LM317 dropping voltage with load - Page 1
Forum section: electronics / beginners

author: Rerouter
author_rank: Super Contributor
timestamp: June 03, 2012, 10:41:47 am
content: 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 thishttp://cdn.head-fi.org/e/e4/e431d7c1_317_337Schematic.pngwith 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)

author: Unknown
author_rank: Unknown
timestamp: June 03, 2012, 11:43:17 am
content: 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?

author: Rerouter
author_rank: Super Contributor
timestamp: June 03, 2012, 12:40:40 pm
content: 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.

author: digsys
author_rank: Unknown
timestamp: June 03, 2012, 01:17:16 pm
content: 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 0.1monoAs is, on an oscilloscope you'll see a 1MHz + square wave on the OP, due to the LM317 taking off. Very common problem.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2012, 02:33:54 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: June 03, 2012, 06:01:30 pm
content: 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?

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2012, 04:21:27 pm
content: Quote from: IanB on June 03, 2012, 06:01:30 pmYes, see my previous post. Large capacitors are poor at dealing with short, high frequency transients because they are non-ideal.

author: free_electron
author_rank: Super Contributor
timestamp: June 04, 2012, 05:13:36 pm
content: Quote from: Rerouter on June 03, 2012, 10:41:47 amif 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 .

author: whonline
author_rank: Newbie
timestamp: June 04, 2012, 05:22:24 pm
content: Your schematic doesn't show your ac source. I assume it is a center tapped transformer. Check the center tap ground connection.

author: Torrentula
author_rank: Regular Contributor
timestamp: June 04, 2012, 05:48:16 pm
content: 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!

author: IanB
author_rank: Super Contributor
timestamp: June 04, 2012, 08:21:06 pm
content: Quote from: Hero999 on June 04, 2012, 04:21:27 pmI 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.

author: Rerouter
author_rank: Super Contributor
timestamp: June 04, 2012, 08:37:11 pm
content: 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

author: tempAccount
author_rank: Newbie
timestamp: June 06, 2012, 04:19:54 am
content: 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.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 06, 2012, 07:47:42 am
content: How hot does it get? Do you have a good heat sink? You could be seeing thermal shutdown.