Components and Devices: Practice Questions & Explanations

Common electronic components and their functions: (1) RESISTOR — limits current and drops voltage; converts electrical energy to heat; color-coded with bands indicating resistance value and tolerance; measured in Ohms; (2) CAPACITOR — stores electrical charge temporarily; passes AC, blocks DC; used for smoothing power supplies, filtering signals, timing circuits; measured in Farads (typically μF or pF); (3) INDUCTOR — stores energy in magnetic field when current flows; resists changes in current; passes DC, restricts AC at high frequencies; measured in Henries; (4) DIODE — allows current in one direction only, blocks reverse direction; used for rectification (converting AC to DC), protection circuits; symbol: triangle pointing to a line; (5) LED (Light Emitting Diode) — diode that emits light when current flows; uses much less energy than incandescent bulbs; long-lasting; specific forward voltage needed; (6) TRANSISTOR — amplifier or switch; controls a large current with a small current/voltage; foundation of all modern electronics; BJT (bipolar junction transistor) and FET (field-effect transistor) types; (7) TRANSFORMER — transfers electrical energy between circuits via magnetic field; can step voltage up or down; works only with AC; primary and secondary windings; (8) FUSE — protective device that breaks the circuit when current exceeds a rated value (the metal element melts); single-use, must be replaced; (9) CIRCUIT BREAKER — like a fuse but resettable mechanically; common in residential breaker panels; (10) SWITCH — manually opens/closes circuit; (11) RELAY — electrically-controlled switch; small electrical signal controls a larger circuit; (12) BATTERY — DC voltage source. Resistor color codes (heavily tested on ASVAB EI): Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Four bands: first two are digits, third is multiplier (10^x), fourth is tolerance (Gold=5%, Silver=10%).

Resistor color code system: 4-band resistor — first two bands are digits, third band is multiplier (number of zeros), fourth band is tolerance. Color values: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Tolerance: Gold=5%, Silver=10%, None=20%. For Red-Red-Orange-Gold: Red=2, Red=2 (two digits = 22), Orange=3 (multiplier = 10³ = 1000), so value = 22 × 1000 = 22,000Ω = 22kΩ. Tolerance Gold = ±5%, meaning actual value is between 20,900Ω and 23,100Ω. Mnemonic for color order: 'Bad Boys Race Our Young Girls But Violet Generally Wins.' Or 'Better Be Right Or Your Great Big Venture Goes West.' Memorize the order — heavily tested. Some examples: Brown-Black-Red-Gold = 1, 0, ×100, 5% = 1,000Ω = 1kΩ ± 5%; Yellow-Violet-Yellow-Gold = 4, 7, ×10,000, 5% = 470,000Ω = 470kΩ ± 5%; Brown-Black-Black-Gold = 1, 0, ×1, 5% = 10Ω ± 5%. 5-BAND resistors (precision): three digit bands, multiplier, tolerance. 6-BAND adds temperature coefficient. Modern surface-mount resistors use printed numbers (e.g., '472' means 47 × 10² = 4700Ω). ASVAB EI definitely tests color codes — practice converting between colors and values both ways. Standard resistor values follow the E-series (E12, E24, etc.) — not arbitrary numbers but standardized increments.

