Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Identify and describe the function of key components.
• Calculate total resistance and apply Ohm's Law.
• Explain capacitor charging/discharging and time constant.
• Analyze simple diode rectifier circuits.
• Explain the basic operation of a BJT as a switch.

Lesson summary

This week, we delve deeper into electronic components and their roles in basic circuits. Electronic circuits are at the heart of nearly every device we use daily, from cell phones to renewable energy systems. Understanding these components is crucial for anyone aspiring to work in electrical engineering, electronics, telecommunications, or related fields, which are all growing sectors in South Africa. Consider the increasing demand for technicians in the renewable energy sector, especially solar panel installation and maintenance, all of which rely on solid understanding of basic electronics.

Lesson notes

2.1 Resistors: Resistors oppose the flow of current. Their resistance is measured in Ohms (Ω). We'll cover fixed resistors and variable resistors (potentiometers and rheostats).

Fixed Resistors: Have a constant resistance value. The value and tolerance are indicated by colour bands.

Colour Code: Remember this mnemonic: Big Boys Race Our Young Girls, But Violet Generally Wins. Black (0), Brown (1), Red (2), Orange (3), Yellow (4), Green (5), Blue (6), Violet (7), Grey (8), White (9). Gold and Silver represent tolerances.

Example: A resistor with colour bands Brown, Black, Red, Gold has a resistance of 1 000 Ω (10 x 10 2 ) with a 5% tolerance.

Variable Resistors: Their resistance can be changed.

Potentiometers: Have three terminals. By adjusting a knob or slider, the resistance between the center terminal and either of the outer terminals changes. Used in volume controls, dimmers.

Rheostats: Have two terminals. One of the outer terminals of a potentiometer is left disconnected, and the wiper (center terminal) and the other outer terminal are used. Used for controlling current in a circuit, like a fan speed controller. 2.2 Capacitors: Capacitors store electrical energy in an electric field. Their capacitance is measured in Farads (F). Common types include electrolytic, ceramic, and film capacitors.

Charging and Discharging: When a capacitor is connected to a voltage source, it charges, accumulating charge on its plates. When the voltage source is removed, the capacitor discharges, releasing the stored energy. Time Constant (τ = RC): The time constant of an RC circuit (Resistor-Capacitor circuit) is the time it takes for the capacitor to charge to approximately 63.2% of the applied voltage or discharge to 36.8% of its initial voltage.

Example: Consider a circuit with a 1000 Ω resistor and a 100 μF capacitor. The time constant is τ = (1000 Ω) (100 x 10 -6 F) = 0.1 seconds. After 0.1 seconds, the capacitor will be approximately 63.2% charged or discharged. 2.3 Inductors: Inductors store energy in a magnetic field when current flows through them. Their inductance is measured in Henries (H). They oppose changes in current. 2.4 Diodes: Diodes allow current to flow in only one direction. They are made from semiconductor materials like silicon.

Forward Bias: When the anode (+) is more positive than the cathode (-), the diode conducts.

Reverse Bias: When the cathode is more positive than the anode, the diode blocks current.

Half-Wave Rectifier: A simple circuit that converts AC voltage to pulsating DC voltage using a single diode. The diode conducts only during the positive half-cycle of the AC input.

Full-Wave Rectifier: A circuit that converts AC voltage to DC voltage using four diodes arranged in a bridge configuration. It uses both the positive and negative half-cycles of the AC input, resulting in a smoother DC output compared to the half-wave rectifier. 2.5 Transistors (Bipolar Junction Transistors - BJTs): Transistors are semiconductor devices that can amplify or switch electronic signals and electrical power.

BJTs have three terminals: Base (B), Collector (C), and Emitter (E). We will focus on their operation as switches.

NPN Transistor as a Switch: When a small current flows into the base, the transistor turns "on," allowing a larger current to flow from the collector to the emitter. When the base current is zero, the transistor turns "off," blocking the current flow from collector to emitter.

Practical Application: Imagine using a transistor to control a light-emitting diode (LED). A small current from a microcontroller can be used to switch on the transistor, which then allows a larger current to flow through the LED, illuminating it. 2.6 Ohm's Law and Series/Parallel Resistor Combinations Ohm's Law: V = IR (Voltage = Current x Resistance)

Series Resistors: Total resistance (R T ) is the sum of individual resistances: R T = R 1 + R 2 + R 3 + ...

Parallel Resistors: The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances: 1/R T = 1/R 1 + 1/R 2 + 1/R 3 + ... For just two resistors in parallel, a shortcut formula is often useful: R T = (R 1 R 2 ) / (R 1 + R 2 )