Electrical Components & Electronics

A fixed resistor provides a specified resistance. For a fixed applied voltage, increasing resistance decreases current. Ohm's law describes the relationship: V = IR and I = V/R.

A resistor can help prevent excessive current from reaching a component. LEDs generally require current limiting, and a resistor is commonly used for this purpose in a simple LED circuit.

A variable resistor has adjustable resistance. A rheostat adjusts resistance in a circuit. A potentiometer usually has three terminals and can provide adjustable voltage, so it is commonly used in volume controls and other adjustable controls.

Resistors convert electrical energy into heat and must have an appropriate power rating. For introductory ASVAB questions, focus on resistance, current limiting, and variable resistance.

When voltage is applied, a capacitor accumulates charge and can later release stored energy into a circuit. Unlike a battery, it generally stores much less energy and can often charge and discharge quickly.

In a simple DC circuit, current may flow briefly while a capacitor charges. Once an ideal capacitor is fully charged to a constant applied DC voltage, current through it stops. An ideal capacitor blocks steady-state DC current.

Capacitors can smooth power-supply fluctuations, filter signals, couple changing signals between circuit stages, and store small amounts of electrical energy. After AC is rectified, a capacitor charges as voltage rises and discharges as it falls, reducing ripple. It does not create energy or independently convert AC into DC.

Current through a coil creates a magnetic field. If current changes, the field changes. An inductor opposes changes in current. Capacitors oppose sudden changes in voltage; inductors oppose sudden changes in current.

When DC is first applied, an inductor resists the initial change in current. Once current is steady, an ideal inductor behaves like a conducting path with no resistance, though real coils have wire resistance. Alternating current continually changes, so an inductor opposes AC; ideal inductive reactance increases as AC frequency increases.

Inductors appear in filters, power supplies, electronic circuits, motors, and some radio-frequency circuits. Coils are also important in transformers and electromagnets.

A switch makes or breaks an electrical connection. An open switch interrupts its conducting path, and a closed switch completes it. A normally open (NO) switch is open when unactivated and closes when activated. A normally closed (NC) switch is closed when unactivated and opens when activated.

A relay is an electrically operated switch. In an electromagnetic relay, current through a coil creates a magnetic field that moves contacts to open or close another circuit. Relays enable remote switching, automated controls, and control of higher-power loads with lower-power control circuits.

Semiconductors are the foundation of diodes, transistors, integrated circuits, computer processors, and solar cells.

Doping adds controlled impurities to change electrical properties. In N-type semiconductor material, electrons are majority mobile charge carriers. In P-type material, holes are majority carriers. Where P-type and N-type regions meet, they form a P-N junction, which is fundamental to many diodes and transistors.

A diode is forward biased when voltage is applied in its normal conducting direction; with sufficient forward voltage, it conducts. Reverse bias applies voltage in the opposite direction, where a normal diode largely blocks current. Diodes are used for rectification, direction control, protection, and signal processing.

An LED emits light when forward biased and operated within its rated current range. It is polarized and commonly uses a resistor to limit current. A rectifier uses diodes to convert AC into unidirectional current. A half-wave rectifier uses one half of the waveform; a full-wave rectifier uses both. Capacitors reduce remaining output ripple.

A transistor can act as an electronically controlled switch: a small control signal turns current through another part of a circuit on or off. It can also control a larger output signal in response to a smaller input signal. An amplifier draws added energy from its power supply; it does not create energy.

A bipolar junction transistor (BJT) has base, collector, and emitter terminals. In a common introductory arrangement, a small base current controls a larger collector-emitter current. A field-effect transistor (FET), including a MOSFET, uses an electric field: voltage at its gate controls current between drain and source.

A resistor is often a zigzag line or rectangle. A variable resistor has an arrow or adjustable contact. A capacitor has two parallel plates, an inductor a coil-like series of loops, and a switch a break with movable contact. The diode bar marks the cathode; LED arrows point outward. A battery is alternating long and short parallel lines. Symbols can vary slightly by drawing standard.

Stores energy in an electric field: capacitor. Stores energy in a magnetic field: inductor. Uses a control signal to electronically switch or amplify: transistor.