Knowing which side of an LED, or Light Emitting Diode, is the positive anode side and which side is the negative cathode side is essential if you want to make the LED emit light. For the LED to emit light, the voltage on the anode must be positive. A simple LED circuit is arranged such that the positive terminal of the battery is connected through a resistor to the anode of the LED. The LED's cathode is connected to the negative terminal of the battery.
Connect the positive terminal of your battery supply to the left lead of a 1,000 ohm resistor. Connect the right lead of the resistor to the left lead of the LED. Connect the right lead of the LED to the negative terminal of the power supply.
Turn on your power supply. Raise the power supply voltage to 1 volt. Observe whether or not the LED emits light. If the LED lights, the positive end, or anode, of the LED is the lead that connects to the resistor.
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Continue raising the voltage in 0.3 volt increments if the LED doesn't light. Observe if the LED lights at each 0.3 volt increment until you reach 3.0 volts or the LED begins to emit light. If the LED doesn't light at or below 3 volts, the LED lead that connects to the resistor is the negative lead, or cathode, and the positive end, or anode, of the LED is the lead that connects to the negative terminal of the battery. If the LED does light, the LED lead that connects to the resistor is the positive end of the LED and the negative end of the LED is the lead that connects to the negative terminal of the battery.
You can visually determine which end of the LED is the positive lead. The shorter lead of the LED is the positive end. However, if the positive LED lead has been cut, this method is not reliable. If you can see through the LED, which is often the case, the positive anode is the smaller of the electrodes within. Review the LED data sheet from the manufacturer. It may have a diagram of the LED that will indicate the positive end of the LED.
Improper use of electronic equipment and components can result in fire, serious injury or death. Always work under the supervision of a safety certified electronic technician or electronic engineer. Obtain a electronic safety certificate before you work with electronic equipment and components.