What is a Thermistor?
A thermistor is a type of resistor whose resistance varies significantly with temperature. The term “thermistor” is a combination of the words “thermal” and “resistor.” Thermistors are used in temperature sensing and temperature control applications due to their high sensitivity to temperature changes.
Types of Thermistors
There are two main types of thermistors, classified by the direction of their resistance change with temperature:
- Negative Temperature Coefficient (NTC) Thermistors: In this type, the resistance decreases as the temperature increases. These are commonly used for temperature measurement and control.
- Positive Temperature Coefficient (PTC) Thermistors: In this type, the resistance increases as the temperature increases. These are typically used for overcurrent protection.
How Thermistors Work
Thermistors are made from ceramic materials that are semiconductive. As temperature changes, the material’s resistance changes in a predictable way. The relationship between resistance and temperature is non-linear, but this characteristic makes thermistors highly sensitive to temperature variations.
The resistance-temperature relationship for NTC thermistors is typically described by the Steinhart-Hart equation:
1/T = A + B * ln(R) + C * (ln(R))^3
Where:
- T is the temperature in Kelvin
- R is the resistance of the thermistor
- A, B, and C are material-specific constants
Applications of Thermistors
Thermistors are used in a wide variety of applications where temperature measurement or control is required:
- Temperature Measurement: NTC thermistors are widely used in digital thermometers and other devices to measure temperature.
- Overcurrent Protection: PTC thermistors are often used as self-resetting fuses in electronic circuits to protect against overcurrent conditions.
- Temperature Compensation: Thermistors are used in circuits that need to adjust for temperature fluctuations, ensuring proper operation over a wide range of temperatures.
How to Choose the Right Thermistor
Choosing the right thermistor involves several factors:
- Temperature Range: The thermistor must operate within the expected temperature range of your application.
- Resistance Value: Select a thermistor with an appropriate nominal resistance at a given reference temperature, usually 25°C.
- Accuracy: Thermistors come in varying accuracy grades; select one that meets the precision requirements of your application.
- Response Time: In dynamic environments, the response time of the thermistor to temperature changes is crucial.
Thermistor Circuit Example
Here’s a simple example of how a thermistor is connected in a temperature sensing circuit:
+-------+-------+
| | |
| Thermistor|
| | |
| | |
+-------+-------+
| |
| ADC |
+---------------+
In this setup, the thermistor is connected in series with an ADC (Analog-to-Digital Converter). The ADC reads the varying voltage drop across the thermistor as its resistance changes with temperature, allowing for temperature measurement.