Maharashtra SSC Electronics and Hardware Pre-Voc/NSQF cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Units: volt, ampere, ohm, watt.

Series circuit: same current through all parts, R_total = R1 + R2 + …; voltage divides in proportion to resistance.

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

Resistor colour code (4-band): digit, digit, multiplier, tolerance. Black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold ±5%, silver ±10%.

Capacitor stores charge in an electric field: Q = C × V; unit farad (F). Series capacitors add like parallel resistors; parallel capacitors add directly.

Inductor stores energy in a magnetic field: V = L × di/dt; unit henry (H). It opposes change of current.

Diode: forward-biased when anode is more positive than cathode by about 0.7 V (silicon) or 0.3 V (germanium); blocks reverse current.

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

Component identification: fixed resistors use colour bands, electrolytic capacitors are polarised (observe polarity), transformers and ICs carry printed part numbers.

Ammeter connects in series with the load and has very low resistance; voltmeter connects in parallel and has very high resistance.

Multimeter functions: DC/AC voltage, DC/AC current, resistance, continuity (beeper), diode test, and sometimes capacitance and transistor hFE.

Continuity test with power off: near-zero ohm reading means a closed path; open circuit reads overload (OL).

Oscilloscope controls: time/div sets horizontal scale, volts/div sets vertical scale; probe ×10 divides signal by 10 at the tip.

Analog multimeter must be zeroed on the ohm range by shorting the probes and adjusting the zero-adjust knob; digital meters self-zero.

Measurement accuracy: analog meter error is read from the scale class (e.g. class 1.5 means ±1.5% of full scale); digital meters state ±(% reading + digits).

CRO measures amplitude, period, frequency (f = 1/T) and phase difference; Lissajous figures compare two frequencies.

Soldering iron temperature and insulation resistance are checked with a megger for insulation and a thermometer or thermal tester for tips.

Intrinsic silicon is a poor conductor; doping with pentavalent atoms gives N-type (electrons), trivalent atoms give P-type (holes).

PN junction: depletion region forms at the junction; forward bias narrows it, reverse bias widens it and only leakage current flows.

Half-wave rectifier uses 1 diode, gives 50 Hz ripple from 50 Hz mains; full-wave centre-tap and bridge use 2 and 4 diodes, giving 100 Hz ripple.

Bridge rectifier output ripple frequency is twice the input frequency; a filter capacitor across the load reduces ripple.

Zener diode works in reverse breakdown to regulate voltage; a series resistor limits current: R = (Vin − Vz)/(Iz + IL).

BJT terminals: emitter, base, collector. β (hFE) = IC/IB; for active region the emitter-base junction is forward biased and collector-base reverse biased.

Transistor as a switch: cut-off (no base current, VCE ≈ VCC) and saturation (VCE ≈ 0.2 V); base resistor RB ≈ (Vin − 0.7)/IB.

MOSFET/IGBT gate is voltage-controlled and has very high input resistance; handle with anti-static precautions.

LED forward drop is typically 1.8–3.3 V depending on colour; series resistor R = (Vsupply − VLED)/ILED.

Common solder alloy for electronics is 60/40 or 63/37 tin-lead, melting near 183–190 °C; lead-free SAC305 melts near 217–220 °C.

Rosin flux removes oxide and improves wetting; it is classified as non-corrosive, unlike acid flux used for sheet metal.

Correct iron tip temperature for tin-lead work is about 300–350 °C; higher temperatures damage pads and lift copper.

A good solder joint is shiny, smooth, concave and cone-shaped, with the component lead visible; a dull, grainy, ball-shaped joint is a cold joint.

Technique: heat the pad and lead together for 1–2 seconds, feed solder to the joint (not the tip), remove solder then iron, and let the joint cool without movement.

Solder bridge (short between adjacent pads) is removed with desoldering braid or a suction pump; excess flux is cleaned with isopropyl alcohol.

ESD-safe soldering uses an earthed tip and wrist strap; never file or sand a plated iron tip - wipe it on a damp sponge or brass wool.

