How to Check LED Cathode & Anode Legs |
Light Emitting Diodes, known to most people as LEDs, have almost entirely replaced incandescent lights in modern electrical equipment. Even modern cars and trucks now use LEDs instead of traditional bulbs. LEDs are physically stronger and smaller than incandescent bulbs, they don't get hot, they resist vibration and they use less power. However, unlike a traditional bulb that can be wired up either way round, LEDs are polarized. They will not illuminate unless they are wired correctly
Instructions 1 Check the legs of the LED. If the diode is new and the legs have not been cut to size, one leg will be longer than the other. The longer leg is the positive anode and the shorter one is the negative cathode.
2 Examine the body of a standard cylindrical LED for a flattened side. This flattened side is located at the base of the colored top, just above the point where the legs leave the body. The flattened area, called the "cathode mark," is directly above the cathode leg.
3 Read the codes printed around the edges of larger "block" LEDs. These LEDs, used for displaying alphanumeric characters, have a common anode or cathode. The appropriate probe is identified by the "A/C." mark. Whether this is the anode or cathode depends on whether the LED is a common anode (SA) or common cathode (SC) device.
Tips & Warnings
Review the technical data sheet that accompanies a new block LED to identify the details of each pin. Connect a 1.5 volt battery across the LED. If the LED illuminates then the prongs are wired in the correct direction. The prong connected to the positive side of the power supply is the positive anode leg. Voltages higher than 3 or 4 volts will usually kill an LED. Do not attempt to identify the cathode and anode by connecting an LED to a voltage greater than 3 volts.
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