ITI/NCVT Mechanic Consumer Electronics (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

Current through a conductor equals voltage across it divided by resistance: I = V/R. Rearranged, V = I × R and R = V/I for DC circuits.

Resistor colour code

Bands give digits then multiplier then tolerance: brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold multiplier 0.1 and tolerance 5 percent.

Capacitor time constant

τ = R × C. A capacitor charged through a resistor reaches 63.2 percent of supply voltage after one τ and is effectively fully charged after five τ.

Transformer turns ratio

Secondary voltage divided by primary voltage equals secondary turns divided by primary turns. A step-down transformer raises current in the same ratio.

Diode forward voltage

A conducting silicon diode drops about 0.7 V, germanium about 0.3 V, Schottky about 0.3 V. An LED drops 1.8 to 3.3 V depending on colour.

Bridge rectifier ripple

A full-wave bridge produces ripple at twice the supply frequency, so 50 Hz mains gives 100 Hz ripple, which is easier to filter than half-wave ripple.

Zener series resistor

Rs equals supply voltage minus Zener voltage, divided by Zener current plus load current. The Zener must stay above knee current to regulate.

Linear regulator dissipation

Power wasted equals input voltage minus output voltage, multiplied by load current. Efficiency equals output voltage divided by input voltage.

SMPS start-up capacitor fault

A dried electrolytic start-up capacitor prevents the controller from starting, giving no output or a hiccup restart. It often reads normal capacitance on a meter.

Class AB amplifier

Two devices conduct slightly more than 180 degrees each, removing the crossover distortion of Class B while keeping efficiency well above Class A.

Decibel voltage gain

Voltage gain in decibels equals 20 times the logarithm of output voltage divided by input voltage. Power gain uses 10 times the logarithm.

Op-amp virtual short

With negative feedback and ideal infinite gain, both input terminals sit at nearly the same potential, so input voltage difference is treated as zero.

Slew rate limit

Slew rate in volts per microsecond caps the highest undistorted frequency: maximum frequency equals slew rate divided by 2π times peak output voltage.

NAND as universal gate

Any logic function can be built from NAND gates alone. Inverters come from a NAND with both inputs tied together.

De Morgan's theorem

The complement of a product equals the sum of complements, and the complement of a sum equals the product of complements. Used to convert gate implementations.

J-K flip-flop

An edge-triggered flip-flop that toggles when both J and K are high, sets when J alone is high and resets when K alone is high, with no invalid state.

ADC resolution

Smallest voltage step equals reference voltage divided by 2 to the power of bit count. An 8-bit converter on 5 V resolves about 19.6 mV.

UART baud rate

Asynchronous serial link where transmitter and receiver must use the same baud rate; framing adds a start bit, optional parity and one or two stop bits.

Amplifier output power

For a sine wave, output power equals RMS output voltage squared divided by load resistance. Peak voltage is 1.414 times the RMS value.

CRT EHT safety

The final anode of a colour CRT holds 20 to 30 kV and keeps charge after switch-off. Discharge to the aquadag earth before any service work.

Earth loop hum

Hum at 50 or 100 Hz caused by multiple earth paths between equipment. Breaking the loop with a single earth reference or isolation transformer removes it.

Motor start capacitor

A capacitor in series with the start winding shifts current phase to create starting torque. Failure gives humming without rotation and risks winding burnout.

Refrigeration cycle order

Compressor raises pressure, condenser rejects heat, expansion device drops pressure, evaporator absorbs heat. Frost on the evaporator with no cooling suggests defrost or capillary fault.

Microwave magnetron supply

The transformer plus high-voltage capacitor and diode form a doubler giving roughly 4 kV. The capacitor holds lethal charge after unplugging and must be discharged.

Carson's rule

FM bandwidth is approximately twice the sum of peak deviation and highest modulating frequency. Broadcast FM with 75 kHz deviation needs about 200 kHz.

Antenna length formula

Wavelength in metres equals 300 divided by frequency in megahertz. A quarter-wave monopole is one quarter of that length; a half-wave dipole is about 0.47 wavelength.

dBm scale

Power relative to one milliwatt in decibels. Minus 50 dBm is strong signal, minus 100 dBm is weak; each 3 dB change halves or doubles power.

Voltmeter and ammeter use

Voltmeter connects in parallel and has high resistance; ammeter connects in series and has low resistance. An ammeter placed across a supply causes a short circuit.

Diode test with DMM

Forward reading of about 0.5 to 0.7 V indicates a good silicon junction; near zero means shorted, open line in both directions means open circuit.

Signal tracing method

Inject a known signal at the input and follow it stage by stage until it disappears. The stage where it vanishes contains the fault.

Insulation resistance test

Measured with a 500 V insulation tester with the appliance unplugged and switches on. Typical pass value is at least 1 megohm from live parts to earth.

Warranty service record

Tag each returned unit with fault description and date, record serial number, diagnosis, parts and post-repair measurements, and safety-test before returning to the customer.