AFCT Electronics: Components and Functions 2 — Questions and Answers

Question 1: What is the primary function of a transistor in an electronic circuit?

To store electrical energy
To amplify or switch electronic signals (Correct answer)
To convert AC to DC
To measure voltage

Correct answer: To amplify or switch electronic signals

Transistors are semiconductor devices that can amplify weak signals or act as electronic switches. They are the fundamental building blocks of modern electronics, from amplifiers to digital logic gates.

Transistors are three-terminal semiconductor devices that come in two main types: bipolar junction transistors (BJT) with base, collector, and emitter terminals, and field-effect transistors (FET) with gate, drain, and source terminals. In amplification mode, a small signal at the control terminal (base or gate) modulates a larger current between the other two terminals. In switching mode, the transistor is either fully on (saturated) or fully off (cutoff), forming the basis of digital logic. A single modern microprocessor contains billions of transistors. BJTs are current-controlled devices, while MOSFETs are voltage-controlled, making MOSFETs more energy-efficient for digital switching applications.

Question 2: What component is used to store energy in an electric field between two conductive plates?

Resistor
Inductor
Capacitor (Correct answer)
Transformer

Correct answer: Capacitor

A capacitor stores energy in the electric field between two conductive plates separated by a dielectric (insulating) material. The capacitance depends on plate area, plate separation, and the dielectric constant.

Capacitors consist of two conductive plates separated by a dielectric (insulator) such as ceramic, polyester, or aluminum oxide. When voltage is applied, electrons accumulate on one plate and deplete from the other, creating an electric field in the dielectric that stores energy. Capacitance (measured in farads) equals C = εA/d, where ε is the dielectric constant, A is plate area, and d is plate separation. Common uses include filtering (smoothing power supply ripple), coupling (passing AC while blocking DC), timing circuits, energy storage, and power factor correction. Types include ceramic (small values, high frequency), electrolytic (large values, polarized), film (precision applications), and supercapacitors (very large energy storage).

Question 3: What does a diode do in a circuit?

Allows current to flow in both directions equally
Allows current to flow in one direction only (Correct answer)
Amplifies the signal passing through it
Stores energy magnetically

Correct answer: Allows current to flow in one direction only

A diode is a semiconductor device that allows current to flow primarily in one direction (forward-biased) while blocking it in the reverse direction. It acts as an electronic one-way valve.

A diode is made from a p-n junction semiconductor. When forward-biased (positive voltage applied to the anode relative to the cathode), current flows freely after overcoming the forward voltage drop (approximately 0.7V for silicon, 0.3V for germanium, and 1.8-3.3V for LEDs depending on color). When reverse-biased, only a negligible leakage current flows until the breakdown voltage is reached. Common diode types include: rectifier diodes (AC to DC conversion), Zener diodes (voltage regulation by operating in controlled breakdown), Schottky diodes (fast switching, lower voltage drop), LEDs (light emission), and photodiodes (light detection). Diodes are essential in power supplies, signal demodulation, voltage clamping, and reverse polarity protection.

Question 4: What is the function of an inductor in an electronic circuit?

To store energy in a magnetic field when current flows through it (Correct answer)
To store energy in an electric field
To convert digital signals to analog
To increase the resistance of the circuit

Correct answer: To store energy in a magnetic field when current flows through it

An inductor stores energy in its magnetic field when current flows through its coil of wire. It opposes changes in current, making it useful in filters, transformers, and energy storage applications.

An inductor is a passive component consisting of a coil of wire, often wound around a core (air, ferrite, or iron). When current flows through the coil, it creates a magnetic field that stores energy. The inductor opposes changes in current (Lenz's law), meaning it resists current increases and tries to maintain current when it decreases. Inductance is measured in henrys (H). The voltage across an inductor equals V = L(di/dt), where L is inductance and di/dt is the rate of current change. Applications include: power supply filtering (smoothing current ripple), energy storage in switch-mode power supplies, RF circuits (tuning and impedance matching), transformers (coupled inductors for voltage conversion), and chokes (blocking high-frequency noise while passing DC).

Question 5: What type of resistor changes its resistance based on temperature?

Potentiometer
Thermistor (Correct answer)
Varistor
Photoresistor

Correct answer: Thermistor

A thermistor is a temperature-sensitive resistor whose resistance changes significantly with temperature. NTC thermistors decrease in resistance as temperature rises, while PTC thermistors increase in resistance.

Thermistors (thermal resistors) exhibit large resistance changes with temperature. Negative Temperature Coefficient (NTC) thermistors decrease in resistance as temperature increases, commonly used in temperature sensing, inrush current limiting, and temperature compensation. Positive Temperature Coefficient (PTC) thermistors increase in resistance with temperature, used as resettable fuses (polyfuses) and self-regulating heaters. Other variable resistors include: photoresistors (LDR) that change with light intensity, varistors (MOV) that change with applied voltage (used for surge protection), and potentiometers that are manually adjusted. Thermistors offer high sensitivity but have a non-linear response, unlike RTDs (resistance temperature detectors) which provide more linear but less sensitive temperature measurement.

Question 6: What is the purpose of a voltage regulator in a power supply circuit?

To convert AC to DC
To maintain a constant output voltage regardless of input voltage or load changes (Correct answer)
To increase voltage above the input level
To measure the voltage of the power source

Correct answer: To maintain a constant output voltage regardless of input voltage or load changes

A voltage regulator maintains a steady, constant output voltage despite variations in input voltage or changes in the load current. This protects sensitive electronic components that require stable voltage levels.

Voltage regulators come in two main types: linear and switching. Linear regulators (like the popular 7805 for 5V output) dissipate excess voltage as heat, making them simple but inefficient when the voltage drop is large. Switching regulators (buck, boost, and buck-boost types) use high-frequency switching with inductors and capacitors to convert voltage more efficiently, often achieving 85-95% efficiency. Key specifications include output voltage, maximum current, dropout voltage (minimum input-to-output difference for linear types), line regulation (output stability vs. input changes), and load regulation (output stability vs. current changes). Modern electronics rely heavily on voltage regulators; a single circuit board may have multiple regulators providing different voltage rails (3.3V, 1.8V, 1.2V, etc.).

AFCT Electronics: Components and Functions 2

What is the primary function of a transistor in an electronic circuit?