ITI/NCVT Mechanic Consumer Electronics (AITT Theory) cheat sheet

Electric shock severity depends on current through the body, not voltage alone: roughly 1 mA is perceptible, 10-20 mA causes muscle tetany (victim cannot let go), 50-100 mA across the heart can cause ventricular fibrillation.

Rule of thumb for body resistance: dry intact skin plus internal tissue is about 1-10 kΩ hand-to-hand; wet or broken skin drops this to a few hundred ohms, so 230 V can drive lethal current.

Protective earth (PE) keeps exposed metalwork at ground potential; the earth conductor must never carry operating current and must never be fused or switched.

Class I appliances rely on PE bonding; Class II (double-insulated, square-in-square mark) rely on reinforced/basic insulation and have no PE; Class III runs at safety extra-low voltage (≤50 V AC).

An RCD/ELCB trips on residual current imbalance (typically 30 mA for personnel protection, 100-300 mA for fire protection); it does not protect against overload or short circuit, so it is used with an MCB.

Isolation rule before servicing: switch off, lock/tag, verify dead with a tested voltage tester on all poles, then discharge capacitors and prove the tester again afterwards.

A 1 kΩ resistor in series with a test probe limits current to about 230 mA at 230 V, reducing arc-flash and shock energy during live measurements.

Lead-acid and Li-ion cells are hazardous waste: never incinerate or landfill; Ni-Cd is restricted under e-waste rules, and Li-ion cells at low state of charge are shipped as Class 9 dangerous goods.

Soldering iron safety: tip temperatures reach 300-400 °C, use a fume extractor for rosin flux, and keep a wet sponge or brass wool plus a fire-safe stand at the bench.

Ohm's law V = I × R; power P = V × I = I²R = V²/R. Series resistances add; parallel conductances add (1/Rt = 1/R1 + 1/R2).

Voltage divider: Vout = Vin × R2/(R1+R2) only when no load current is drawn; loading the tap lowers Vout.

Capacitor impedance Zc = 1/(2πfC); it blocks DC and passes AC. Time constant τ = RC; after 1τ a capacitor reaches 63.2 % of final voltage, after 5τ it is effectively charged.

Inductor impedance Zl = 2πfL; it passes DC and opposes changes in current. Energy stored = ½LI²; capacitor energy = ½CV².

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold = ±5 %, silver = ±10 %, 4th band is multiplier.

Diode forward drop is about 0.7 V for silicon and 0.3 V for germanium; a Schottky is about 0.3 V and an LED 1.8-3.3 V depending on colour.

Transformer relation: Vs/Vp = Ns/Np = Ip/Is. A step-down transformer raises secondary current; VA rating is set by core and winding size, not by the load.

A relay coil is inductive, so a flyback diode across it clamps the switch-off spike; a triac or SCR latches on once gate current fires it and turns off only at zero crossing (triac) or when current falls below holding level (SCR).

BJT regions: cutoff (Vbe < 0.6 V), active (Ic = βIb), saturation (Vce ≈ 0.2 V). For switching, drive the base hard into saturation to minimise dissipation.

MOSFET is voltage-controlled with very high gate resistance; it needs gate voltage above threshold and a gate resistor plus pull-down to avoid spurious turn-on.

Half-wave rectifier: output ripple frequency = supply frequency, low efficiency. Full-wave/bridge: ripple = 2× supply frequency, PIV of each bridge diode = peak secondary voltage.

Filter capacitor sizing rule: C ≈ Iload/(f_ripple × Vripple). Larger C reduces ripple but raises surge current through the rectifier.

Zener regulator: series resistor Rs = (Vin − Vz)/(Iz + Iload); the Zener must be biased above its knee current to regulate.

Linear regulator dissipation P = (Vin − Vout) × Iload; efficiency = Vout/Vin, so a 5 V output from 12 V is at best about 42 % efficient.

SMPS topologies: buck steps down, boost steps up, buck-boost inverts. Switching frequency (tens of kHz) allows small magnetics; feedback via optocoupler maintains isolation.

Common SMPS faults: dried electrolytic start-up capacitor (no or hiccup start), shorted chopper/MOSFET from surge, open current-sense resistor, and blown fuse caused by the chopper, not the fuse itself.

