WBBSE Madhyamik Electronics (Vocational) cheat sheet

Ohm's law: V = I × R; rearranged I = V/R and R = V/I. V in volts, I in amperes, R in ohms.

Power: P = V × I = I²R = V²/R. Energy consumed = P × t; 1 unit (kWh) = 1000 W running for 1 hour.

Series circuit: same current through all elements; R_total = R1 + R2 + …; voltages divide in proportion to resistance.

Parallel circuit: same voltage across all branches; 1/R_total = 1/R1 + 1/R2 + …; currents divide inversely with resistance.

Kirchhoff's current law: sum of currents entering a node equals sum leaving. Voltage law: algebraic sum of voltages round a closed loop is zero.

Conductor resistance R = ρL/A: rises with length, falls with area, and increases with temperature for metals (positive temperature coefficient).

AC quantities: RMS = peak/√2 ≈ 0.707 × peak; peak = 1.414 × RMS; frequency f = 1/T; mains supply in India is 230 V RMS, 50 Hz.

Capacitive reactance Xc = 1/(2πfC) falls as frequency rises; inductive reactance XL = 2πfL rises with frequency; at resonance XL = Xc.

Resistor colour code (B-B-ROY-G-BV): black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.

Four-band resistor: band 1 = first digit, band 2 = second digit, band 3 = multiplier (power of ten), band 4 = tolerance (gold ±5%, silver ±10%).

Common resistor types: carbon film and metal film (general use), wire-wound (high power), and potentiometers/presets (variable).

Capacitor codes: 104 means 10 × 10⁴ pF = 100 nF = 0.1 µF; 103 = 10 nF; 102 = 1 nF.

Ceramic capacitors are non-polarised and suit high-frequency work; electrolytic capacitors are polarised, have large values, and must observe +/− polarity.

Capacitor testing with a multimeter on ohms range: the reading should climb towards infinity as it charges; a steady low reading indicates a short.

Inductors store energy in a magnetic field (W = ½LI²); they oppose changes in current and are used in filters, chokes, and transformers.

Transformer relation: Vp/Vs = Np/Ns = Is/Ip. A step-down transformer reduces voltage and increases available current.

Silicon and germanium are the common semiconductor materials; silicon diodes have a forward drop of about 0.7 V, germanium about 0.3 V.

A PN junction conducts when forward biased (P to +, N to −) and blocks when reverse biased; reverse breakdown is normal for Zener diodes.

Rectifier diode (e.g. 1N4007): 1 A forward current, 1000 V peak reverse voltage. Check with a multimeter diode test: about 0.6–0.7 V forward, open circuit reverse.

A Zener diode is operated in reverse breakdown to give a stable reference voltage; a series resistor limits current: Rs = (Vin − Vz)/Iz.

BJT has three terminals (emitter, base, collector), two junctions, and comes in NPN and PNP types; it is current controlled: Ic = β × Ib.

BJT regions: cutoff (both junctions off, acts as open switch), active (amplifier), saturation (both junctions on, acts as closed switch).

For a silicon BJT, VBE ≈ 0.7 V in the active region; the base-emitter junction behaves like a forward-biased diode.

JFET is voltage controlled: drain current is set by the gate-source voltage, and the gate junction is always reverse biased for normal operation.

Half-wave rectifier: one diode, output ripple frequency = supply frequency (50 Hz), efficiency about 40.6%.

Full-wave centre-tap rectifier: two diodes, ripple frequency 100 Hz, efficiency about 81.2%; each diode must withstand 2 × peak voltage.

Bridge rectifier: four diodes, 100 Hz ripple, efficiency about 81.2%; needs no centre tap and suits transformer secondary windings well.

Filter rule: a capacitor in parallel with the load smooths ripple; larger C and lighter load give smaller ripple. A choke (L) in series also filters.

Amplifier gain: voltage gain Av = Vout/Vin; power gain in dB = 10 log₁₀(Pout/Pin); voltage gain in dB = 20 log₁₀(Vout/Vin).

Class A amplifier conducts for the full 360° cycle with low distortion but low efficiency (max about 25%); Class B conducts 180°; Class AB reduces crossover distortion.

Common-emitter amplifier gives voltage gain and 180° phase inversion; common-collector (emitter follower) gives unity voltage gain and current gain with no inversion.

Oscillator needs positive feedback with loop gain ≥ 1 and 360° total phase shift; the Wien bridge and phase-shift types use RC networks to set frequency.

Number conversion: binary 1011 = 8+0+2+1 = 11 decimal; hexadecimal digits 0–9 and A–F correspond to 4-bit groups.

Boolean identities: A + 0 = A, A · 1 = A, A + 1 = 1, A · 0 = 0, A + A = A, A · A = A, A + Ā = 1, A · Ā = 0.

De Morgan's theorems: (A + B)′ = A′ · B′ and (A · B)′ = A′ + B′.

Basic gates: AND (output 1 only if all inputs 1), OR (output 1 if any input 1), NOT (inverter); NAND and NOR are universal gates.