Capacitor structure: two conductive plates separated by an insulating material (the 'dielectric' — air, ceramic, plastic, electrolyte). When voltage is applied, positive charge accumulates on one plate and negative on the other; energy is stored in the electric field between them. Behavior in circuits: (1) DC: capacitor initially conducts as it charges, but once fully charged it blocks DC; like an open circuit at steady state; (2) AC: capacitor continuously charges and discharges as voltage alternates; appears as a frequency-dependent impedance (Xc = 1/(2πfC)); lower impedance at higher frequencies. Applications: (1) POWER SUPPLY FILTERING — large capacitors smooth ripple from rectified AC, providing cleaner DC; (2) DECOUPLING — small capacitors near IC power pins filter noise; (3) TIMING — capacitor charges through a resistor at a predictable rate (RC time constant = R × C); used in oscillators and timers (555 timer is famous); (4) AC COUPLING — passes AC signal between stages while blocking DC bias differences; (5) TUNING — variable capacitors in radio receivers select frequencies; (6) ENERGY STORAGE — large capacitors in flash photography, defibrillators, electric vehicles. Units: Farad (F) — large; typical practical values are microfarad (μF = 10⁻⁶ F), nanofarad (nF = 10⁻⁹), picofarad (pF = 10⁻¹²). Types: ceramic (small, stable), electrolytic (large capacitance, polarized — must be installed correctly), tantalum (compact, polarized), film (high quality, audio), supercapacitor (very large, energy storage). Safety: large capacitors store dangerous charge even after circuit is unplugged; discharge before working on equipment. CAUTION: electrolytic capacitors are polarized — backwards installation can cause failure, leakage, or explosion.

Diode: semiconductor device that allows current in one direction (FORWARD) and blocks it in the reverse direction. Terminals: ANODE (+) and CATHODE (-). Current flows from anode to cathode when forward-biased (anode positive relative to cathode); blocked when reverse-biased. Voltage drop: silicon diodes have ~0.6-0.7V forward voltage drop when conducting; germanium ~0.3V; Schottky ~0.2V; LEDs vary 1.5V (red) to 3.5V (white/blue). Symbol: triangle pointing to a vertical line; current direction follows the triangle's point. Stripe on diode body marks cathode end. Applications: (1) RECTIFICATION — convert AC to pulsing DC: HALF-WAVE rectifier uses one diode (allows only half of AC cycle); FULL-WAVE uses two diodes with center-tapped transformer; BRIDGE rectifier uses four diodes for full-wave from any AC source (most common); (2) PROTECTION — protect circuits from reverse polarity; (3) FREE-WHEELING — protect against voltage spikes from inductive loads (motors, relays) — diode connected across inductor allows the current to circulate safely when switched off; (4) VOLTAGE REGULATION — ZENER DIODES are designed to operate in reverse breakdown at a precise voltage; used for voltage references; (5) SIGNAL DEMODULATION — extracting audio from radio carrier waves; (6) LEDs — special diodes that emit light; used for indicators, displays, lighting (highly efficient compared to incandescent); (7) PHOTODIODES — convert light to current (opposite of LEDs); (8) LASER DIODES — emit coherent light; used in laser pointers, optical disc readers, fiber optic communications. ASVAB EI may show schematic symbols — recognize the diode triangle-and-line symbol with current flow direction.

Transformer: two coils of wire wound around a common iron core; AC current in the PRIMARY coil creates a changing magnetic field; the changing magnetic field induces an AC voltage in the SECONDARY coil. No direct electrical connection between primary and secondary — energy transfers through magnetic field. Turns ratio: V_secondary / V_primary = N_secondary / N_primary, where N = number of turns. STEP-UP transformer (more turns on secondary): increases voltage, decreases current proportionally. STEP-DOWN transformer (fewer turns on secondary): decreases voltage, increases current proportionally. Power (P = VI) is approximately conserved (minor losses to heat); higher voltage = lower current for the same power. Why transformers matter: ELECTRICAL TRANSMISSION uses very high voltage (250-765 kV) for long distances because higher voltage = lower current = less I²R power loss in wires; transformers step down to lower voltages near use (distribution lines 4-25 kV; household 120/240V US, 230V UK). Transformers ONLY work with AC because they require a CHANGING magnetic field to induce voltage in the secondary — DC creates a constant field that doesn't induce. Common applications: (1) UTILITY POWER GRID — countless transformers from generators (10s of kV) → transmission lines (100s of kV) → distribution (kV) → service drops (240V) → appliances (5-120V); (2) ELECTRONIC POWER SUPPLIES — step down household 120V AC to lower AC for further conversion to DC (older devices); (3) AUDIO — impedance matching, isolation; (4) WELDING — step down to provide high current at low voltage; (5) DOORBELLS — step down to 16-24V for the chime. Losses: COPPER LOSS (I²R in wire), CORE LOSS (eddy currents and hysteresis in iron) — minimized by using laminated cores and good materials. Modern switched-mode power supplies use small high-frequency transformers, more efficient than older 60 Hz transformers.

