RBSE Sec Voc Electronics and Hardware cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series circuit: current is the same everywhere, resistances add, voltages divide in ratio to resistance.

Parallel circuit: voltage is the same across all branches, currents add, total resistance is less than the smallest branch.

Kirchhoff's current law: sum of currents entering a node equals the sum leaving; voltage law: sum of drops around a closed loop equals the applied EMF.

Conductor resistance R = ρL/A, so resistance rises with length and falls with cross-sectional area.

Alternating current: RMS value = peak ÷ √2 (about 0.707 × peak); average value of a full sine wave is zero.

Capacitive reactance Xc = 1/(2πfC) falls with frequency; inductive reactance Xl = 2πfL rises with frequency.

Resonance occurs when Xl = Xc, giving minimum impedance in a series tuned circuit.

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% tolerance band.

Four-band resistor: first two bands are digits, third is the multiplier (number of zeros), fourth is tolerance.

Capacitor codes: 104 means 10 × 10⁴ pF = 100 nF; ceramic capacitors drift with temperature and voltage, electrolytics are polarized.

Electrolytic capacitors are polarized: reversing the polarity damages them and can cause rupture.

Inductor impedance rises with frequency; a ferrite core increases inductance, and the value is stated in henries (H, mH, µH).

Potentiometer is a three-terminal variable resistor used as a voltage divider; a rheostat uses two terminals to control current.

Transformer turns ratio: Vp/Vs = Np/Ns; a step-down transformer has fewer secondary turns and higher secondary current.

Relay coil voltage and contact rating are separate specifications; the contact rating limits the load it may switch.

Diode: conducts when anode is positive relative to cathode (forward bias, about 0.7 V drop for silicon, 0.3 V for germanium).

A bridge rectifier uses four diodes to give full-wave rectification; output ripple frequency is twice the input frequency.

Zener diode operates in reverse breakdown to regulate voltage; a series resistor limits current through it.

Bipolar transistor: NPN turns on when base is about 0.7 V above emitter; current gain β = Ic/Ib.

Transistor regions: cutoff (no collector current), active (amplifier use), saturation (switch fully on, Vce near 0.2 V).

MOSFET is voltage-controlled; gate current is negligible but the gate is static-sensitive and needs a bleed resistor.

LED requires a series resistor: R = (Vsupply − Vled) / Iled.

SCR latches on once gate-triggered and turns off only when anode current falls below the holding current.

AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts.

NAND and NOR are universal gates - any logic function can be built from either alone.

XOR outputs 1 when inputs differ; it is the basis of half-adder sum output, with AND giving the carry.

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

Binary to decimal: each bit has weight 2ⁿ from the right starting at 2⁰ = 1.

A flip-flop stores one bit; a D flip-flop copies input to output on the clock edge, a JK flip-flop toggles when J = K = 1.

A multiplexer selects one of many inputs; a decoder converts binary code to one active output line.

Seven-segment display uses a decoder/driver such as the 7447 for common-anode or 7448 for common-cathode types.

Ammeter is connected in series and has very low resistance; voltmeter is connected in parallel and has very high resistance.

Multimeter on ohms range requires the circuit to be unpowered, otherwise readings are false and the meter may be damaged.

Oscilloscope: measure peak-to-peak voltage from vertical divisions × volts/div, and period from horizontal divisions × time/div; f = 1/T.

Continuity test confirms a low-resistance path; a beep typically indicates below roughly 30–50 Ω depending on the meter.

A clamp meter measures AC current by sensing the magnetic field around a conductor and needs only one conductor inside the jaw.

Analog multimeter must be zeroed on the ohms range before measuring, and read on the correct scale to avoid parallax error.

Logic probe indicates high, low, and pulsing states; it is used for digital troubleshooting, not for analog levels.

Insulation resistance is measured with a megger at high test voltage with the circuit isolated and discharged.

Solder iron tip temperature for leaded solder is typically 300–350 °C; lead-free solder needs roughly 350–380 °C.

