* Question
The characteristics and parameters of the thermistor
* Answer
A thermistor is a type of resistor whose resistance varies significantly with temperature. Thermistors are commonly used in temperature sensing and control applications due to their sensitivity to temperature changes. They can be classified into two types based on their resistance-temperature relationship:
1. NTC (Negative Temperature Coefficient) Thermistors:
– In NTC thermistors, the resistance decreases as the temperature increases. They are widely used for temperature measurement, control, and compensation in a variety of applications.
2. PTC (Positive Temperature Coefficient) Thermistors:
– In PTC thermistors, the resistance increases with an increase in temperature. These are typically used for overcurrent protection and heating applications.
Key Characteristics and Parameters:
1. Temperature Coefficient:
– Defines how much the resistance of the thermistor changes with temperature. For NTC thermistors, this value is negative (resistance decreases with temperature), while for PTC thermistors, it is positive.
2. Resistance (RT):
– This is the resistance of the thermistor at a specific temperature, commonly specified at 25°C (R25). For example, an NTC thermistor might have a resistance of 10kΩ at 25°C.
3. Beta Value (β):
– The Beta (β) parameter describes the relationship between the thermistor’s resistance and temperature. It is a material constant used to calculate the resistance of the thermistor at different temperatures using the following equation:
RT=R25*eβ（1/T-1/T25）
– Where RT is the resistance at temperature T, R25 is the resistance at 25°C, andβis the material constant.
4. Dissipation Constant (δ):
– This constant defines the power required to raise the temperature of the thermistor by 1°C. It is measured in milliwatts per degree Celsius (mW/°C). Higher values indicate that the thermistor can handle more power without a significant change in its temperature.
5. Time Constant (τ):
– This parameter indicates the speed at which the thermistor responds to changes in temperature. It is defined as the time taken for the thermistor to register 63.2% of the total temperature change and is measured in seconds. A smaller time constant indicates a faster response.
6. Operating Temperature Range:
– Thermistors are designed to operate within a specific temperature range, which varies based on the type and application. For example, NTC thermistors can have ranges from -55°C to +200°C or higher depending on the material used.
7. Tolerance:
– This parameter defines the precision of the thermistor’s resistance at a given temperature, typically at 25°C. It is usually expressed as a percentage, such as ±1%, ±5%, or ±10%.
8. Self-heating Effect:
– Thermistors can exhibit a self-heating effect when subjected to high current. This can lead to inaccurate temperature readings, especially in NTC thermistors, where the resistance decreases as temperature rises.
9. Maximum Rated Power:
– The maximum power dissipation the thermistor can handle without degrading. Exceeding this power can result in permanent damage.
Common Applications:
– Temperature Sensing: NTC thermistors are commonly used in temperature sensors for home appliances, HVAC systems, and medical devices.
– Overcurrent Protection: PTC thermistors are used in circuit protection to limit current in case of faults.
– Temperature Compensation: Used to stabilize electronic circuits affected by temperature changes.
– Inrush Current Limiting: NTC thermistors are used to limit inrush current in power supplies and other electronic devices.
Thermistors are valued for their high sensitivity, compact size, and versatility in both consumer and industrial applications.
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