A diode is a two-terminal part that passes current in one direction and blocks it in the other. The terminals are the anode (current enters) and the cathode (current leaves, marked with a stripe or bar). The symbol's triangle points the way conventional current flows.
The one-way valve
A diode is a PN junction. Forward-biased, it conducts once the voltage passes its threshold; reverse-biased, almost nothing flows.
Below its threshold a diode is nearly off. Above it, current rises steeply.
Once conducting, a diode drops roughly its threshold voltage whatever the current, so it dissipates (forward voltage × current) as heat. Excess current overheats the junction, a common way for a diode to fail. Every diode has a maximum forward current and a peak inverse voltage (PIV), the most reverse voltage it can block before breaking down.
The family
Figure E6-2 symbols, with a hook to recognize each and what it does.
The SCR and triac in the gallery are not true diodes but switches built from several junctions: they latch on when triggered and are used to control AC power.
Zener regulation
An ordinary diode is not meant to run in reverse breakdown. A Zener is built to: above its rated voltage it conducts in reverse and holds that voltage across itself.
Illustrative ideal Zener. The Zener soaks up whatever current the load does not take, so the output voltage barely moves.
Put a Zener across the load behind a series resistor and it takes whatever current the load leaves over. Input voltage changes land on the resistor, not the output. The resistor must limit the current so the Zener stays inside its power rating. For anything demanding, an IC regulator does the same job better; see Power supplies and regulators.
In practice
Rectifying: a diode turns the alternating current of a transformer into pulses of one polarity. A capacitor then smooths them.
Detecting: a diode with a capacitor strips the RF from an AM signal and leaves the audio; see Detectors and demodulators.
Mixing: Schottky diodes in a balanced ring are a classic mixer.
Relay protection: a coil that is suddenly switched off produces a large voltage spike. A diode across the coil, reverse-biased in normal operation, absorbs it.
Reverse polarity: a series diode in a 12 V supply line stops a reversed connection from reaching the radio, at the price of its forward drop.