Circuit diagrams

A circuit diagram shows how electrical components are connected. It uses standard symbols for components instead of pictures of the physical devices. Lines represent conducting wires, and the places where lines join show how the wires are connected.

For example, a cell is represented by a long and a short parallel line, a lamp by a circle containing a cross, and a switch by a symbol that shows whether its contacts are open or closed. Each component connects to the rest of the circuit at its terminals.

Tracing paths in a circuit diagram

To interpret a circuit diagram, identify its components and follow the wires from one terminal to the next. A steady current requires a closed conducting path with a source such as a cell. If there is a break in a path, current cannot flow in that path. An open switch creates such a break; closing the switch completes the connection across it.

A diagram can also be drawn from a description: select the correct symbol for each component, then join their terminals with lines to match the stated connections. The shape or position of a line on the page need not match the physical layout. What matters is which terminals and wires are connected.

Series paths and parallel branches

Components are in series when they lie on the same single current path, one after another. For example, a cell, switch and lamp connected in one loop form a series circuit. Opening the switch breaks that loop, so current cannot flow through the lamp.

Components are in parallel when they lie on separate branches connected between the same two junctions. Each branch provides a separate path between those junctions. A break in one branch prevents current in that branch, but another branch can still carry current if it remains part of a closed conducting path.

Electrical sources in circuits

An electrical source provides energy to a circuit by maintaining a potential difference between its terminals. The potential difference is the energy transferred per unit of electric charge. When the circuit is complete, charge flows and the source transfers energy electrically to the circuit.

Different sources obtain this energy in different ways:

• A cell converts stored chemical energy into electrical energy.
• A battery consists of two or more cells.
• A power supply provides an electrical output to a circuit.
• A generator converts mechanical energy, such as energy from motion, into electrical energy.

Electrical source symbols and polarity

A cell is represented in a circuit diagram by two parallel lines of unequal length. The longer line marks the positive terminal and the shorter line marks the negative terminal. A battery symbol repeats this cell pattern.

Power supplies and generators may be represented by source symbols or labelled terminals. A generator may be shown as a circle containing the letter G.

The polarity and type of output depend on the source. A direct output has a fixed polarity, whereas the polarity of an alternating output reverses repeatedly.

Fixed and variable resistors

A resistor opposes current in a circuit. For a given potential difference across a resistor, a greater resistance produces a smaller current.

A fixed resistor has a resistance intended to remain constant. A variable resistor can be adjusted to change its resistance. If the potential difference across it stays the same, increasing its resistance decreases the current through it; decreasing its resistance increases the current.

Variable resistor adjustment

When a variable resistor is used with two terminals, moving its control changes the resistance in the conducting path between them. This changes the current in the circuit, provided the other circuit conditions remain unchanged.

Resistor circuit symbols

A fixed resistor is commonly represented by a rectangle. A variable resistor uses the same resistor symbol with a diagonal arrow through it. The arrow indicates that the resistance can be adjusted; it does not show the direction of current.

Potential dividers

A potential divider splits an input potential difference between resistances connected in series. The output potential difference is taken across one part of the arrangement, so it is a fraction of the input.

How the input potential difference is divided

Consider two resistances, $$R_1$$ and $$R_2$$, connected in series across an input potential difference $$V_{\text{in}}$$. The output, $$V_{\text{out}}$$, is measured across $$R_2$$.

When no significant current is drawn from the output, the same current flows through both resistances. The potential difference across each resistance is proportional to its resistance, so the fraction of the input across $$R_2$$ is $$R_2$$ divided by the total resistance:

The larger the share of the total resistance represented by $$R_2$$, the larger the share of the input potential difference across it. This equation applies to the output across $$R_2$$ and assumes that the output draws no significant current.

Variable potential dividers

A variable potential divider has a resistive track connected across the input and a movable contact called a slider. The slider divides the track into two sections and provides an additional connection. The output is taken between the slider and one end of the track.

Moving the slider changes the resistance of each section. This changes the fraction of the input potential difference across the section between the slider and the chosen end, and therefore changes the output. With no significant current drawn from the output, moving the slider from one end of the track to the other changes the output from zero to the full input potential difference.

The circuit symbol shows a resistive element with a variable contact pointing to it. The contact represents the slider and forms the additional connection.

NTC thermistors

An NTC thermistor is a resistor whose resistance changes with its temperature. NTC stands for negative temperature coefficient: as the thermistor’s temperature increases, its resistance decreases. As it cools, its resistance increases.

If the potential difference across the thermistor is held fixed, a decrease in resistance causes the current through it to increase. A rise in temperature can therefore produce an increase in current, giving a temperature-dependent electrical response.

NTC thermistor symbol

The circuit symbol is based on a resistor symbol crossed by a diagonal line with a short bar. An NTC thermistor is different from a manually adjusted variable resistor: its resistance changes in response to temperature rather than being set by a person.

Light-dependent resistors

A light-dependent resistor (LDR) is a resistor whose resistance changes with the intensity of the light falling on it. As light intensity increases, its resistance decreases. As light intensity decreases, its resistance increases.