Transistor: semiconductor device that AMPLIFIES or SWITCHES electronic signals. Invented at Bell Labs in 1947 (Bardeen, Brattain, Shockley — Nobel Prize 1956). Foundation of all modern electronics. Two main types: (1) BJT (Bipolar Junction Transistor) — three terminals: Base, Collector, Emitter; small current at base controls larger current between collector and emitter; comes in NPN and PNP polarities; (2) FET (Field-Effect Transistor) — three terminals: Gate, Drain, Source; voltage at gate controls current between drain and source; very high input impedance (doesn't draw current at gate); MOSFETs are most common — used in nearly all digital electronics. As AMPLIFIER: small input signal (e.g., audio from microphone, signal from antenna) varies the larger current flowing through the transistor; output is amplified version of input. Used in audio amplifiers, radio receivers/transmitters, RF circuits. As SWITCH: small base current (BJT) or gate voltage (FET) turns the transistor fully ON or fully OFF; replaces mechanical switches in electronic circuits; very fast (billions of switches per second); essential for digital electronics. Integrated circuits (ICs) pack millions to billions of transistors on a single silicon chip: microprocessor (modern CPU) has tens of billions of transistors; memory chips have similar counts. Moore's Law (Gordon Moore, 1965): transistor density on chips doubles approximately every 2 years — held remarkably true for decades. Modern transistor gate sizes: 3-5 nanometers (about 10-20 atoms wide). Transistor configurations: COMMON EMITTER (amplifies both current and voltage), COMMON BASE (amplifies voltage), COMMON COLLECTOR / EMITTER FOLLOWER (amplifies current). Identification: schematic symbol shows arrow indicating emitter direction (NPN arrow out, PNP arrow in). Transistor ratings: maximum voltage, maximum current, maximum power dissipation, gain (hFE for BJT, transconductance for FET). Common BJTs: 2N2222, 2N3904 (general purpose), 2N3055 (power). Common MOSFETs: 2N7000, IRF series (power switching).

Integrated Circuit (IC): an electronic circuit fabricated on a single piece of semiconductor material (usually silicon). Contains anywhere from a few to billions of transistors plus other components — all interconnected in a single piece. Invented by Jack Kilby (Texas Instruments) and Robert Noyce (Fairchild Semiconductor) in 1958-1959. Common form factors: DIP (Dual In-line Package — through-hole), SOIC (Small Outline IC — surface mount), QFP (Quad Flat Pack), BGA (Ball Grid Array), and many specialty packages. Types of ICs: (1) ANALOG/LINEAR — process continuous signals: op-amps (operational amplifiers like 741, LM358), voltage regulators (7805, LM317), audio amplifiers, comparators; (2) DIGITAL — process binary signals: logic gates (74-series, 4000-series CMOS), flip-flops, counters; (3) MIXED-SIGNAL — combine analog and digital: ADCs (analog-to-digital converters), DACs, signal processors; (4) MICROPROCESSORS (CPUs) — programmable processors: Intel, AMD, ARM cores; modern CPUs have 10-50 billion transistors; (5) MICROCONTROLLERS — CPU + memory + I/O in one chip: Arduino's ATmega328, ESP32, STM32; (6) MEMORY — store data: DRAM (main computer memory), SRAM (cache), flash (SSDs, USB drives, smartphones), EEPROM; (7) ASIC (Application-Specific) — custom chips for specific functions: cryptocurrency miners, AI accelerators, video codecs; (8) FPGA (Field-Programmable Gate Array) — reconfigurable hardware. Manufacturing: photolithography — light patterns on photoresist coated silicon define structures; multiple layers built up; can be thousands of process steps; cleanroom environment; multi-billion-dollar fabrication facilities. Moore's Law: transistor count doubles ~every 2 years (held since 1965, slowing recently). Current process nodes: 3-5 nm (TSMC, Samsung). One modern smartphone has more computing power than NASA's entire infrastructure used during Apollo. ASVAB EI tests basic IC recognition, common types (741 op-amp, 555 timer, 7400-series logic), and concept of integration.

