Current through a diode goes one way only. An LED is a diode that lights up.
A diode lets current flow one way and blocks it the other way, like a one-way valve. An LED (light-emitting diode) is a diode that gives off light while current flows.
1. Which way round
- The anode is the positive side. The cathode is the negative side.
- On a diode, the stripe marks the cathode.
- On an LED, the shorter leg and the flat edge on the rim mark the cathode.
Current flows from anode to cathode. Put it in backwards and nothing flows.
2. Forward voltage
A diode uses up a small voltage when it conducts. A silicon diode drops about 0.6 to 0.7V. A Schottky diode drops less, often around 0.3 to 0.5V. LEDs drop more, roughly 1.8 to 3.4V depending on colour. Check the datasheet.
3. Why an LED needs a resistor
An LED doesn't limit its own current. Connect one straight across a battery and it can burn out in a moment. A series resistor sets the current:
Resistor = (supply voltage − LED forward voltage) ÷ LED current.
For a 12V supply, a 2V red LED and 10mA: (12 − 2) ÷ 0.010 = 1,000 Ω. Work it out with the LED Resistor Calculator.
4. Where diodes turn up
- Blocking diodes and bypass diodes in solar panels.
- Across relay coils to soak up spikes.
- Reverse polarity protection in devices.
- Rectifiers in chargers that turn AC into DC.
5. Testing a diode
Use the diode test setting on your meter. One way it shows the forward voltage (around 0.5 to 0.7 for silicon). The other way it shows OL. Both ways OL means it's open. Both ways near zero means it's shorted.
6. Quick summary
- Current flows anode to cathode only.
- The stripe or flat side marks the cathode.
- LEDs always need current limiting.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician. Low-voltage DC can still deliver very high current, so treat batteries with respect.
