Testing LEDs With a Multimeter




Introduction: Testing LEDs With a Multimeter

About: I am work in a high voltage laboratory, the largest laboratory owned by a university. I am in love with electricity. I have been fortunate enough to work with a variety of green technology including fuel cel...

I have been using this method for many years. It is a quick and simple way to test LED. I buy LEDs in bulk, when they come in the colors aren't marked so this i the method I use to sort them out(yes I buy clear bodied LED)


Digital Multimeter with diode function
resistor (I used a 1K )
Test clips ( you do not have to use these, I did so I could take the photos usually I just used the DMM leads)


Determine the Anode and Cathode sides of the LED, if it is a new LED the long leg should be the anode(+)and the short side is the cathode (-). You can also look inside of the LED and see the larger electrode which is your cathode and the smaller electrode is the anode (+). 

Take you resistor and wrap it around the positive(+) lead of your meter, wrap it tight so it does not come off.

Take your test clips, if you are using them, and clip the positive(+) to the predetermined anode(+) of the LED and the negative(-) to the cathode(-)

Turn your meter on and set it to diode scale, should be marked by a diode symbol. If you used the clips you should see your diode emitting otherwise touch the positive(+) lead of the meter(which should have the resistor) to the anode (+) and the negative (-) lead to the cathode(-) and you should have light!

 If your diode does not light up make sure that you have you polarity correct and you meter is set to the proper function.

 I will admit that I have done this test without a resistor and have had no problems. This meter only outputs 3VDC on the diode scale, my fluke outputs 8VDC. It is rule of thumb when working with LED to limit the current with a resistor. 

 I hope you found this helpful, until next time.

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    7 years ago on Introduction

    You might add that the flat side of regular LEDs is the cathode.

    I think most DMMs output only a limited current on the diode test range, typically about 1mA, so the resistor may not be necessary.

    I suspect even your Fluke will have this limit, so that the rest of the volts stay inside the meter when a current is flowing.

    Older or cheaper DMMs may have a diode test function limited to 2V, so may not light white or blue LEDS which usually need at least 2.5V to start glowing.

    It is worth mentioning that most analog test meters have a reversed polarity on the resistance ranges (to simplify the internal circuit), and they may need the resistor because the output current can be higher.