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A
To convert DC to AC
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B
To store electric charge
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C
To amplify a signal
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D
To allow current to flow in only one direction (acts as a one-way valve for electricity)
Why this is the answer
Diode: semiconductor device that allows current in one direction (FORWARD) and blocks it in the reverse direction. Terminals: ANODE (+) and CATHODE (-). Current flows from anode to cathode when forward-biased (anode positive relative to cathode); blocked when reverse-biased. Voltage drop: silicon diodes have ~0.6-0.7V forward voltage drop when conducting; germanium ~0.3V; Schottky ~0.2V; LEDs vary 1.5V (red) to 3.5V (white/blue). Symbol: triangle pointing to a vertical line; current direction follows the triangle's point. Stripe on diode body marks cathode end. Applications: (1) RECTIFICATION — convert AC to pulsing DC: HALF-WAVE rectifier uses one diode (allows only half of AC cycle); FULL-WAVE uses two diodes with center-tapped transformer; BRIDGE rectifier uses four diodes for full-wave from any AC source (most common); (2) PROTECTION — protect circuits from reverse polarity; (3) FREE-WHEELING — protect against voltage spikes from inductive loads (motors, relays) — diode connected across inductor allows the current to circulate safely when switched off; (4) VOLTAGE REGULATION — ZENER DIODES are designed to operate in reverse breakdown at a precise voltage; used for voltage references; (5) SIGNAL DEMODULATION — extracting audio from radio carrier waves; (6) LEDs — special diodes that emit light; used for indicators, displays, lighting (highly efficient compared to incandescent); (7) PHOTODIODES — convert light to current (opposite of LEDs); (8) LASER DIODES — emit coherent light; used in laser pointers, optical disc readers, fiber optic communications. ASVAB EI may show schematic symbols — recognize the diode triangle-and-line symbol with current flow direction.
Source: ASVAB EI, Diodes