This post will cover some basic on LEDs and how to test an LED.
LED is the acronym for Light-Emitting Diode, which is a semiconductor diode (a p-n junction) that emits light when positive current flows from the anode to the cathode of the LED. Anode is the + or positive side of the diode. Cathode is the - or negative side of the diode.
The symbol for the diode is an arrow head touching a vertical line. The arrow shows the direction of current that will cause light to be emitted by the LED. Occasionally, the symbol for an LED will also include arrows that represent light leaving the diode.
Most of the package is not the actual LED, but wires and a plastic lens. The longer lead on the package is the anode or + side of the diode. The wire next to the flat on the package base is the cathode when looking at the LED from the bottom. LED Side view Bottom view
How to Test a Diode with a Multimeter
Connect the black lead to the COM terminal on the multimeter.
Connect the red lead to the Ω terminal, unless your particular model differs.
Turn the dial to the diode symbol on the multimeter. This allows for electric current to travel in one direction (the arrow) and not the other.
Turn the multimeter on. The display window should indicate either 0L or OPEN.
Connect the black probe to the cathode end of the LED, which usually is the shorter end and/or cut flat at its bottom. Connect the red probe to the anode end of the LED.
What the test results tell you:
If it occurs that the multimeter display doesn’t change from 0L or OPEN, then it may be that you connected the probes in the wrong order, or that the connections are not secure. Make sure the steps above are followed accurately. Otherwise, it may indicate that the particular LED is damaged. If the voltage in the display is below 400 mV, then it is possible that the cathode and anode are touching, or the probes are touching. This is termed a short circuit—when current passes directly from the cathode to the anode, instead of passing through the LED.
If the steps are followed properly and the LED is undamaged, however, the display should indicate a value of approximately 1600 mV.
When you are testing your LED, take notice of its brightness. If you are already in a lit room, then shade the LED with your hands. A lower efficiency LED will grow dimly, or may just gleam faintly, whereas a higher efficiency LED will glow clearly.