By Gareth Downes‑Powell
Updated Mar 24, 2022
LEDs—light‑emitting diodes—are among the most popular electronic components because they are inexpensive, low‑power, reliable, and long‑lasting. As polarized diodes, LEDs allow current to flow only in one direction, so they must be connected with the correct polarity. Testing an LED is simple: apply a low‑voltage source, such as a battery or a multimeter, and observe whether the diode lights.
Attach a crocodile clip to the LED’s anode (the longer lead). If both leads are cut to the same length, connect the clip to either lead and swap them if needed. Clip the other end of the same lead to the resistor’s one side.
Connect another clip to the LED’s cathode (the shorter lead) and clip its other end to the negative terminal of a 9‑V battery.
Take a third clip, attach one end to the free side of the resistor, and touch the other end to the battery’s positive terminal. The LED should illuminate, confirming it is functional. If it does not light, reverse the LED leads and reconnect the resistor to the battery’s positive side. Persistent failure indicates a faulty LED.
Set the multimeter to the diode‑test mode, or, if unavailable, select the lowest resistance setting. The meter will supply a small voltage sufficient to forward‑bias the LED.
Connect the meter’s positive lead to the LED’s anode. If you are unsure which lead is the anode, connect to any lead and swap later if necessary.
Connect the meter’s negative lead to the cathode. A dim glow indicates the LED is working. If it remains dark, reverse the connections; if it still does not light, the LED is defective.
Use a 9‑V battery with a 1 kΩ resistor or a multimeter in diode‑test mode to quickly verify an LED’s operation. Always include a current‑limiting resistor to avoid damage.
Never connect a battery directly to an LED without a proper current‑limiting resistor. Doing so can destroy the diode instantly.
