Circuit diagrams and circuit components Revision Notes

Cells and batteries are sources of electrical energy in circuits. A cell converts chemical energy into electrical energy, producing a potential difference (voltage) typically around 1.5 V. A battery is a combination of two or more cells connected in series to provide a higher voltage.

Power supplies convert mains electricity into a stable DC voltage for circuits, often adjustable. Generators convert mechanical energy into electrical energy, producing an alternating voltage.

Standard symbols: A cell is represented by a pair of parallel lines, the longer line indicating the positive terminal. A battery is multiple cells in series. Power supplies and generators have distinct symbols showing their function.

• Cell: single chemical energy source, ~1.5 V.
• Battery: multiple cells in series, higher voltage.
• Power supply: converts AC to DC, adjustable voltage.
• Generator: converts mechanical to electrical energy, AC output.

Resistors oppose the flow of electric current, limiting current and dividing voltage in circuits. Fixed resistors have a constant resistance value, measured in ohms (Ω).

Thermistors are temperature-dependent resistors; specifically, Negative Temperature Coefficient (NTC) thermistors decrease resistance as temperature increases. This property allows them to be used in temperature sensing and control circuits.

Light-dependent resistors (LDRs) change resistance based on light intensity. Their resistance decreases as light intensity increases, useful in light sensing applications such as automatic lighting controls or alarms.

• Fixed resistor: constant resistance, limits current.
• NTC thermistor: resistance decreases as temperature rises.
• LDR: resistance decreases as light intensity increases.
• Resistance measured in ohms (Ω).

Switches control the flow of current by opening (off) or closing (on) the circuit. They are essential for controlling devices safely and conveniently.

Heaters convert electrical energy into heat energy, typically using a resistive element that heats up when current passes through.

Lamps convert electrical energy into light energy and some heat. The brightness depends on the current and voltage supplied, and the filament resistance increases as it heats up, affecting current flow slightly during operation (non-ohmic behaviour). Lamps are often used as indicators or light sources in circuits, and their symbol is a circle with a cross inside it or a filament shape depending on the diagram style. Motors convert electrical energy into mechanical energy, causing rotation or movement. They are used in fans, toys, and appliances. Bells convert electrical energy into sound energy, typically using an electromagnet to strike a bell when current flows. Transformers change the voltage of alternating current (AC) in circuits. They consist of two coils (primary and secondary) wound on a magnetic core. The voltage ratio between primary and secondary coils depends on the turns ratio. Transformers only work with AC, not DC.

• Switch: opens/closes circuit to control current.
• Heater: converts electrical energy to heat.
• Lamp: converts electrical energy to light and heat; filament resistance increases with temperature.
• Motor: converts electrical energy to mechanical energy (motion).
• Bell: converts electrical energy to sound via electromagnet.
• Transformer: changes AC voltage levels; voltage ratio = turns ratio.

Ammeters measure electric current flowing through a circuit and are connected in series. Their internal resistance is very low to avoid affecting the current.

Voltmeters measure potential difference (voltage) across components and are connected in parallel. Their internal resistance is very high to minimise current draw.

Fuses protect circuits from excessive current by melting a thin wire inside when current exceeds a safe limit, breaking the circuit and preventing damage or fire. The fuse rating (in amperes) must be chosen appropriately for the circuit's normal current range to avoid nuisance blowing or insufficient protection. After a fuse blows, it must be replaced; it is a one-time protective device. Relays are electrically operated switches that use a small current to control a larger current. They consist of an electromagnet coil and one or more switch contacts. When current flows through the coil, it magnetically closes or opens the contacts, allowing control of high-power circuits safely and remotely.

• Ammeter: measures current, connected in series, low resistance.
• Voltmeter: measures voltage, connected in parallel, high resistance.
• Fuse: melts to break circuit if current too high, protects wiring and devices.
• Relay: uses small current to switch larger current, electrically operated switch.

Circuit diagrams use standard symbols to represent components and lines to represent wires connecting them. When drawing:

Use straight lines for wires, avoiding unnecessary bends.

Place components clearly with their symbols oriented consistently (e.g., positive terminal of cells at the top). Label components if necessary (e.g., R1 for resistor 1). Show the direction of current flow (conventional current from positive to negative). For ammeters and voltmeters, indicate correct connection (ammeter in series, voltmeter in parallel). Include switches in open or closed positions as required by the circuit function. For transformers, draw two coils with a core between them, indicating primary and secondary sides. For thermistors and LDRs, use their specific symbols and note their variable resistance behaviour in explanations. Understanding how each component behaves in the circuit allows prediction of current, voltage, and overall circuit operation.

• Use standard symbols and clear connections.
• Show correct polarity and orientation.
• Connect ammeters in series, voltmeters in parallel.
• Indicate switch positions clearly.
• Label components for clarity.
• Depict transformers with primary and secondary coils.

A diode is a semiconductor device that allows electric current to flow in only one direction. It acts like a one-way valve for current, blocking current flow in the reverse direction. This property is called rectification.

When a diode is forward biased (positive voltage on the anode relative to the cathode), it conducts current. When reverse biased, it ideally does not conduct.

This behaviour protects circuits from damage due to reverse polarity and is used in converting alternating current (AC) to direct current (DC).

• Diode allows current flow in one direction only.
• Forward bias: diode conducts current.
• Reverse bias: diode blocks current.

In circuit diagrams, components are represented by standard symbols to show their function and connections.

The diode symbol consists of a triangle pointing towards a vertical line. The triangle represents the anode (positive side), and the line represents the cathode (negative side).

An LED symbol is similar but includes two small arrows pointing away from the diode, indicating light emission when current flows through it.

• Diode symbol: triangle pointing to a line.
• LED symbol: diode symbol plus two arrows indicating light emission.
• Anode is the triangle side; cathode is the line side.

An LED is a special type of diode that emits light when current passes through it in the forward direction.

When forward biased, electrons recombine with holes in the semiconductor material, releasing energy as photons (light).

LEDs are used as indicators and in lighting because they convert electrical energy efficiently into visible light, with low heat production compared to traditional bulbs.

• LED emits light when forward biased.
• Light is produced by electron-hole recombination.
• LEDs are energy-efficient light sources.

Current flows from the positive terminal to the negative terminal of a power supply externally, but inside the circuit, electrons flow from negative to positive.

For a diode or LED to conduct, the anode must be at a higher potential than the cathode (forward bias).

If connected in reverse (reverse bias), the diode or LED blocks current, and the LED will not light up.

• Current flows forward through diode/LED only if anode is positive relative to cathode.
• Reverse bias prevents current flow.
• LED lights only when forward biased.

To draw a diode in a circuit diagram, sketch a triangle pointing to a line, with the triangle side connected to the positive side of the circuit.

For an LED, draw the diode symbol and add two small arrows pointing away from the diode to indicate light emission.

Label the anode and cathode if necessary to clarify polarity, especially in practical circuits where correct orientation is essential for operation.

• Draw diode as triangle pointing to line.
• Add arrows for LED to show light emission.
• Indicate anode and cathode to show correct polarity.