Health hazards: solder fumes are flux fumes, so use fume extraction or work in a ventilated area; wash hands after handling lead solder.

PCB assembly order: smallest and lowest-height components first (resistors, diodes), then ICs and sockets, then tall electrolytic capacitors and connectors.

Polarised parts - electrolytic capacitors, diodes, LEDs, ICs - must be placed with correct orientation: stripe/cathode band, notch or dot for pin 1.

PCB copper side is the solder side; component side carries the silkscreen legend. Through-hole leads are bent slightly to hold parts before soldering.

Wire gauge is chosen by current: typical 1 mm² cable carries roughly 10 A in free air; long runs need heavier gauge to limit voltage drop.

Colour convention for mains wiring in India: phase red/brown, neutral black/blue, earth green or green-yellow.

Good wiring practice: route wires in straight lines, use cable ties and grommets, keep mains and signal wires separated, and tin stranded ends before terminal screws.

Insulation resistance is checked with a megger between live and earth; a reading in megohms indicates sound insulation.

Continuity and short checks on an assembled board are done before power-up; verify supply rails and polarity with a multimeter.

PC assembly order: install CPU and cooler on the motherboard, fit RAM, mount the board in the case with standoffs, then add PSU, drives and expansion cards.

CPU socket alignment is shown by the triangle on the corner of the processor and the socket; never force the retention lever.

Thermal paste goes in a small pea-sized amount at the centre of the CPU heat spreader; excess paste is harmful, not helpful.

RAM modules are keyed and seat only one way; release clips must click. DDR generations are not interchangeable.

SATA data cable connects drive to motherboard; SATA power comes from the PSU. M.2 NVMe drives fit the M-key slot and are secured with one screw.

BIOS/UEFI is entered with Del or F2 at boot; boot order, XMP/memory profile and virtualisation are configured there.

Upgradation checks: motherboard socket and chipset support for the new CPU, RAM type and maximum capacity, PSU wattage headroom, and drive interface.

Disassembly practice: power off, unplug, press the power button to discharge, use an anti-static wrist strap, and keep screws and parts organised by location.

Data units: 1 byte = 8 bits, 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB, 1 TB = 1024 GB.

Number systems: binary base 2, octal base 8, decimal base 10, hexadecimal base 16; hex digits A–F represent 10–15.

Main memory is volatile RAM; ROM is non-volatile. Cache is faster than RAM and sits between CPU and memory.

CPU parts: ALU performs arithmetic and logic, control unit sequences instructions, registers hold operands; clock speed in GHz.

Input devices: keyboard, mouse, scanner, microphone. Output devices: monitor, printer, speaker, projector.

Storage: HDD uses rotating magnetic platters, SSD uses flash memory with no moving parts, and optical drives use laser read/write.

Ports: USB (universal serial bus, plug-and-play), HDMI (digital audio and video), VGA (analog video), RJ-45 (Ethernet), 3.5 mm audio jack.

Firmware is software stored in ROM/flash that starts the hardware; BIOS/UEFI performs POST at power-on.

OS functions: process management, memory management, file system management, device management and user interface.

Windows file systems: NTFS (current, supports permissions and large volumes), FAT32 (4 GB file limit); Linux commonly uses ext4.

Command prompt basics: ipconfig shows IP configuration, ping tests reachability, ipconfig /all shows MAC and DNS, tracert shows the route.

IPv4 address is 32 bits in four octets; private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.

Subnet mask separates network and host portions; default gateway is the router address used to reach other networks.

DNS resolves domain names to IP addresses; DHCP automatically assigns IP, mask, gateway and DNS to clients.

Topologies: star uses a central switch (most common in LANs), bus uses a shared backbone, ring passes data in a loop, mesh offers redundant paths.

Wi-Fi standards 802.11n/ac/ax differ in band and speed; 2.4 GHz has longer range, 5 GHz has more channels and higher speed.

Hardware firewall or router NAT hides internal private addresses behind one public address.