Transistor bias: voltage-divider bias is the most stable for BJT amplifiers; the emitter resistor provides negative feedback against β and temperature variation.

Class A conducts 360°, Class B 180° per device, Class AB slightly more than 180° to remove crossover distortion, Class D switches at high frequency for efficiency above 80 %.

Gain in dB: voltage gain (dB) = 20 log(Vout/Vin); power gain (dB) = 10 log(Pout/Pin). −3 dB point is where power falls to half, i.e. 0.707 of mid-band voltage.

Op-amp ideal rules: infinite open-loop gain and input impedance, zero output impedance; with negative feedback the two input terminals are at equal potential (virtual short).

Inverting amplifier gain = −Rf/Rin; non-inverting gain = 1 + Rf/Rin; voltage follower gain = 1 with very high input impedance.

Slew rate limits high-frequency large-signal output: max frequency without distortion f = SR/(2πVpeak).

Negative feedback widens bandwidth, lowers distortion and output impedance, and stabilises gain at the cost of reduced open-loop gain.

Tone control (Baxandall) shelves treble and bass around a mid-band reference; a loudness contour boosts bass and treble at low volume to compensate for equal-loudness hearing curves.

Logic gates: AND outputs 1 only when all inputs are 1; OR when any input is 1; XOR when inputs differ. NAND and NOR are universal gates.

De Morgan: (A·B)' = A' + B' and (A+B)' = A'·B'. Use it to convert between AND-OR and NAND-only implementations.

Boolean simplification via Karnaugh map groups powers of two (1, 2, 4, 8) of adjacent cells; larger groups give simpler terms.

Flip-flops: SR is level-sensitive and has an invalid state when both inputs are 1; JK removes that; D is transparent latch or edge-triggered; T toggles on each clock.

Counters: a ripple counter of n flip-flops divides by 2^n; modulus = 2^n, so a divide-by-10 counter needs 4 flip-flops with reset at count 10.

A 4-bit binary number has 16 states; an 8-bit DAC with 5 V reference has resolution 5/255 ≈ 19.6 mV per LSB.

Microcontroller essentials: GPIO, ADC (SAR type, resolution = Vref/2^n), timers, UART (asynchronous, baud rate must match), I²C (2-wire, addresses), SPI (4-wire, full duplex).

8051 basics: 8-bit core, 4 KB on-chip ROM in 8051, 128 bytes RAM, two 16-bit timers, 4 ports of 8 bits, and an active-low EA pin selecting internal or external program memory.

Audio power amplifier output power P = Vrms²/Rload; a ±35 V rail amplifier driving 8 Ω gives about 76 W peak, roughly 38 W RMS sine.

Distortion checks: THD below 0.1 % is typical of good hi-fi; hum at 50/100 Hz points to supply filtering or earth loop, while hiss indicates a noisy preamp stage.

Amplifier protection circuits sense DC offset at the output and open the speaker relay; a stuck relay usually means a failed output device or driver.

Cassette/turntable: RIAA equalisation boosts bass and cuts treble on playback; wow is slow pitch drift (once per revolution), flutter is fast (once per capstan).

CRT television: the EHT anode runs at 20-30 kV and retains charge after switch-off; discharge it to the aquadag spring with a proper probe before touching the tube.

CRT deflection: vertical yoke coils at field rate (50/60 Hz) and horizontal at line rate (15.6 kHz for 625-line PAL); loss of horizontal drive can destroy the line output transistor.

LCD/LED TV backlight: an open LED in one series string blanks the whole panel; test with a backlight tester and check the boost converter and LED driver IC.

Colour TV signals: luminance (Y) carries brightness, chrominance (C) carries hue and saturation; PAL uses 4.43 MHz subcarrier with phase alternation to cancel hue errors.

Electric iron: thermostat bimetallic strip opens the element circuit at set temperature; a blown element or welded contact gives no heat or continuous overheating.

Induction motor (fan, washing machine, compressor) direction reverses by swapping two supply leads to the run and start windings, not by reversing the mains.

Single-phase motors need a start winding with capacitor (permanent-split or capacitor-start); a failed capacitor causes humming without rotation and possible winding burnout.

Washing machine: inlet valve fills on pressure-switch signal, drain pump empties, and the door interlock must be closed before the motor can run.

