OCR Specification focus:
‘Use correct symbols for variable resistors, LDRs, thermistors, and potentiometers.’
In electrical circuits, variable and dependent devices play a vital role in adjusting and controlling current, voltage, and resistance. Understanding their symbols and functions is essential for constructing and interpreting circuit diagrams accurately.
Variable and Dependent Devices in Circuit Diagrams
Overview of Variable and Dependent Components
Electrical circuits often require components whose resistance or output changes with conditions such as light intensity, temperature, or user adjustment. These are known as variable or dependent devices.
Variable devices are those intentionally adjusted by the user to alter resistance or voltage in a circuit.
Dependent devices automatically respond to environmental changes, such as temperature or light intensity.
Both types must be represented with standard circuit symbols for clarity, ensuring that anyone interpreting the diagram understands their role and behaviour.
Variable Resistors
Symbol and Purpose
A variable resistor (or rheostat) allows manual adjustment of resistance to control the current in a circuit. Its circuit symbol is based on the resistor symbol with an arrow diagonally across it, representing variability.
A clean comparison of the fixed resistor, variable resistor (rheostat), and potentiometer symbols. The angled arrow across the resistor denotes variability; the wiper arrow to the midpoint denotes a potentiometer’s adjustable output tap. Includes the fixed resistor for context (extra to syllabus). Source.
Variable Resistor: A resistor whose resistance can be altered manually to control current or voltage in a circuit.
Variable resistors are commonly used in laboratory setups or as components in devices requiring adjustable output, such as light dimmers or volume controls.
Types of Variable Resistors
Rheostats – control current directly in circuits with high currents.
Potentiometers – function as voltage dividers, adjusting the output voltage rather than current directly.
The circuit symbol must make it clear which configuration is intended. A potentiometer is represented with a resistor symbol and an arrow connecting to the midpoint, indicating a variable output tap.
Potentiometers
Symbol and Function
A potentiometer is a three-terminal device used to vary voltage in a circuit, functioning as a voltage divider.
Potentiometer: A variable resistor with three terminals, used to produce an adjustable voltage output that depends on the position of a sliding or rotating contact.
When drawing a potentiometer in a circuit diagram:
The resistive track is drawn as a resistor symbol.
An arrow or wiper shows the adjustable contact point.
The output is taken from the wiper, producing a fraction of the total voltage across the resistor.
Potentiometers are used in applications such as volume controls, sensor calibration, and analogue signal tuning.
Thermistors
Symbol and Behaviour
A thermistor is a temperature-dependent resistor whose resistance changes with temperature. It is represented by a resistor symbol with a diagonal line and a small temperature indicator (often the letter “T” or a slope symbol).
Thermistor: A component whose resistance varies predictably with temperature, used to sense or compensate for thermal changes.
Thermistors are of two main types:
NTC (Negative Temperature Coefficient) – resistance decreases as temperature increases.
PTC (Positive Temperature Coefficient) – resistance increases as temperature increases.
The OCR specification often focuses on NTC thermistors, as they are widely used in temperature sensors and protection circuits.
Application and Circuit Usage
Used in temperature measurement and control circuits.
Common in overcurrent protection, battery charging circuits, and environmental monitoring.
In circuit diagrams, thermistors are drawn using the resistor symbol with the temperature marker, clearly distinguishing them from fixed resistors.
Light-Dependent Resistors (LDRs)
Symbol and Principle of Operation
An LDR (Light-Dependent Resistor) is a photoresistor whose resistance decreases as light intensity increases. Its symbol is the resistor symbol with two arrows pointing towards it, representing incident light.
Standard light-dependent resistor (photoresistor) symbol. The inward arrows denote incoming light that lowers resistance in typical LDRs. Layout is uncluttered and suitable for quick recognition in circuit diagrams. Source.
Light-Dependent Resistor (LDR): A component whose resistance decreases as the light intensity falling on it increases.
This makes LDRs valuable in automatic lighting and sensing circuits, where changes in light intensity are converted into electrical signals or control actions.
Common Uses of LDRs
Streetlights that turn on automatically in darkness.
Camera exposure controls to detect brightness levels.
Security systems using light interruption detection.
Correct symbol usage ensures that light-dependent behaviour is easily recognised when reading or constructing circuit diagrams.
Relationship Between Variable and Dependent Devices
Although variable resistors, potentiometers, thermistors, and LDRs differ in operation, they share the fundamental property of resistance variation. This variation affects the current (I) and potential difference (V) across the circuit, following Ohm’s law (V = IR).
