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December 15, 2015

Troubleshoot LEDs With Multimeter

We put up some Christmas lights on our house. These were some cheap LED icicle¬†lights. That night I noticed some of them were not working. So I grabbed a few¬†of the LEDs and notices that one was bad. As I jiggled it, others came on. I pulled out that LED, and it had a broken leg. I thought I would just solder a new leg on. But upon closer inspection of the bulb, it had NO cathode or anode marking. The only way to tell would have been that one leg would be longer then the other… but one was broken off. DOH! So, I was not sure what direction to put it back into the holder. I can up with a way to test almost any LED with a multimeter!

My Fluke multimeter has a “Continuity Test”. It makes a BEEP when the 2 probes are a dead short. Is this mode the probes have a small amount of voltage on them. So if you take the black probe and touch it to one leg of the LED and the red probe to the other leg, the LED will light up. If it does not, swap the probes. If the LED lights up, then you can see what leg is cathode or anode by the red (anode) or black (cathode). Since the voltage is very low on the probes, there is almost no likelihood of destroying the led with reversed probes.

This works great for small SMD LEDs too. I had a mixed up pile of re, blue, and white 1206 SMD LEDs. Using this method, I was able to sort them very quickly. Also works for testing LEDs in circuit.

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