Anode and Cathode Explained
The longer lead is usually the anode, but always verify with a meter.
What the two terminals are
The anode is the positive terminal, where conventional current enters the LED. The cathode is the negative terminal, where current leaves. Current flows from anode to cathode, never the other way during normal use.
Ways to tell them apart
- Leg length: on a new through-hole LED the longer leg is the anode. Trimmed legs lose this clue.
- Flat edge: the rim of the plastic body often has a flat side next to the cathode.
- Inside the lens: look from the side. The larger metal piece inside is usually the cathode, and the smaller one with the thin wire is the anode.
- SMD markings: surface-mount parts often have a dot, notch, green tint or tab on the cathode end. Check the datasheet to be sure.
Testing with a multimeter
Set the meter to diode test and touch the probes to the legs. With the red probe on the anode and the black on the cathode, the LED may glow faintly and the meter shows its forward voltage. Reversed, the display normally shows an open circuit.
Testing with a battery
A 3 V coin cell with a resistor of a few hundred ohms in series is a quick check for small red or yellow LEDs. If nothing lights, swap the legs once.
If you connect it backwards
The LED will stay dark. Small LEDs can usually survive a brief reverse connection at low voltage, but most are only rated for around 5 V in reverse, so higher voltages can damage them.
Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.