Before any appliance repair: disconnect mains, discharge capacitors (especially SMPS and CRT), and verify dead with a tester.

Electric iron: heating element open circuit means no heat; thermostat contacts and bimetallic strip control temperature; check cord and plug first.

Electric fan: common faults are capacitor failure (fan hums but does not start), bearing noise, and winding burnout; a good capacitor reads charging on the ohm range.

Mixer grinder: check carbon brushes, coupler, overload protector and switch; a burnt smell usually indicates winding or armature damage.

SMPS faults: first check fuse, then bridge rectifier, main switching transistor and output electrolytic capacitors (bulged top means replace).

Refrigerator and air conditioner: thermostat, overload protector, start relay and compressor winding resistance are checked in sequence; never open the sealed refrigerant circuit without certification.

Microwave oven: high-voltage capacitor and transformer store lethal energy; the door interlock switches are safety-critical and must not be bypassed.

Preventive maintenance: clean filters and vents, tighten terminals, check earthing, and test insulation with a megger at scheduled intervals.

Electric shock severity depends on current path and magnitude: about 1 mA is perceptible, 10–30 mA causes muscle contraction, and 50 mA and above can be fatal.

First aid for electric shock: switch off supply or use an insulated object to separate the victim, then call for help and start CPR if breathing has stopped.

Earthing gives a low-resistance path so that fault current trips the MCB/ELCB; earth wire resistance should be low (typically under 1 ohm for installation).

MCB protects against overload and short circuit; ELCB/RCCB protects people against earth leakage, typically tripping at 30 mA.

Fuse rating must match the circuit; a blown fuse is replaced with the same rating, never with thick wire.

Fire classes: A solid, B liquid, C gas, D metal, E electrical. CO2 and dry powder are used on electrical fires; water is never used.

PPE for hardware work: safety glasses when cutting or drilling, insulated gloves for live work, anti-static wrist strap for boards, and closed shoes.

Safe workshop habits: keep the workbench dry, do not overload sockets, use tools with insulated handles, and report damaged cords immediately.

Chemical and battery safety: lead-acid batteries release hydrogen (no sparks nearby) and electrolyte is corrosive; lithium cells must not be punctured or shorted.

Screwdriver types: flat (slotted) and Phillips (cross) in numbered sizes; a wrong size damages screw heads and slips.

Precision tools: long-nose pliers for reaching and bending, side cutters for wire, tweezers for SMD parts, and a desoldering pump for through-hole removal.

A multimeter, soldering station, hot-air rework station, magnifier lamp and anti-static mat form a basic electronics workbench.

Drill bits: HSS for metal, masonry for concrete, and the bit diameter should match the screw or wall plug; mark the point with a centre punch first.

Measurement and marking tools: steel rule, vernier caliper (least count 0.02 mm typical), try square and scriber.

Hand tool care: wipe off oil, keep cutting edges sharp, store in a tool roll, and never use pliers as a hammer.

Torque practice: tighten screws in a diagonal pattern on covers and heatsinks to avoid warping or cracking.

Lockout/tagout: isolate the supply, lock the switch, tag it, and verify dead before working on equipment.

Business entity options in India: sole proprietorship (simplest), partnership, LLP, private limited company; each differs in liability and compliance.

GST registration is required above the turnover threshold (commonly ₹40 lakh for goods and ₹20 lakh for services for normal states); GSTIN is a 15-character number.

Udyam registration gives MSME status and access to schemes such as credit guarantee and subsidy.

A business plan covers product/service, market and customers, competition, pricing, cost, break-even and funding requirement.

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

Working capital funds day-to-day stock and wages; a repair shop's main costs are rent, technician wages, spares and tools.

Invoicing must show GSTIN, invoice number and date, HSN/SAC code, taxable value, tax rate and total; keep records for the statutory period.

Customer acquisition for a hardware service business: local reputation, warranty on repairs, transparent estimates, and AMC (annual maintenance contracts).

Digital presence: Google Business listing, WhatsApp support, and online reviews matter as much as signage for a small repair shop.

