Thermistors

Thermistor Basics

A thermistor is a type of resistor whose resistance changes significantly with temperature. It is mainly used as a temperature sensor in electrical circuits.

Thermistors are made from semiconductor materials such as metal oxides (e.g., manganese, cobalt, or nickel oxides) pressed into a bead or disc shape and coated for protection.

Most thermistors used in GCSE circuits have a Negative Temperature Coefficient (NTC). This means their resistance decreases as the temperature increases. This property allows thermistors to detect temperature changes by measuring resistance. There are also Positive Temperature Coefficient (PTC) thermistors, whose resistance increases with temperature, but these are less common in GCSE contexts.

Resistance and Temperature

For an NTC thermistor, as the temperature rises, the resistance falls. This happens because heating gives the semiconductor material more energy, allowing more charge carriers to move freely and conduct electricity better.

The graph of resistance against temperature for an NTC thermistor is a curve that slopes downwards:

• At low temperatures, resistance is high.
• As temperature increases, resistance decreases rapidly.

This non-linear relationship is important for accurate temperature sensing.

Thermistors are widely used in temperature sensing circuits because they provide a simple way to convert temperature changes into electrical signals (changes in resistance).

For instance, if a thermistor has a resistance of 10 k 3 at 20DA and 5 k 3 at 40DA, the resistance halves when the temperature doubles. This change can be detected by measuring voltage across the thermistor in a circuit.

Uses of Thermistors

Thermistors are commonly used as temperature sensors in many everyday devices:

• Thermostats: Thermistors detect room temperature and control heating systems by switching the heater on or off to maintain a set temperature.
• Fire alarms: Thermistors sense rapid temperature rises caused by fire and trigger alarms.
• Automotive temperature monitoring: In cars, thermistors monitor engine temperature to prevent overheating and control cooling fans.

Their fast response to temperature changes and small size make thermistors ideal for these applications.

Investigating Thermistor Resistance

To investigate how a thermistors resistance changes with temperature, you can set up a simple circuit:

• Connect the thermistor in series with a fixed resistor and a power supply.
• Measure the potential difference (voltage) across the thermistor using a voltmeter.
• Use a variable resistor or a known resistor to form a voltage divider circuit.
• Change the temperature of the thermistor by heating it gently (e.g., with warm water) or cooling it (e.g., with ice water).
• Record the voltage across the thermistor at different temperatures.

From the voltage readings and known circuit values, you can calculate the resistance of the thermistor at each temperature using the formula for resistors in series and the voltage divider rule. The resistance of the thermistor $R_T$ can be calculated using the formula:

$$R_T = R \times \frac{V_T}{V_R}$$

where $R$ is the resistance of the fixed resistor, $V_T$ is the voltage across the thermistor, and $V_R$ is the voltage across the fixed resistor.

For example, if the total voltage supplied is 12 V and the voltage across the thermistor is measured at 4 V, and the fixed resistor is 10 k 3, the thermistor resistance $R_T$ can be found by:

$$V_{total} = V_R + V_T \quad \Rightarrow \quad 12 = V_R + 4$$

So, $V_R = 8$ V.

Using Ohms law for the fixed resistor:

$$I = \frac{V_R}{R} = \frac{8}{10,000} = 0.0008 \text{ A}$$

Since current is the same through series components, the thermistor resistance is:

$$R_T = \frac{V_T}{I} = \frac{4}{0.0008} = 5,000 \Omega = 5 \text{ k}\Omega$$