Electrotechnological Components: Principles and Applications

Overview

Electrotechnological systems are built from discrete electronic and electrical components. Each component has a defined principle of operation — the physical or electronic mechanism by which it works — and specific applications where it is best suited. This knowledge underpins circuit design, analysis, and troubleshooting in VCE Systems Engineering.

KEY TAKEAWAY: Know the circuit symbol, function, and at least one application for each component listed in the study design.

Resistors

Principle: Resist the flow of electric current by converting electrical energy to heat. Obey Ohm’s Law: $V = IR$.

Types:
- Fixed resistor: Constant resistance value
- Variable resistor (rheostat): Adjustable resistance; controls current
- Potentiometer: Voltage divider; provides variable output voltage
- Thermistor (NTC): Resistance decreases as temperature increases
- Light-Dependent Resistor (LDR): Resistance decreases as light intensity increases

Applications: Current limiting (protecting LEDs), voltage dividers, sensor circuits (thermistors as temperature sensors, LDRs as light sensors)

Circuit symbol: Rectangle (IEC) or zigzag line (ANSI)

EXAM TIP: Thermistors and LDRs are both resistors that respond to a physical quantity — they are transducers (convert physical signal to electrical signal).

Capacitors

Principle: Store electrical charge (and therefore energy) in an electric field between two conducting plates separated by an insulating dielectric. Capacitance $C = Q/V$, unit: Farad (F).

Types:
- Ceramic/film capacitors: Small values; general purpose, non-polarised
- Electrolytic capacitors: Large values; polarised (must be connected correctly)

Behaviour:
- Block DC (once fully charged, no current flows)
- Pass AC (charge/discharge continuously)
- Smooth out voltage ripple in power supplies
- Introduce time delays in timing circuits ($\tau = RC$)

Applications: Power supply filtering, timing circuits (555 timer), coupling/decoupling in amplifiers, motor start capacitors

VCAA FOCUS: Understanding that capacitors block DC and pass AC is a key examination concept, as is their role in smoothing rectified AC to DC in a power supply.

Diodes

Principle: Allow current to flow in one direction only (forward bias). Made from a p-n semiconductor junction. The forward voltage drop is approximately 0.6–0.7 V for silicon diodes.

Types:
- Signal diode (1N4148): Low current, signal protection
- Rectifier diode: Converts AC to DC (high current)
- Zener diode: Maintains constant reverse voltage; used in voltage regulation
- LED (Light-Emitting Diode): Emits light when forward biased
- Photodiode: Generates current when illuminated

Applications: Rectification (AC→DC conversion), voltage clamping, indicator lights (LEDs), optical sensors (photodiodes), protection against reverse polarity

COMMON MISTAKE: LEDs must always have a series resistor to limit current — without it, the LED will be destroyed. $R = (V_{supply} - V_{LED}) / I_{LED}$.

Transistors

Principle: Three-terminal semiconductor devices that act as electronically controlled switches or amplifiers.

BJT (Bipolar Junction Transistor):
- Terminals: Base (B), Collector (C), Emitter (E)
- NPN type (most common): Small base current ($I_B$) controls large collector current ($I_C$)
- Current gain: $h_{FE} = I_C / I_B$ (typically 50–300)
- As a switch: Base current turns on large collector-emitter current
- As an amplifier: Varies output current proportional to input

MOSFET:
- Voltage-controlled; gate draws virtually no current
- Better for microcontroller interfacing (logic-level control)
- Applications: Motor drivers, power switching

Applications: Switching relays and motors from microcontroller outputs, audio amplification, logic gates

APPLICATION: In a typical transistor switch circuit, the microcontroller output (3.3 V or 5 V) drives the transistor base through a resistor, allowing the transistor to switch a higher-voltage/current load (motor, relay) on and off.

Relays

Principle: An electromagnetic switch — a small electrical current through a coil creates a magnetic field that physically moves a set of contacts to open or close a higher-power circuit.

Structure: Electromagnetic coil + armature + contacts (Normally Open NO, Normally Closed NC, Common COM)

Why use a relay?
- Electrically isolates control circuit from load circuit
- Allows low-voltage logic (5 V) to control mains voltage (240 V AC)
- Contacts can switch large currents

Applications: Automotive systems (starter motor), industrial control panels, microcontroller-driven high-current loads

STUDY HINT: Relays are often driven by a transistor switch, not directly from a microcontroller, because relay coils draw more current than a digital output pin can safely supply.

Switches

Principle: Mechanically make or break an electrical circuit.

Types:
- SPST (Single Pole Single Throw): Simple on/off
- SPDT (Single Pole Double Throw): Selects between two circuits
- DPDT (Double Pole Double Throw): Controls two circuits simultaneously
- Push-button (momentary): On only while pressed
- Toggle: Latches on or off
- Limit switch (microswitch): Triggered by physical contact with a moving part
- Reed switch: Closes in the presence of a magnetic field

Applications: User input, end-of-travel detection (limit switches in CNC machines), door/window sensors (reed switches)

Sensors

Principle: Transducers that convert a physical quantity into an electrical signal (voltage or current).

Actuators

Principle: Convert electrical energy into a physical action (motion, sound, light, heat).

VCAA FOCUS: For any integrated system, be prepared to identify the sensors (inputs) and actuators (outputs), describe their operating principles, and explain why they were selected for the application.