Inductor: a coil of wire (often around a magnetic core) that stores energy in a magnetic field. Energy stored: E = ½LI², where L is inductance (Henries) and I is current. Key properties: (1) Resists CHANGES in current — voltage across inductor = L × (di/dt); when current is steady, voltage drop is ~0 (just the wire resistance); rapid changes in current produce large voltage spikes; (2) Passes DC easily — once current is steady, inductor looks like a wire (just its DC resistance); (3) Opposes AC — impedance increases with frequency: X_L = 2πfL (reactance, in ohms); at high frequencies, inductors become like 'open circuits'; (4) Stores energy in magnetic field — opposite of capacitor which stores in electric field. Applications: (1) FILTERS — inductors in series block high frequencies (low-pass filter); used in audio crossovers, power supply filtering; (2) TRANSFORMERS — magnetic coupling between two coils; (3) MOTORS, GENERATORS, SOLENOIDS — convert between electrical and mechanical energy via magnetic field; (4) RELAYS — coil energizes to operate switch contacts; (5) IGNITION COILS — automotive; pulse the primary, get high-voltage spark from secondary; (6) RADIO TUNING — LC circuit (inductor + capacitor) resonates at specific frequency; (7) ENERGY STORAGE — buck/boost switching converters store and release energy in inductor; (8) CHOKES — block AC noise from power lines while passing DC. Construction: wire wound around an air or magnetic core. Core materials: AIR (low inductance, used at high frequencies); FERRITE (intermediate, common in switching power supplies); IRON or LAMINATED STEEL (high inductance, used in transformers and 60 Hz applications). Inductance increases with: more turns, larger cross-sectional area, magnetic core material, closer winding. Symbol: looped or coiled line; sometimes with bars indicating iron core. Units: HENRY (H), millihenry (mH), microhenry (μH), nanohenry (nH). Typical values: μH range for RF circuits, mH for audio crossovers, H for power applications. Inductor and capacitor behave oppositely: capacitor — passes AC, blocks DC; inductor — passes DC, blocks AC (high frequency). LC circuits combining both can resonate at a specific frequency: f = 1/(2π√LC) — fundamental to radio receivers, oscillators, filters.

A diode is a semiconductor device that allows current to flow in one direction only — it acts as a one-way valve for electrical current.

The TRANSISTOR (invented 1947, Bell Labs) is arguably the most important invention of the 20th century. TWO KEY FUNCTIONS: SWITCH: A small base current turns the collector-emitter current ON or OFF — this is how billions of transistors in microprocessors perform digital logic (0 or 1); AMPLIFIER: A small base signal controls a proportionally larger collector current — this is how audio amplifiers, radio receivers, and signal processing work. Modern CPUs contain billions of transistors on a single chip. Types: BJT (Bipolar Junction Transistor) — current-controlled; MOSFET (Metal-Oxide-Semiconductor FET) — voltage-controlled, used in digital circuits.

DC (Direct Current): Electrons flow in one consistent direction — positive terminal to negative terminal (conventional current). Source: batteries, solar cells, fuel cells. Used in: all electronics, electric vehicles, USB devices. AC (Alternating Current): Direction of electron flow reverses periodically. US standard: 60 Hz (60 cycles per second = 120 direction reversals/second); European standard: 50 Hz. Used in: power grid transmission (more efficient for long-distance), motors, household appliances. CONVERSION: DC → AC: Inverter; AC → DC: Rectifier (diodes). The power adapter for your laptop converts AC wall power to DC for the device.

