• Anode

    • Positive side

    • Round lens case at base

    • fat part of diagram triangle

    • Longer lead - not always reliable

    • Thin blade inside plastic lens package

  • Cathode

    • Negative side

    • Flat spot on lens case at base

    • pointy part of diagram triangle plus vertical line

    • Shorter lead - not always reliable

    • Thicker blade inside plastic lens package

Common LED Electrical Properties

Voltage Drop

  • red = 1.7V

  • yellow/green = 2.2V

  • white = 3.6V

Measuring Directly With A Power Source And Voltmeter

Put negative lead of LED on negative terminal of the battery. Measure the voltage from the positive lead of the LED to the positive lead of the battery. Subtract this value from the voltage between the two battery terminals. This will be the LED’s voltage drop.

Ideal Operating Current

It’s good to know this based on the rating as they were purchased. But if you don’t know assume:

  • 20mA is typical

Calculating Resistor Value

LEDs and diodes generally need a resistor (or some kind of resistance) to keep the circuit from essentially being a short.

Resistance = (Voltage_PS - Voltage_LED) / Desired_Current