A Light Emitting Diode or LED is a small device that emits light when electrical current passes through it. LEDs come in different shapes and sizes, called packages. The smallest LED package is of SMD or Surface Mount Device type. An SMD LED does not have leads rather terminals that are small metal pieces on both of its ends. These terminals are used to connect the LED with the pads on Printed Circuit Boards or PCBs. LEDs can be tested with the help of a multimeter.
Insert the black multimeter lead into the multimeter socket labelled COM and the red one in the socket labelled V or OHM.
Turn the multimeter on and set it to the continuity testing mode. This can be done by turning its knob to the point labelled with a diode symbol and/or a few curved lines. The diode symbol is an arrow with a solid triangle pointing to a line.
Place the SMD LED on a flat and dry surface and touch its two terminals with the two multimeter lead tips at the same time such that the red lead touches the terminal on the left, and the black lead touches the terminal on the right. Note if you hear a continuous beep from the multimeter. Then touch the LED terminals with the leads again but this time connect the black lead to the terminal on the left and the red lead to the terminal on the right of the LED. If you do not hear beep in both instances, it means that the LED is open in both directions or in other words the LED has gone bad. If you hear beep in both cases, it also means that the LED has gone bad. If you hear a beep in one case but not in the other, the LED is good.
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