Thermistors are a type of sensor resistor whose resistance value changes with temperature variations. They are classified into two types based on their temperature coefficient: Positive Temperature Coefficient (PTC) thermistors and Negative Temperature Coefficient (NTC) thermistors. PTC thermistors increase in resistance as temperature rises, while NTC thermistors decrease in resistance as temperature increases. Both types are semiconductor devices and offer several key characteristics that make them essential in various applications.

Key Characteristics of Thermistors

1. High Sensitivity: Thermistors exhibit higher resistance temperature coefficients than metals by 10 to 100 times, capable of detecting minute temperature changes as small as 10^-6°C.
2. Wide Operating Temperature Range: Standard thermistors can operate between -55°C and 315°C, high-temperature variants can exceed 315°C (up to 2000°C), and low-temperature thermistors are suitable for -273°C to -55°C.
3. Compact Size: Their small size allows for temperature measurement in spaces that other thermometers cannot access, such as gaps, cavities, and blood vessels.
4. Versatility in Resistance: Resistance values can be chosen between 0.1Ω and 100kΩ, offering flexibility in design.
5. Ease of Manufacturing: Thermistors can be produced in various shapes and in large quantities.
6. Stability and Overload Capacity: They provide reliable performance and can handle significant overloads.

You may like: Advantages and Disadvantages of Thermistor 2024

Working Principle

Thermistors remain inactive under normal conditions. When environmental temperature and current fall within a specific range, the thermistor’s heat dissipation power matches its heating power, leading to potential activation. The activation time shortens significantly with increasing current and higher ambient temperatures, resulting in lower holding and activation currents.

PTC Thermistors

PTC thermistors exhibit the PTC effect, where resistance increases with temperature. This linear PTC effect is common in most metals. Non-linear PTC effects occur in materials that undergo phase transitions, causing resistance to surge dramatically within a narrow temperature range. Conductive polymers often display this non-linear PTC effect, making them useful in overcurrent protection devices.

Applications of Polymer PTC Thermistors

Polymer PTC thermistors, also known as self-resetting fuses, are ideal for overcurrent protection due to their unique positive temperature coefficient characteristics. They function similarly to conventional fuses by being connected in series within a circuit. Under normal operation, the thermistor’s temperature and resistance are low, allowing current to flow unimpeded. However, in the event of an overcurrent, the thermistor’s temperature rises sharply, causing its resistance to spike and reducing the circuit’s current to a safe level.

Impact of Environmental Temperature

The performance of polymer PTC thermistors, including holding current (Ihold), activation current (Itrip), and activation time, is influenced by environmental temperature. They respond to both self-heating and cooling dynamics, and their resistance can recover to approximately 1.6 times the initial value within seconds to minutes after activation, allowing for repeated use. Smaller thermistors recover faster than larger ones.

Basic Characteristics

Thermistors follow a resistance-temperature relationship that can be approximated by the formula: R=R0exp⁡{B(1T−1T0)}R = R_0 \exp \{ B \left( \frac{1}{T} — \frac{1}{T_0} \right) \}R=R0exp{B(T1−T01)}

Where:

• RRR is the resistance at temperature TTT (in Kelvin).
• R0R_0R0 is the resistance at reference temperature T0T_0T0 (in Kelvin).
• BBB is a constant that varies with material composition.

Calculating Resistance Values

For practical applications, the B value can be adjusted using a temperature-dependent function: BT=CT2+DT+EB_T = CT² + DT + EBT=CT2+DT+E

Where CCC, DDD, and EEE are constants derived from resistance-temperature data points. This adjustment reduces errors between calculated and measured values, ensuring more accurate resistance calculations across a broad temperature range.

Example Calculation

To determine the resistance of a thermistor with a 25°C resistance of 5kΩ and a B value variation of 50K between 10°C and 30°C:

1. Calculate constants CCC, DDD, and EEE using data points (T0, R0), (T1, R1), (T2, R2), and (T3, R3).
2. Apply these constants to the formula BT=CT2+DT+E+50B_T = CT² + DT + E + 50BT=CT2+DT+E+50 to find BTB_TBT.
3. Substitute BTB_TBT into the resistance formula to compute RRR for the temperature range 10°C to 30°C.

Conclusion

Thermistors are versatile and highly sensitive temperature sensors essential in various applications, from industrial processes to medical devices. Understanding their characteristics, working principles, and application methods is crucial for leveraging their full potential in temperature monitoring and protection systems.