitors, mine only has an output capacitance of 15uF

[tempAccount]

Hey, I was having this exact same problem and found this thread when googling for an answer. If you assume that the standard wiring diagram has the correct pinout, it may be incorrect but still appear to work somewhat. Instead, use the datasheet to find the correct pinout, which for me was adjust, output, input.

[ejeffrey]

How hot does it get?  Do you have a good heat sink?  You could be seeing thermal shutdown.