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LM317 adjustable voltage regulator testing method
Some time ago, I got a question from a member to show how to test the LM317 adjustable voltage regulator. I decide to make a simple tester and to show my method of testing this little but very handy component.
The LM317 is a three pin voltage regulator, which comes in different package and different load current up to 1.5A. For a higher load, the regulator must be equipped with a passive or active heat sink to cool down the temperature of the IC.
I usually make two tests when I have to test this or other type of regulator IC:
– short circuit test
– voltage regulating testing
The best and most effective testing technique to test this IC is out of the PCB. For the pin out of this IC please refer to the exactly IC datasheet what you have to test. In this document, I will use the LM317T regulator and the pin out is usually the same with other LM317 family.
Here is the pin out of the LM317T:
Short circuit testing:
Take out from the PCB the LM317 and use your well used DMM to check for shorten pins between all the pins. No pins have to be in short. Otherwise, the IC is shorted and bad. Change the IC with new one with the same load performance.
Voltage regulation testing:
This is the most complicated part. Here is the adjustable resistor calculating formula how to calculate the relation between the adjust resistor and the Vout:
If you build the above simple circuit, you can test your LM317 easily. Close one desired switch, read the output voltage and compare with this table.
If you close on same time more then one switch then you have to manual calculate the R2 value with the parallel resistance equation first, and then use the above equation to calculate the output voltage and compare the result with the output voltage shown by your DMM. In your calculations and measurement you will get some small differences, this is because the tolerance of the resistors, the LM317 and the precision of the DMM. Don’t worry about that, that is normal and is around +-5%.
Somebody would say does putting a potentiometer in place of the R2 would be an easier way to adjust the voltage. Yes, that would be but then I suggest to put in series another resistor around 100 Ohm or so with the potentiometer. This is because in some datasheet, they take attention not to short out the Adj pin and that will happen if you put the potmeter in 0 Ohm position. The I-Adj can rise over the Iadj_max and can kill the IC.
Never use the test circuit for in-circuit testing purpose. The input voltage of the tester could damage some components on the PCB. Whenever you test the LM317, take it out from the PCB and connect it to the test circuit.
I hope you enjoy this tutorial and will save lot of repair time.
This article was prepared for you by Christian Robert Adzic from Novi Knezevac-Serbia.
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