Thermistors are temperature-dependent variable resistors that exhibit a significant change in resistance with temperature. They are widely used in microcontroller-based applications for accurate and reliable temperature measurement due to their high sensitivity, durability, and cost-effectiveness. This comprehensive guide delves into the key specifications, characteristics, and practical applications of thermistor temperature sensors.
Resistance and Tolerance
The resistance of a thermistor is typically specified at a reference temperature, usually 25°C. The resistance value can vary depending on the tolerance, which is usually expressed as a percentage. For example, a 10 kΩ thermistor with a ±5% tolerance means the resistance can range from 9.5 kΩ to 10.5 kΩ at 25°C.
Thermistors are available in a wide range of resistance values, from a few ohms to several megohms, depending on the application and temperature range. Common resistance values include 2.2 kΩ, 10 kΩ, 100 kΩ, and 1 MΩ.
Beta (B) Constant
The Beta (B) constant is a crucial parameter that represents the relationship between the resistance and temperature over a specified temperature range. It is used to calculate the temperature from the measured resistance value. The Beta constant is typically in the range of 3000 K to 4500 K for most thermistors.
The relationship between resistance (R) and temperature (T) can be expressed using the Steinhart-Hart equation:
1/T = (1/T₀) + (1/B) * ln(R/R₀)
Where:
– T₀ is the reference temperature (usually 25°C or 298.15 K)
– R₀ is the resistance at the reference temperature
– B is the Beta constant
By rearranging the equation, the temperature can be calculated from the measured resistance:
T = 1 / ((1/T₀) + (1/B) * ln(R/R₀))
Operating Temperature Range
Thermistors have a limited temperature range in which they can operate without damage. The manufacturer’s data sheet specifies the minimum and maximum operating temperatures, which can vary widely depending on the thermistor type and construction.
Common operating temperature ranges for thermistors include:
– Negative Temperature Coefficient (NTC) thermistors: -55°C to 150°C
– Positive Temperature Coefficient (PTC) thermistors: -55°C to 300°C
– High-temperature thermistors: -55°C to 500°C
It is essential to ensure that the thermistor is used within its specified operating temperature range to avoid permanent damage or performance degradation.
Thermal Time Constant
The thermal time constant (τ) is the time it takes for a thermistor to reach 63.2% of the difference between the old and new temperatures when the temperature changes. It is an important parameter that determines the response time of the thermistor.
The thermal time constant is influenced by factors such as the thermistor’s size, construction, and the surrounding medium. Typical thermal time constants for thermistors range from a few milliseconds to several seconds, depending on the application.
The thermal time constant can be calculated using the formula:
τ = (ρ * c * V) / (h * A)
Where:
– ρ is the density of the thermistor material
– c is the specific heat capacity of the thermistor material
– V is the volume of the thermistor
– h is the heat transfer coefficient of the surrounding medium
– A is the surface area of the thermistor
Thermal Dissipation Constant
Thermistors are subject to self-heating as they pass current through them. The thermal dissipation constant (θ) is the amount of power required to raise the thermistor’s temperature by 1°C, and it is typically specified in milliwatts per degree Celsius (mW/°C).
The thermal dissipation constant is an important parameter in determining the maximum power that can be applied to the thermistor without causing significant self-heating, which can affect the accuracy of the temperature measurement.
The thermal dissipation constant can be calculated using the formula:
θ = (T₂ – T₁) / P
Where:
– T₂ is the final temperature of the thermistor
– T₁ is the initial temperature of the thermistor
– P is the power dissipated in the thermistor
Maximum Allowable Power
The maximum power dissipation that a thermistor can handle without damage is specified in Watts (W). This parameter is crucial in ensuring the thermistor is not operated beyond its safe limits, which could lead to permanent damage or performance degradation.
The maximum allowable power for a thermistor is typically in the range of a few milliwatts to several watts, depending on the size, construction, and intended application. Exceeding the maximum allowable power can cause the thermistor to overheat, leading to inaccurate temperature measurements or even device failure.
Resistance-Temperature Table
The manufacturer’s data sheet for a thermistor provides a resistance-temperature table, which lists the resistance values corresponding to different temperatures within the thermistor’s operating range. This table is essential for converting the measured resistance to the corresponding temperature.
Here’s an example resistance-temperature table for a 10 kΩ NTC thermistor:
| Temperature (°C) | Resistance (kΩ) |
|---|---|
| -40 | 195.0 |
| -20 | 65.66 |
| 0 | 25.92 |
| 20 | 10.00 |
| 40 | 4.374 |
| 60 | 2.033 |
| 80 | 0.9934 |
| 100 | 0.5133 |
By measuring the resistance of the thermistor and using the resistance-temperature table, the corresponding temperature can be determined.
Nonlinear Behavior and Polynomial Approximation
Thermistors exhibit a nonlinear relationship between resistance and temperature, which means the change in resistance is not a straight line but rather a curve. This nonlinear behavior can be approximated using a third-order polynomial equation, known as the Steinhart-Hart equation, as mentioned earlier.
The Steinhart-Hart equation provides a more accurate representation of the resistance-temperature relationship compared to a linear approximation, especially over a wide temperature range. This nonlinear behavior is a result of the thermistor’s material composition and construction, which determines the rate of change in resistance with temperature.
Advantages and Applications of Thermistor Temperature Sensors
Thermistors offer several advantages that make them a popular choice for temperature measurement in various applications:
- High Sensitivity: Thermistors can detect small changes in temperature with high accuracy, making them suitable for precise temperature control and monitoring.
- Durability: Thermistors are rugged and can withstand harsh environmental conditions, such as vibrations, shocks, and exposure to chemicals.
- Low Cost: Thermistors are generally inexpensive compared to other temperature sensing technologies, making them a cost-effective solution for many applications.
- Fast Response Time: Thermistors have a relatively fast response time, allowing for quick detection of temperature changes.
- Compact Size: Thermistors are available in small, compact packages, making them suitable for integration into various electronic devices and systems.
Thermistor temperature sensors are widely used in a variety of applications, including:
- Temperature control and monitoring in HVAC systems
- Thermal protection and overload detection in motors and transformers
- Temperature measurement in consumer electronics, such as smartphones, laptops, and home appliances
- Process control and monitoring in industrial equipment
- Medical devices for body temperature measurement and monitoring
- Automotive applications, such as engine and cabin temperature monitoring
Conclusion
Thermistor temperature sensors are a versatile and cost-effective solution for accurate temperature measurement in a wide range of applications. By understanding the key specifications, characteristics, and practical considerations of thermistors, engineers and designers can effectively integrate these sensors into their microcontroller-based systems to achieve reliable and precise temperature monitoring and control.
References
- Philip Kane, “Temperature Measurement with NTC Thermistors,” Jameco Electronics, accessed May 9, 2024, https://www.jameco.com/Jameco/workshop/TechTip/temperature-measurement-ntc-thermistors.html
- “Thermistor Basics,” Wavelength Electronics, accessed May 9, 2024, https://www.teamwavelength.com/thermistor-basics/
- “How to Measure Temperature with an NTC Thermistor,” Digi-Key Electronics, accessed May 9, 2024, https://www.digikey.com/en/maker/projects/how-to-measure-temperature-with-an-ntc-thermistor/4a4b326095f144029df7f2eca589ca54
- “Thermistor Temperature Sensors: Theory and Application,” Omega Engineering, accessed May 9, 2024, https://www.omega.com/en-us/resources/thermistor-temperature-sensors
- “Thermistor Selection Guide,” Murata Manufacturing, accessed May 9, 2024, https://www.murata.com/en-us/products/thermistor/ntc/guide
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