AQA GCSE HIGHER PHYSICS - Uses of Thermistors and LDRs

Applications of Thermistors and LDRs

This tutorial will explore the uses of thermistors and LDRs in practical circuits.

Thermistors: Temperature Control

What is a thermistor?

A thermistor is a resistor whose resistance changes significantly with temperature. There are two types:

• Negative Temperature Coefficient (NTC) thermistors: Resistance decreases as temperature increases.
• Positive Temperature Coefficient (PTC) thermistors: Resistance increases as temperature increases.

Applications of Thermistors:

• Temperature Control: NTC thermistors are commonly used in temperature control systems. They can be used to:
  • Turn on a heater: When the temperature falls below a set point, the thermistor's resistance increases, allowing more current to flow through a heater, raising the temperature.
  • Turn off a heater: When the temperature reaches a set point, the thermistor's resistance decreases, reducing the current flow through the heater and turning it off.
  • Control fans: NTC thermistors can also be used to control fans in devices like computers. The thermistor senses the temperature of the device and turns on the fan when the temperature gets too high.
• Overheating Protection: NTC thermistors are used in circuits to detect overheating and trigger a safety mechanism. For example, in a power supply, if the temperature of the device increases excessively, the thermistor's resistance decreases, interrupting the current flow and preventing damage.

Example Circuit:

A simple temperature control circuit using an NTC thermistor can be built using a transistor as a switch.

       +-----------+
       |           |
       |   NTC     |
       |  Thermistor |
       |           |
   ---  +-----------+  ---
       |           |
       |     +---+
       |     |   |
       |     |   |
       |     +---+
       |
       |  Resistor
       |
       |
       |    +-----+
       |    |     |
       |    |Transistor |
       |    |     |
       |    +-----+
       |      |
       |     +---+
       |     |   |
       |     |   |
       |     +---+
       |
       |      +---
       |      |
       |     +---+
       |     |   |
       |     |   |
       |     +---+
       |
       |  Resistor
       |
       |    +-----+
       |    |     |
       |    |    Load |
       |    |     |
       |    +-----+
       |
       |
       |
       |    +-----+
       |    |     |
       |    |  +--- |
       |    |  |   |
       |    |  +--- |
       |    |     |
       |    +-----+
       |      |
       |      +---
       |
       +-------------+
          Power Supply 
       +-------------+

Explanation:

• Temperature Changes: As the temperature increases, the resistance of the NTC thermistor decreases. This increases the current flowing through the base of the transistor.
• Transistor Switching: The increased base current turns the transistor "on," allowing current to flow through the load (e.g., a heater).
• Temperature Control: The circuit can be designed to control the temperature within a specific range by adjusting the resistance values and choosing the appropriate components.

LDRs: Light-Sensitive Circuits

What is an LDR?

An LDR (Light Dependent Resistor) is a type of resistor whose resistance changes depending on the intensity of light falling on it. The resistance is high in darkness and decreases as the light intensity increases.

Applications of LDRs:

• Automatic Lighting: LDRs are used in automatic street lights and security lights. When it gets dark, the resistance of the LDR increases, turning on the lights.
• Light Meters: LDRs are used in light meters to measure the intensity of light.
• Camera Exposure Control: Some cameras use LDRs to automatically adjust the aperture and shutter speed for optimal exposure.
• Security Alarms: LDRs can be used in simple security alarm systems. When light falls on the LDR, its resistance decreases, triggering an alarm if someone interrupts the light beam.

Example Circuit:

A simple circuit using an LDR can be used to turn on a light when it gets dark.

      +-----+
      |     |
      |  LDR |
      |     |
      +-----+
        |
       +---+
       |   |
       |   |
       +---+
       |
    Resistor
       |
       |
       |    +-----+
       |    |     |
       |    |Transistor |
       |    |     |
       |    +-----+
       |      |
       |     +---+
       |     |   |
       |     |   |
       |     +---+
       |
       |  Resistor
       |
       |    +-----+
       |    |     |
       |    |    Load |
       |    |     |
       |    +-----+
       |
       |
       |
       |    +-----+
       |    |     |
       |    |  +--- |
       |    |  |   |
       |    |  +--- |
       |    |     |
       |    +-----+
       |      |
       |      +---
       |
       +-------------+
          Power Supply 
       +-------------+

Explanation:

• Light Changes: When it gets dark, the resistance of the LDR increases.
• Transistor Switching: The increased resistance allows more current to flow through the base of the transistor, turning it "on."
• Light Activation: The "on" transistor allows current to flow through the load (e.g., a light bulb), turning it on.

Key Points:

• Thermistors and LDRs are passive components that change their resistance based on external factors.
• Thermistors are temperature-sensitive, while LDRs are light-sensitive.
• These components are used in a wide range of applications, including temperature control, light sensing, and security systems.

Further Exploration:

• Research specific applications of thermistors and LDRs in real-world devices.
• Experiment with building circuits using these components to understand their behavior.
• Explore the different types of thermistors and LDRs available and their characteristics.