ITI/NCVT Mechanic Consumer Electronics (AITT Theory) key terms

A procedure for switching off and isolating a machine's energy source, then attaching a personal lock and warning tag so nobody can re-energise it while someone is working on it.

Equipment whose exposed metal parts are connected to the protective earth conductor, so a fault that energises the casing trips the supply instead of shocking the user.

A construction using basic insulation plus a supplementary layer, marked with a square-within-a-square symbol, that removes the need for a protective earth connection on Class II appliances.

A transformer with separate primary and secondary windings that breaks the direct conductive path to the mains, limiting shock current when servicing live equipment from an isolated bench supply.

The relationship stating that current through a conductor equals the voltage across it divided by its resistance; written V = I × R, it underpins almost every circuit calculation in servicing.

The rate at which a component converts electrical energy into heat, calculated as P = V × I or I²R; it determines the wattage rating a resistor or semiconductor must have.

Two or more resistors in series across a supply, where the voltage across each is proportional to its resistance; used for biasing, sensing and setting reference levels.

The voltage measured between two points in a circuit, representing the energy transferred per unit charge as it moves between them; always measured in parallel with the component.

A low-value, high-current resistor placed in parallel with a meter movement so that most current bypasses the coil, extending the measurable range without damaging it.

A polarised capacitor with a liquid or gel electrolyte giving large capacitance in a small body; used for smoothing and coupling, and prone to drying out and bulging with age.

The small internal resistance present in every real capacitor, especially electrolytics; a rise in ESR causes poor smoothing and erratic operation even when capacitance still reads correctly.

A diode designed to operate in reverse breakdown at a precise voltage, holding that voltage fairly constant; used for simple regulation, clamping and reference generation.

Four diodes arranged so that both half-cycles of an AC input are converted into unidirectional output, producing full-wave rectification with a ripple at twice the mains frequency.

The residual AC variation superimposed on a DC supply after rectification; it is reduced by smoothing capacitors and is a common cause of hum in audio equipment.

A circuit or IC that holds its output voltage constant despite changes in input voltage or load current; three-terminal types such as the 78xx series are widely used in consumer equipment.

A supply that chops the input at high frequency, transforms it in a small ferrite transformer and regulates by pulse-width control, giving high efficiency and wide input range.

A control method that varies the proportion of on-time in a fixed switching period; the average output follows the duty cycle, so it sets motor speed, lamp brightness or supply voltage.

The fraction of one switching period during which a signal or load is active, usually expressed as a percentage; it determines average power delivered by a PWM stage.

An LED and phototransistor pair that passes the output-voltage error signal back across the isolation barrier of a mains SMPS without an electrical connection.

The steady DC voltages and currents set at a transistor's terminals so it operates in the intended region, such as active, saturation or cutoff, without distorting the signal.

A push-pull output stage where each device conducts for slightly more than half the cycle, trading a little efficiency for the removal of crossover distortion; standard in audio power amps.

A notch or kink in the output waveform occurring at the moment one output device hands over to the other, caused by insufficient bias in a class B or AB stage.

Returning a fraction of the output signal to the input in opposing phase, which reduces gain but improves bandwidth, linearity and stability of an amplifier.

The maximum rate at which an amplifier output can change, usually quoted in volts per microsecond; too low a value rounds off fast transients and causes high-frequency distortion.

The constant figure obtained by multiplying an op-amp's open-loop gain by the frequency at which it is measured; it predicts the closed-loop bandwidth available at a chosen gain.

Two rules of Boolean algebra allowing a NAND to be rewritten as an inverted OR and a NOR as an inverted AND, used to simplify and convert logic expressions.

A grid layout of a truth table in which adjacent cells differ by one variable, allowing groups of ones to be circled and simplified into minimal logic expressions.

A table listing every possible combination of logic inputs alongside the required output for each, defining the behaviour of a gate or combinational circuit.

A combinational device that selects one of several data inputs according to address lines and routes it to a single output, often used to expand the input capacity of a microcontroller.

A chain of flip-flops that moves data one position per clock pulse; used for serial-to-parallel conversion, driving displays and extending a microcontroller's output pins.

A circuit that samples a continuous voltage and produces a proportional binary number, characterised by resolution in bits, sampling rate and reference voltage.

A two-wire serial bus using clock and data lines with device addressing, widely used inside televisions and appliances to link microcontrollers, memory and sensors.

A four-wire synchronous serial link with separate data-in and data-out lines, offering higher speed than I²C and used for flash memory, tuners and display modules.

A measurement of resistance from the plug's earth pin to exposed metalwork, confirming a low-impedance protective path on a Class I appliance, typically below one ohm.

A test applying a high DC voltage, usually 500 V, between live parts and accessible metal to confirm the insulation resistance exceeds a specified minimum, commonly one megohm.

Adjusting a probe's trimmer capacitor against the scope's calibration output so a square wave displays with flat tops, preventing waveform and amplitude errors at high frequency.

A combined instrument measuring voltage, current, resistance and often continuity or capacitance; correct function and range selection is essential to avoid damage or false readings.

A variable autotransformer that supplies an adjustable AC voltage from zero to above mains, useful for gently powering up suspect equipment while watching current draw.

An incandescent lamp wired in series with equipment under test; it limits fault current and its brightness indicates whether the device is drawing excessive current.

Reducing a component's permitted power or current as ambient temperature rises, ensuring junction or surface temperatures stay within safe limits during continuous operation.

A metal structure with fins that increases surface area to carry heat away from power semiconductors, usually mounted with thermal compound for good contact.