If the potential difference across the LDR is kept fixed, a decrease in resistance allows a larger current to flow. The LDR therefore provides an electrical response that depends on the light falling on it.

LDR circuit symbol

The circuit symbol for an LDR is a resistor with arrows pointing towards it, commonly enclosed by a circle. The inward-pointing arrows represent light falling on the LDR and distinguish it from a component that emits light.

Electrical output components

Electrical output components transfer electrical energy into forms such as thermal energy, light, movement and sound. They operate when current flows through them.

• A heater transfers electrical energy into thermal energy, warming its surroundings.
• A lamp transfers electrical energy into light. It also transfers some energy by heating.
• A motor transfers electrical energy into mechanical movement, usually by turning a shaft.
• A bell uses electrical energy to move its mechanism, producing sound.

Output components in circuit diagrams

Circuit diagrams use symbols or labels to distinguish output components. A lamp is commonly shown as a circle containing a cross, while a motor is shown as a circle containing M. A bell is shown by a bell symbol. A heater is represented as a heating element, sometimes with a label identifying it.

Ammeters and voltmeters

An ammeter measures the current through a component or circuit path. A voltmeter measures the potential difference between two points, such as the two ends of a component.

Connecting an ammeter

An ammeter is connected in series with the path whose current is being measured. The circuit is arranged so that the current in that path passes through the ammeter as well as the component.

An ideal ammeter has negligible resistance. It therefore does not significantly change the current it is measuring. Its circuit symbol is a circle containing A.

Connecting a voltmeter

A voltmeter is connected in parallel across the component or section being measured. Its two terminals are connected to the two points between which the potential difference is required.

An ideal voltmeter has very high resistance, so it draws negligible current. Its circuit symbol is a circle containing V.

Circuit switches

A switch controls whether a conducting path is complete. When a switch is closed, its contacts are connected, so current can flow through that path if the rest of the circuit is complete. When the switch is open, its contacts are separated, breaking the path and preventing current from flowing through it.

Switches in circuit diagrams

A switch symbol shows two contacts and a movable connection. An open switch is drawn with a gap between the movable connection and one contact. A closed switch is drawn with the connection joining the contacts, leaving no gap.

Switches in parallel branches

Parallel branches provide separate paths for current. Opening a switch in one branch breaks that branch’s path, so current cannot flow through it. Current can still flow in another branch if that branch has a complete path through the rest of the circuit.

Fuses

A fuse protects a circuit by breaking the conducting path if the current becomes too large. It is connected in series with the part of the circuit it protects, so the current through that part also passes through the fuse.

Inside the fuse is a thin conducting element. Current heats the element; if the current exceeds the fuse’s rating sufficiently, the element melts. This leaves a gap in the conducting path, creating an open circuit and stopping the current.

In a circuit diagram, a fuse is commonly shown as a small rectangle with a line through it. Once the element has melted, the fuse must be replaced to restore the connection.

Relays

A relay is an electrically operated switch. It allows a current in one circuit to control a switch in a separate circuit. The switched circuit may carry a larger current than the control circuit.

When current flows through the relay’s coil, the coil produces a magnetic field. This field moves an armature, a movable part that operates the switch contacts. The contacts change state: contacts that were open may close, allowing current through the switched circuit, while contacts that were closed may open.

When the coil is no longer energised, the armature and contacts return to their normal state: the state they have when no current flows through the coil.

Relay diagrams

A relay diagram shows the coil and the switch contacts. A dashed line often links them to show that the coil operates the contacts mechanically; it is not an electrical connection. The control circuit containing the coil and the circuit controlled by the contacts need not be electrically connected to each other.

Transformers

A transformer transfers energy from one coil of wire to another through a changing magnetic field. The input coil is called the primary coil, and the output coil is called the secondary coil. The coils are usually wound around a magnetic core, which helps link them through the magnetic field.

An alternating potential difference across the primary coil produces a changing magnetic field in the core. This changing field induces an alternating potential difference across the secondary coil. If the secondary coil is connected to a circuit, energy can be transferred to that circuit.

The coils are not normally directly electrically connected: the changing magnetic field links them.

Step-up and step-down transformers

A transformer can change the size of an alternating potential difference:

• A step-up transformer produces a larger potential difference across the secondary coil than across the primary coil.
• A step-down transformer produces a smaller potential difference across the secondary coil than across the primary coil.

Transformer circuit symbol

The circuit symbol for a transformer shows two coils facing one another. Parallel lines between the coils often represent the magnetic core.

Diodes and light-emitting diodes

A diode allows conventional current to flow through it in one direction but prevents current in the opposite direction. When it is connected in the permitted direction, it is forward-biased. Reversing it makes it reverse-biased, preventing current through it.

The diode’s circuit symbol shows the permitted direction of conventional current. Its triangular part points towards a bar; the bar marks the cathode of the diode.

A light-emitting diode (LED) is a diode that emits light when current flows through it in the permitted direction. Its symbol is the diode symbol with two arrows pointing outwards. These arrows represent the light emitted, not the direction of current.

If an LED is reversed so that it is reverse-biased, it prevents current through it and does not emit light.