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Why did my lm317 stop working? Answered

I am building an lm317 power supply that has a 510 ohm resistor and a 10k potentiometer. For power I have a 12v 1A wall wart (really more like 14-15v ). I built it on a breadboard and then from there moved it to a pcb (the pre cut kind from radio shack) and everything went fine. However when I tested it before i closed up the project box, it would only supply about 14.06v to 15.22v. Im not sure what is wrong, all the connections look fine and nothing really changed from when it worked.


The lm317 must have a resistor of about 220 to 280 ohms from pin 1 (the control pin) to pin 2 ( the output pin). Also, your variable resistor must on pin 1 must have one end connected to ground. If that variable resistor opens up or looses it's ground, then the volts will go to max. Other than that, the only thing to be carefull of on the LM317 is if you bolt it to a metal ground. The metal tab of the LM317 must dissipate heat to the metal, but NOT actually touch the metal electrically. Just use an OHM METER to check it. Place your meter ground probe (black) onto the metal chassis. Place your RED probe onto the LM317 metal tab. It should NOT measure close to zero ohms. A couple hundred ohms or thousand is OK... but not 0.2 or such. (use the lowest ohms scale such as R X 200 for that test. If it measures bad then you must place an insulator kit with plastic washer on the screw of the LM317 to eliminate the short to ground.

I checked the resistance and there was 0.8 ohms, but the heat sink is not connected to anything else. I can still adjust the voltage, but only from 14.4v to 15.2v.

What does "checked the resistance" mean, in this context ? You said you had 510 Ohm from Vout to adj, which isn't actually correct, but it "should" work with it.

I checked the resistance between the heat sink ant the metal vout tab. When it was working the 510 ohm resistor was fine.

Post a couple of pictures of your circuit, like Framistan and I said, you probably lost a ground.

Sorry about any blurriness and the upside down part, I used a gopro because it was the only camera I had available. The 3 wires coming off of it are for the switch witch is just between the power source and Vin. The potentiometer was temporarily disconnected because i tested it to make sure that it wasn't the problem.


Sorry, they don't help. I can't see the component side very well.

The soldering isn't very good, and that could well cause you problems, with excessive amounts which could be shorting pins out, and excess lengths left on component legs.


5 years ago

I don't understand your question. What is it doing now? When was it working? What did you consider working?

I will note though that you likely measured the unloaded voltage of your power supply. Under load it will drop much closer to 12.

It was working yesterday, and stopped about an hour later. When it was working I could get 1.2v to about 14v.

You've lost a ground.