Testing a LM317T Voltage Regulator

You can conduct a basic-performance test by connecting:

Pin 1: To common ground (Supply - , DMM - , and load, if any)

Pin 2: To the DMM + probe (DMM set for DC voltage). You can also connect a load (a few Ohms) between this pin and ground.

Pin 3: To any DC supply from +7 to 40 V

(You can reach a datasheet at: http://www.fairchildsemi.com/ds/LM/LM317.pdf)

The voltage at pin 2 should be close to 1.25 V, independently of the load and the input voltage.

This is just a quick-and-dirty test, not ensuring fully functionality. But you don't get anything better when you check a transistor by simply measuring its hFE with a DMM.

Besides, the LM-317 is a tough regulator (including current limitation and thermal shutdown), so there is a good chance of finding your pieces in a good condition.

LM317 is a positive variable output voltage. To make sure that is working, the best way is to test it in a real life configuration.

Take a 120 ohm resistor and connect it between pin 2 (output) and pin 1 (adj). A 360 ohm resistor between pin 1 (adj) and ground. Connect pin 3 (input voltage which could be up to +40V). The output voltage should be close to 5V. If you connect a load 5 ohm or higher between pin 2 (output) and ground, the voltage on it should be a 5 V constant.

You might want to swap the good side regulator for the bad one and see if the problem follows the regulator, if it doesn't, then suspect another component.

I gave points to both fernandotasso and indel since their answers are both valid for testing the LM317. I can't think of a simpler/better way to test it.

I just want to add that, you can't test the LM317 like a transistor (although it looks like one) because its internals is a circuit, not a simple transistor. I haven't noted how a good LM317 looks like with a resistance check (then again, I don't know how much variation there will be from one device to the next).

regards,

Vulcan