Diodes and LEDs
A resistor treats current the same in both directions. A diode does not. A diode is a one-way valve for electricity: it lets current through one way and blocks it the other way. That single trick protects circuits, turns AC into DC, and — in the form of the LED — lights up almost every gadget you own.
The big idea & what you will learn
Everything in this lesson comes from one fact: a diode conducts in only one direction, and while it conducts it "steals" a small fixed voltage (about 0.7 volts for the common silicon type). Hold onto those two ideas and the rest is just consequences.
- Which way current flows through a diode, and how the stripe tells you.
- The forward voltage drop and how to work out the voltage left for the rest of the circuit.
- What happens in reverse: tiny leakage, then breakdown — and why the Zener diode makes breakdown useful.
- The real jobs diodes do: reverse-battery protection, rectifying AC to DC, flyback protection, and steering power.
- LEDs: forward voltages by colour, and the current-limiting resistor recipe (worked three times).
- How to read LED polarity, why brightness is controlled with PWM, and how to test a diode with your multimeter.
1. The diode: a one-way valve for current
A diode has two legs. One is called the anode (the "in" side) and the other the cathode (the "out" side). Current is allowed to flow from anode → cathode, and is blocked in the other direction.