XOR gives output 1 for an odd number of 1 inputs; it is used in adders and parity circuits.

Half adder: Sum = A ⊕ B, Carry = A · B. Full adder adds a carry-in and is built from two half adders plus an OR gate.

Combinational circuits (adders, multiplexers, decoders) have no memory; sequential circuits (flip-flops, counters, registers) store state and are clock driven.

A JK flip-flop with J = K = 1 toggles on each clock; a D flip-flop simply copies input D to output Q at the clock edge.

Multimeter: connect in series to measure current, in parallel to measure voltage; select the correct AC/DC range above the expected value.

Measuring resistance requires the circuit to be unpowered; zero the analogue meter first and choose a range near mid-scale for best accuracy.

A CRO displays voltage against time. Time period = time/div × horizontal divisions; voltage = volts/div × vertical divisions.

CRO controls: intensity and focus adjust the trace, trigger stabilises the waveform, and the probe is switched ×1 or ×10.

Signal generator supplies sine, square, and triangular waves over a set frequency and amplitude for testing amplifier response.

Soldering: use a 25–40 W iron, tin the tip, heat the joint and apply solder to the joint (not the iron), and clean with flux.

PCB practice: keep tracks short and wide for current, avoid solder bridges, and check continuity with a multimeter after assembly.

Use a series lamp or isolation transformer while testing mains-powered circuits; never probe live mains with an ordinary multimeter on the wrong range.

Linear regulated supply chain: transformer → rectifier → filter → regulator (e.g. 7805 for +5 V, 7812 for +12 V, 7905 for −5 V).

The 78xx series needs input at least 2–3 V above output; it limits current internally and must be heatsinked at higher currents.

SMPS (switch-mode power supply) is more efficient and lighter than a linear supply; it is common in TVs, chargers, and computers.

Transformer faults: open winding gives no output, shorted turns cause overheating and blown fuses; check winding continuity with a multimeter.

First checks on a dead appliance: mains plug and fuse, power cord continuity, transformer output, and DC rail voltage at the filter capacitor.

No picture with sound in a TV often points to the backlight or display driver; no sound with picture points to the audio amplifier stage.

Electric iron, heater, and fan faults are often open heating elements, broken cords, or worn brushes; test continuity before replacing parts.

A fuse must be replaced with the same rating; a blown fuse repeatedly indicates a short circuit, not a faulty fuse.

Current is the hazard: about 1 mA is perceptible, 10–30 mA can cause muscle contraction, and 50 mA across the heart can be fatal.

Body resistance falls when skin is wet, so shock risk rises sharply in damp conditions; use rubber-soled footwear and insulated tools.

Earth wire (green/yellow) connects metal casings to ground; a blown earth connection makes an appliance chassis live.

Use an MCB or fuse for overload and short-circuit protection and an ELCB/RCCB (30 mA) for shock protection.

First aid for electric shock: switch off supply or separate the victim with dry insulating material, then call help and give CPR if trained.

Never work on live circuits; isolate, lock/tag, discharge capacitors, and confirm dead with a tester before touching.

E-waste contains lead, mercury, and cadmium; return it to authorised recyclers and never burn cables to recover copper.

Store lithium batteries and capacitors safely, keep flammables away from soldering stations, and ventilate areas used for flux and solder fumes.

Service-shop costing: charge = parts cost + labour + overhead + profit margin; issue a written estimate before starting repair.

Keep a job register with customer name, appliance, fault reported, parts replaced, charge, and warranty period.

Customer handling: listen fully, explain the fault in simple language, give a realistic time and cost, and return replaced parts on request.

Self-employment options include repair services, PCB assembly, inverter and stabiliser sales, LED lighting installation, and e-waste collection.

For a small unit, register as a sole proprietor, obtain GST registration if turnover crosses the threshold, and keep purchase and sale records.

Basic tools for a start-up: multimeter, soldering station, CRO or DSO, signal generator, bench supply, hand tools, and a component stock.

Warranty on repairs should be stated in writing; honour it to build repeat business and referrals.

Skill pathways after Madhyamik include ITI electronics, polytechnic diploma, and vocational training; apprenticeships give supervised shop-floor experience.

Measurement discipline: always confirm the meter function and range before connecting, and verify the meter on a known source.

Component identification: read resistor colour bands and capacitor codes before fitting, and confirm polarity of diodes and electrolytic capacitors.

Signal path thinking: trace from supply to load stage by stage (transformer, rectifier, filter, regulator, load) when fault-finding.

Digital basics: convert between binary, decimal, and hex; test logic levels (0 V ≈ logic 0, 5 V ≈ logic 1) with a multimeter or logic probe.

Safety priority order: isolate supply, discharge stored energy, verify dead, then work; restore supply only after reassembly.

Documentation: keep a schematic and a record of measured voltages at key test points for every repaired unit.

Environmental duty: segregate e-waste, avoid burning plastics or cables, and dispose of solder dross and batteries through authorised channels.

Business basics: separate personal and business money, price for profit, and keep a simple ledger of income and expenses.