Diodes

A diode is a one-way valve for current. Apply voltage in the forward direction and it conducts; apply it in reverse and it blocks. That simple behavior turns out to be useful in a huge range of applications — rectification, voltage regulation, protection, and light emission.

How They Work

A diode is a P-N junction — a piece of semiconductor doped to be positive (P-type) on one side and negative (N-type) on the other. Current flows easily from anode (P side) to cathode (N side) when forward-biased, but not the other way around. The symbol's triangle points in the direction of conventional current flow, with the bar at the cathode.

There's a voltage drop across the diode when conducting — the forward voltage (Vf). For common silicon diodes this is about 0.6–0.7V. This isn't optional: that voltage is always dropped across the diode when it conducts, and you need to account for it in circuit calculations.

Types

Rectifier diodes — general purpose, handle significant current. The 1N4001–1N4007 series is ubiquitous. Used to convert AC to DC. Relatively slow switching.
Signal diodes — small, fast, lower current. The 1N4148 is the classic signal diode. Used for signal steering, demodulation, clipping.
Schottky diodes — lower forward voltage (0.2–0.3V) and much faster switching than silicon diodes. Used in switching power supplies and high-frequency circuits. Common for flyback protection where the lower voltage drop matters.
Zener diodes — designed to conduct in reverse at a specific voltage (the Zener voltage). Used for voltage regulation and voltage references. Unlike regular diodes, operating in reverse breakdown is normal and intentional.
LEDs (Light Emitting Diodes) — emit light when forward-biased. Forward voltage varies by color: ~2V red, ~2.1V yellow, ~3.3V blue/white. Always need a current-limiting resistor.

Rectification

The most fundamental diode circuit — converting AC to DC. A single diode gives half-wave rectification (passes only the positive half of the AC wave):

V_DC ≈ V_RMS × 0.45     (half-wave)

Example: 12V RMS AC input
V_DC ≈ 12 × 0.45 = 5.4V DC (before filtering)

A bridge rectifier (four diodes arranged as a bridge) gives full-wave rectification — both halves of the AC cycle are used:

V_DC ≈ V_RMS × 0.9      (full-wave)

Example: 12V RMS input
V_DC ≈ 12 × 0.9 = 10.8V DC (before filtering)

The output of either circuit is pulsating DC, not smooth DC. A large filter capacitor after the bridge smooths it out into something usable for most electronics.

Zener Voltage Regulation

Connect a Zener in reverse with a series resistor and you get a simple voltage reference:

         R_series
Vin ----[  470Ω ]----+---- Vout (stabilized at V_zener)
                     |
                  [Zener]
                     |
                    GND

Not great for high-current loads — efficiency is poor and the regulation isn't tight. But for a simple voltage reference or protecting a logic input, it works fine.

Protection

Two common protection uses:

Flyback diode — across any inductive load (relay, motor, solenoid) connected to a switching transistor. When the transistor turns off, the inductor's stored energy produces a voltage spike. The diode clamps it, protecting the transistor. Connect cathode to the positive supply, anode to the transistor side.
Reverse polarity protection — a diode in series with the power input blocks reverse voltage if the supply is connected backwards. There's a Vf drop, but the circuit survives.