Diodes & LEDs
The one-way valve of electronics — and its glowing cousin that you must never connect without a resistor.
Builds on: 1.2 Ohm's Law1.4 Power & Energy
Semiconductors in sixty seconds
Silicon sits between conductor and insulator — nearly useless pure, magical when doped. Sprinkle in atoms with a spare electron and you get n-type material (mobile negative carriers); atoms one electron short give p-type (mobile positive “holes”). The entire modern world is built on what happens where a p region meets an n region: a junction that conducts in one direction only.
The diode
A diode is one p–n junction with two legs: the anode (+ side, the triangle in the symbol) and cathode (− side, the bar). Push conventional current from anode to cathode (forward) and it flows freely — but only after the voltage across the junction exceeds the forward voltage Vf, about 0.7 V for silicon. Reverse it and essentially nothing flows. Unlike a resistor’s gentle straight line, the diode’s I–V curve is a hockey stick: flat, flat, flat, then nearly vertical. Diodes rectify AC into DC, protect circuits from reversed batteries, and clamp inductive kicks (your flyback diode from Lesson 2.4).
LEDs: diodes that pay you in photons
In a light-emitting diode, each electron crossing the junction drops an energy step and emits that energy as a photon. The step size sets both the colour and the forward voltage: red ≈ 1.8 V, green ≈ 2.2 V, blue/white ≈ 3.0–3.2 V. Physical markers: the long leg and the rounded side are the anode; short leg and flat side, the cathode.
The most important calculation in hobby electronics
Past Vf the curve is nearly vertical — the LED itself does almost nothing to limit current. Connect one straight across a battery and the current skyrockets until the LED dies (took our simulated one ~a millisecond). The fix: a series resistor sized with Ohm’s law. The resistor sees whatever voltage is left over:
Every LED in every circuit needs something limiting its current — usually 10–20 mA for the small ones. This rule has no exceptions, and the lab below lets you learn it the fun way (on a simulated casualty).
⚡ Lab — Light It — or Cook It
An LED, a supply and a resistor you choose. The LED can genuinely die in here.
- With a red LED on 9 V, find the resistor that gives 15 mA. Compare with the recipe readout.
- Now slide R down and watch the operating point climb the hockey stick… 💀
- Reverse the LED — the curve explains what you see.
- Switch to blue at low supply voltages. Why does it stay dark longer than red?