Refrigerator cycle: compressor raises pressure and temperature, condenser rejects heat, capillary/expansion valve drops pressure, evaporator absorbs heat. Frost on the evaporator with no cooling usually means a defrost-timer/heater fault or a blocked capillary.

Microwave oven: the magnetron needs about 4 kV from the transformer plus doubler; the high-voltage capacitor and diode can retain lethal charge after unplugging, so discharge before service.

Air conditioner: cooling capacity in tonnes (1 tonne ≈ 3.5 kW); a dirty condenser raises head pressure and current, while low refrigerant causes low suction pressure and frost at the service valve.

Heating appliances: power P = V²/R; a 230 V, 1000 W element draws about 4.35 A and has R ≈ 53 Ω cold, higher when hot.

Modulation: AM varies carrier amplitude with sidebands at fc ± fm; FM varies frequency with deviation proportional to audio amplitude and is more resistant to amplitude noise.

FM deviation and bandwidth: Carson's rule BW ≈ 2(Δf + fm). Broadcast FM uses 75 kHz deviation giving about 200 kHz channel spacing.

Antenna basics: a half-wave dipole is about 0.47 λ long; a quarter-wave monopole over a ground plane is 0.25 λ. Length in metres = 300/f(MHz) × factor.

Mobile handset blocks: antenna switch/diplexer, RF transceiver, baseband processor, power amplifier with power control, SIM interface, and power management IC.

A phone that draws no charge current but runs on battery points to a charge-port or charging IC fault; a phone that heats and drains quickly often has a shorted PA or a swollen battery.

SIM interface is a smart-card link (clock, reset, data, Vcc); no SIM detected is often a bent SIM connector or a missing supply rail, not a network fault.

Bluetooth 2.4 GHz ISM band uses frequency hopping; Wi-Fi 2.4 GHz channels 1, 6 and 11 are non-overlapping, while 5 GHz offers more channels and shorter range.

Signal strength is measured in dBm; −50 dBm is strong, −100 dBm is weak. Each 3 dB change halves or doubles power, and each 6 dB roughly halves range in free space.

Ammeter is connected in series and has very low resistance; voltmeter is in parallel and has very high resistance. Connecting an ammeter across a supply shorts it.

Measure resistance only with the circuit unpowered and capacitors discharged; in-circuit readings are lower than marked value because parallel paths shunt the component.

Diode test on a DMM shows about 0.5-0.7 V forward for silicon; a reading near 0 V means short, OL in both directions means open.

Oscilloscope: probe ×10 divides signal and raises input impedance; compensate the probe on the calibration square wave before measuring amplitude or rise time.

CRO timebase: period = time/div × horizontal divisions; frequency = 1/period. Use the vertical position and AC/DC coupling to keep the trace on screen.

Signal tracing divides faults in half: inject a signal at the input and follow stage by stage until it disappears. Voltage mapping against a known-good set of node voltages localises the faulty stage.

Intermittent faults: apply gentle flexing, heat or cold spray to suspect joints and components while monitoring; cracked solder joints on connectors and power devices are common.

Soldering quality: a good joint is shiny, concave and wets both pad and lead; a dull ball or a spike indicates cold solder, insufficient flux or a dirty surface.

Cable sizing must satisfy current-carrying capacity, voltage drop (typically ≤3 % for a final circuit) and protective-device coordination; earth continuity is verified after installation.

Earthing and polarity checks before energising: earth-to-neutral resistance, insulation resistance (≥1 MΩ at 500 V for typical appliance circuits) and correct line/neutral at the socket.

Antenna installation: mount for line of sight, keep the down-lead short, bond the mast to earth, and aim using signal strength or BER rather than guesswork.

Preventive maintenance schedule: clean condenser coils and filters every 1-3 months for cooling appliances, check door gaskets, lubricate moving parts, and verify earth continuity annually.

Customer handover should include operating instructions, safety warnings, warranty terms and a written service report listing the fault found, parts replaced and measured values.

When a repair is uneconomical, state the reason in measurable terms (part cost versus replacement value) and record the customer's decision rather than deciding silently.

Goods returned under warranty must be tagged with fault description and date; never return a device to the customer without a functional and insulation safety test.

Documentation: record serial number, model, symptoms, diagnosis, parts, and post-repair measurements; this record is the basis for repeat-fault analysis and warranty claims.