EQUATION
—-----------------------------------------------------------------
Ohm’s Law (V = IR)
V = Potential difference across the component (volts, V)
I = Current through the component (amperes, A)
R = Resistance of the component (ohms, Ω)
—-----------------------------------------------------------------
When these devices are included in circuits, their changing resistance modifies current flow, allowing automatic control or user adjustment of electrical parameters.
Best Practice in Diagram Representation
Key Conventions for Accuracy
When drawing variable or dependent devices:
Use straight lines for connecting wires.
Connect at right angles where possible.
Label components clearly with their type and value (e.g., “10 kΩ NTC thermistor”).
Ensure arrows and additional markings (light or temperature symbols) are clearly shown and correctly oriented.
Maintain consistent scale and spacing for clarity.
Avoiding Common Errors
Students must avoid:
Using non-standard or ambiguous symbols.
Confusing the thermistor symbol with that of a resistor or diode.
Reversing the direction of light arrows in LDR symbols.
Following these conventions ensures that circuits can be universally understood and accurately built from diagrams.
FAQ
A variable resistor and a rheostat are closely related, but they differ in configuration and use.
A variable resistor generally refers to any resistor whose resistance can be adjusted, including potentiometers used for voltage division.
A rheostat specifically controls current by varying resistance in series with the load. It typically uses two terminals (one end and the wiper), while potentiometers use three. Rheostats are often found in laboratory setups or older electrical devices for coarse current control.
Arrows indicate change or adjustability in circuit diagrams.
In a variable resistor or potentiometer, the arrow shows that resistance can be manually altered.
In dependent components such as thermistors or LDRs, the arrowed markings (light or temperature symbols) show that their resistance depends on external conditions.
These arrows provide a visual cue to the reader that the component’s electrical properties are not fixed.
Yes. The function depends on how the potentiometer is connected.
When all three terminals are used, it acts as a voltage divider, outputting a fraction of the total potential difference.
When only two terminals (one end and the wiper) are used, it functions as a variable resistor, controlling current in the circuit.
Thus, a single component can serve either role depending on wiring configuration.
Both components change resistance in response to environmental conditions, enabling feedback and automation.
Thermistors are used in thermostats or temperature-sensitive alarms; as temperature rises, resistance falls, triggering a control circuit.
LDRs are used in light-sensitive switches; as light intensity increases, resistance drops, allowing automatic control of lighting or camera exposure.
Their changing resistance can directly influence current or voltage in control circuits without human input.
Accurate symbols ensure universal understanding, safe construction, and effective troubleshooting.
Technicians and examiners must instantly recognise a component’s function from its symbol.
Misinterpreting a thermistor or LDR symbol could lead to incorrect wiring, inaccurate measurements, or component damage.
Standard symbols also support clear communication between design, simulation, and practical testing stages in professional engineering practice.
Practice Questions
Question 1 (2 marks)
State the correct circuit symbols for:
(a) a thermistor
(b) a light-dependent resistor (LDR).
Mark Scheme:
(a) 1 mark for a resistor symbol with a diagonal line and a temperature marker (or sloping line/‘T’ symbol).
(b) 1 mark for a resistor symbol with two arrows pointing towards it, representing incident light.
Question 2 (5 marks)
A student is designing a simple temperature and light sensor circuit using a variable resistor, a thermistor, and an LDR.
(a) Explain how the resistance of the thermistor and LDR change in response to temperature and light intensity. (2 marks)
(b) Describe how using these components in the circuit allows automatic control of current or voltage without manual adjustment. (2 marks)
(c) State why it is important to use the correct circuit symbols for these components when drawing the circuit diagram. (1 mark)
Mark Scheme:
(a)
1 mark for stating that the resistance of an NTC thermistor decreases as temperature increases.
1 mark for stating that the resistance of an LDR decreases as light intensity increases.
(b)
1 mark for explaining that as environmental conditions change, the variable resistance of the components alters the current or potential difference in the circuit.
1 mark for stating that this allows automatic adjustment or feedback without manual control, e.g. dimming lights when it is bright or increasing heater power when it is cold.
(c)
1 mark for stating that using correct circuit symbols ensures clarity, safety, and universal understanding when building or interpreting the circuit diagram.
,_and_Potentiometer_symbols.svg){kind=link}