A CV/resume should carry contact details, a skills summary, education, practical experience and references - kept to one or two pages.

Interview basics: research the employer, dress neatly, carry documents, answer with concrete examples, and ask questions about the role.

Communication skills for technicians: explain the fault and the cost in plain language, confirm the customer's approval before repair, and give a written job sheet.

Workplace rights and duties: wages, working hours, safety equipment and grievance channels; a technician must follow the employer's safety policy.

Green skills: reduce, reuse and recycle - e-waste goes to authorised recyclers, never to ordinary bins.

E-waste contains lead, mercury and cadmium; informal acid leaching and open burning are hazardous and illegal.

Energy efficiency: LED lamps use far less power than incandescent; a 9 W LED gives light comparable to a 60 W bulb.

5S workplace practice: sort, set in order, shine, standardise, sustain - reduces wasted time and lost tools.

Sustainability in repair work: repair rather than replace, recover reusable parts, and dispose of batteries and CRTs separately.

Before soldering a PCB, verify component values with a multimeter and confirm orientation of diodes, electrolytic capacitors and ICs against the silkscreen.

Flux residue on a finished board is cleaned with isopropyl alcohol and a brush; ionic residue can cause leakage and corrosion over time.

Wire stripping: strip only the length needed, avoid nicking the conductor, and tin stranded wire before fitting it into a terminal block.

Crimp terminals must match wire gauge; a loose crimp heats up under load and can start a fire.

Cable routing inside an enclosure: keep mains conductors short and separated from signal wiring, and secure with cable ties and clamps.

A bridge rectifier fitted on a PCB has four terminals marked AC, AC, + and −; connecting DC reversed will destroy the board.

After assembly, do a visual inspection under magnification for solder bridges, lifted pads, tombstoned SMD parts and cold joints.

Functional test order: insulation and continuity check, then power-up with a series bulb or current-limited supply, then measure supply rails.

Motherboard form factors: ATX, micro-ATX and mini-ITX differ in size and expansion slots; the case must match the form factor.

PSU connectors: 24-pin ATX main, 4/8-pin EPS CPU, 6/8-pin PCIe for graphics cards, SATA power for drives.

RAM types: DDR3, DDR4 and DDR5 are keyed differently and are not interchangeable; laptop uses SO-DIMM, desktop uses DIMM.

Expansion slots: PCIe x16 is normally used for graphics, PCIe x1 for sound or network cards; NVMe SSDs use PCIe lanes.

Cooling: CPU cooler, case intake and exhaust fans; airflow should go front-to-back or bottom-to-top, and dust filters need cleaning.

POST beeps or debug LEDs indicate faults: no display often points to RAM seating, CPU power or graphics card.

CMOS battery (CR2032) keeps the real-time clock and BIOS settings; a dead battery causes lost time and settings.

Peripheral interfaces: USB-A, USB-C, HDMI, DisplayPort, RJ-45 and audio jacks; know which are on the rear panel and which are front-panel headers.

No internet but local network works: check the default gateway with ipconfig, ping the gateway, then ping 8.8.8.8, then ping a domain name to isolate DNS.

An APIPA address (169.254.x.x) means DHCP failed; check the cable, switch port and DHCP server.

Limited connectivity on Wi-Fi: forget and rejoin the network, check the password, and confirm the router's DHCP pool is not exhausted.

Slow PC: check startup programs, disk health (SMART), RAM usage in Task Manager, and malware; a failing HDD causes long boot times.

Windows repair tools: Safe Mode, System Restore, chkdsk, sfc /scannow and the built-in reset option.

Printer not found on the network: verify its IP, ping it, and reinstall the driver with the correct port.

Ethernet cable wiring: T568B order is white-orange, orange, white-green, blue, white-blue, green, white-brown, brown; both ends must match for a straight cable.

Cable length limit for twisted-pair Ethernet is 100 m per segment; beyond that use a switch or fibre.

Universal fault-finding order: supply and plug, fuse/MCB, switch, cord and connector, then internal components.