An INDUCTOR (also called a coil or choke) is a coil of wire that stores energy in a magnetic field. KEY PROPERTY: Inductors OPPOSE CHANGES IN CURRENT (Lenz's Law) — this is called inductive reactance. When current increases: the magnetic field builds, inducing a back-EMF that opposes the increase; When current decreases: the collapsing field releases energy trying to maintain current. APPLICATIONS: Power supply filtering (smooths ripple); radio tuning circuits (with capacitors, forms LC resonant circuits); transformers (mutual inductance between coils); electric motors and generators. COMPARE with CAPACITOR: Capacitors oppose changes in VOLTAGE and store energy in an electric field; inductors oppose changes in CURRENT and store energy in a magnetic field.

A RESISTOR OPPOSES (limits) the flow of electric CURRENT, controlling how much current flows and dropping voltage in a circuit. ASVAB Electronics Information tests components. Resistance is measured in OHMS (Ω). Resistors protect components, set current levels, and divide voltage. Other components: CAPACITOR (stores electrical energy); INDUCTOR (stores energy in a magnetic field, opposes current changes); TRANSISTOR (amplifies/switches); DIODE (allows current one direction). Knowing the resistor limits/opposes current is fundamental, commonly tested electronics knowledge.

A CAPACITOR STORES electrical energy (charge) in an electric field and releases it when needed. ASVAB Electronics Information tests components. Capacitors store charge on two plates separated by an insulator (dielectric); capacitance is measured in FARADS. Uses: smoothing/filtering power supplies, timing circuits, storing energy, blocking DC while passing AC. Unlike a resistor (which limits current) or battery (chemical storage), a capacitor stores energy electrostatically and can charge/discharge quickly. Knowing the capacitor stores and releases electrical energy is commonly tested.

A TRANSFORMER increases ('step-up') or decreases ('step-down') AC VOLTAGE using electromagnetic induction between two coils (primary and secondary windings). ASVAB Electronics Information tests components. The voltage change depends on the ratio of coil turns: more turns on the secondary = step-up (higher voltage); fewer = step-down (lower voltage). Transformers work only with AC (changing current creates the changing magnetic field needed). Used in power distribution (high voltage for transmission, stepped down for homes) and adapters. Knowing the transformer changes AC voltage is commonly tested.

A resistor limits or controls the flow of electric current in a circuit by providing a specific amount of resistance, measured in ohms. Resistors are used to set current levels, divide voltage, and protect sensitive components from too much current. A capacitor stores electrical charge; a transistor can amplify signals; and a diode (or rectifier) converts AC to DC. Knowing the basic job of each common component — resistor (limits current), capacitor (stores charge), diode (one-way valve), transistor (amplify/switch) — is core electronics content.

A diode allows current to flow in only one direction and blocks it in the reverse direction, acting like a one-way valve for electricity. This property makes diodes useful for converting alternating current (AC) to direct current (DC) in a process called rectification. A light-emitting diode (LED) is a special diode that emits light when current flows through it in the forward direction. Distinguishing the diode's one-way behavior from a resistor (limits current both ways) or a capacitor (stores charge) is common on the Electronics subtest.

A capacitor stores electrical energy in an electric field between two conductive plates separated by an insulator (dielectric). It can charge up and then release that energy, which makes capacitors useful for smoothing out voltage, filtering signals, and providing short bursts of power. Capacitance is measured in farads. Unlike a battery, a capacitor stores energy electrostatically and can charge and discharge very quickly. Knowing that a capacitor stores charge — distinct from a resistor (limits current) or inductor (stores energy in a magnetic field) — is standard component knowledge.

A transformer increases (steps up) or decreases (steps down) alternating-current voltage using two coils of wire wound around a shared iron core. The ratio of turns between the primary and secondary coils determines how much the voltage changes. Transformers work only with AC because they rely on a changing magnetic field. They are essential to the power grid, stepping voltage up for efficient long-distance transmission and down to safe levels for homes. Note a transformer changes voltage but does not convert AC to DC (that is a rectifier's job, using diodes).