Good solder joint is shiny, concave, and smooth; a dull grainy joint indicates a cold joint caused by insufficient heat.

Heat the pad and lead together, then apply solder to the joint - never to the iron tip.

Solder flux removes oxide and improves wetting; rosin flux is standard for electronics, acid flux is not.

ESD precautions: wrist strap with 1 MΩ resistor, grounded mat, and store ICs in conductive foam.

PCB track side is the copper side; component side carries the body. Observe polarity marks for electrolytics and diodes.

Desoldering uses a solder sucker or desoldering braid; add fresh solder first to improve heat transfer on old joints.

Dry joints, solder bridges, and lifted pads are the three most common hand-soldering defects to inspect for.

POST beep codes indicate faults before video initializes; continuous beeps usually point to RAM or power problems.

RAM types are not interchangeable: DDR, DDR2, DDR3, and DDR4 have different notch positions and voltages.

SATA III peak is 6 Gb/s; IDE/PATA is obsolete and uses a 40-pin ribbon cable.

SMPS colour code (ATX): yellow = +12 V, red = +5 V, orange = +3.3 V, black = ground, purple = +5 V standby.

Thermal paste goes between CPU and heatsink in a thin layer; excess paste reduces cooling performance.

CMOS battery (CR2032) maintains BIOS settings and real-time clock when power is removed.

Printer maintenance: laser printers use toner and a drum, inkjets use cartridges and are prone to nozzle clogging if unused.

Backup strategy: keep at least two copies on different media, one off-site, and verify restores periodically.

Linear supply stages: transformer → rectifier → filter → regulator; the regulator dissipates (Vin − Vout) × I as heat.

SMPS achieves high efficiency by switching at high frequency, so the transformer and filter can be much smaller.

Ripple is the residual AC on a DC output; larger filter capacitance and higher load resistance reduce it.

Half-wave rectifier ripple frequency equals the supply frequency; full-wave and bridge give twice the supply frequency.

78xx series regulators give fixed positive output (7805 = +5 V); 79xx series give fixed negative output.

SMPS output capacitors hold charge after power-off - discharge and verify before servicing.

Crowbar and overvoltage protection circuits clamp the output to protect the load if the regulator fails.

UPS types: offline switches over on failure, online double-conversion isolates the load continuously.

Current, not voltage, causes injury: roughly 1 mA is perceptible, 10–30 mA can cause muscle lock, and 50 mA and above can be fatal.

Rule for mains work: isolate, lock out/tag out, test for dead with a proven tester, then work.

Use one hand where possible when probing live circuits to avoid a hand-to-hand path across the heart.

First aid for electric shock: isolate the supply before touching the casualty, then call for help and start CPR if trained.

Class C fire extinguisher (CO₂) is used on live electrical fires; never use water.

Soldering safety: fume extraction, safety glasses, iron in a stand, and never catch a falling iron.

ESD damage can be latent - a device may fail later, so handling precautions are mandatory even if the part still works.

Lead solder waste and dross are hazardous waste and must not go into general refuse.

GST invoice must show GSTIN, invoice number and date, HSN/SAC code, taxable value, tax rate and amount, and place of supply.

E-way bill is generally required for movement of goods above the prescribed value threshold; it carries a validity period based on distance.

Quotation states price, validity, payment terms, and delivery; a proforma invoice precedes supply and is not a demand for payment.

Purchase order is the buyer's authorization to supply; delivery challan accompanies goods without transferring ownership.

Break-even quantity = fixed cost ÷ (selling price − variable cost per unit).

Working capital = current assets − current liabilities; it funds day-to-day operations.

A small enterprise can register on the Udyam portal to access MSME benefits and priority lending.

Employability basics: a resume lists skills and measurable achievements, and an interview answer should follow situation–task–action–result structure.

Warranty card and test certificate are commonly supplied with electronic hardware and support after-sales claims.