Heating appliances (iron, heater, geyser): open element reads infinite resistance; a good element shows a few tens of ohms.

Motor appliances (fan, mixer, washing machine): check supply, capacitor, brushes, switch and thermal cut-out before suspecting the winding.

Washing machine: inlet valve, water level sensor, door interlock, motor capacitor and drain pump are the usual fault points.

TV/monitor: no power - check fuse, SMPS output rails and backlight; no picture with sound - suspect backlight or panel, not the main board.

Inverter and UPS: battery voltage, fuse, charging section and output MOSFETs; a swollen battery must be replaced, not topped up.

Test lamps and series-bulb testing protect a board under repair from a short circuit on first power-up.

Record the fault, the tests done and the parts replaced on a job card; this supports warranty claims and repeat repairs.

Only a licensed electrician may work on fixed wiring; a pre-voc technician works on appliances and low-voltage equipment under supervision.

Live working is avoided; if unavoidable, use insulated tools, stand on an insulating mat, and keep one hand free.

Never touch a person in contact with a live conductor; switch off the supply first or use dry insulating material.

Earth leakage protection (RCCB, 30 mA) is the primary protection against fatal shock in wet areas and workshops.

Portable tools must be double-insulated or earthed; check the cord and plug before every use.

Electrical fire: isolate the supply, use CO2 or dry powder extinguisher, and never throw water.

Burns from a soldering iron or hot component: cool under running water for at least 10 minutes and seek medical help for serious burns.

Ventilation is mandatory when soldering, using solvents or charging lead-acid batteries; keep a first-aid box and fire extinguisher accessible.

Vernier caliper reading = main scale reading + (vernier coincidence × least count); typical least count 0.02 mm.

Micrometer screw gauge least count = pitch ÷ number of circular scale divisions; typical 0.01 mm.

Multimeter on the ohm range must be used with the circuit unpowered, otherwise the reading is meaningless and the meter may be damaged.

Current measurement needs the meter in series; a common mistake is putting the meter across the supply in current mode, which blows the fuse.

Oscilloscope probe compensation is adjusted on the calibration square wave before accurate measurements.

Soldering iron wattage: 15–25 W for PCB work, 40–60 W for heavier joints and connectors.

Anti-static precautions: wrist strap with 1 MΩ resistor to earth, conductive mat, and storage of boards in anti-static bags.

Tool inventory and shadow boards prevent lost tools and speed up fault-finding in a workshop.

Job costing for a repair = parts cost + labour time × hourly rate + overhead share + margin.

Depreciation of tools and test equipment is a real cost and must be recovered in the service price.

GST on most electronic goods and repair services is commonly 18%; the invoice must show CGST/SGST separately for intra-state supply.

Cash flow differs from profit: stock purchases and credit sales can leave a profitable shop short of cash.

Warranty policy: define the period and what it covers (workmanship only, or parts too) in writing to avoid disputes.

Record keeping: purchase bills, job cards, invoices and warranty claims; digital records reduce loss and simplify tax filing.

Marketing for a small repair business: service guarantee, turnaround time, pickup and drop, and transparent estimates build repeat customers.

Growth options: add AMC contracts, sell accessories, offer installation services, and train an apprentice to increase capacity.

Punctuality, safe work habits and honesty about a fault's cause are the core employability traits in a service workshop.

A job sheet records customer details, reported fault, diagnosis, parts used, charges and signature on delivery.

Teamwork: escalate faults beyond your skill level rather than risking damage or injury.

Right to information and grievance: an employee may ask for written terms of employment and must be told about hazards of the job.

Green practice: switch off tools and lights when idle, use power-saving modes, and prefer rechargeable batteries over disposable cells.

E-waste handling: collect boards, batteries and CRTs separately and hand them to authorised recyclers with a receipt.

Energy audit basics: identify the largest loads, replace old tube lights with LED, and fix leakage and loose connections that waste power.

Sustainability also means repairability: choose products with available spares and document repairs so equipment lasts longer.