Introduction to Thermister
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically ?90 °C to 130 °C.[1]
Basic operation
Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:
:
\Delta R=k\Delta T \,
where
\Delta R = change in resistance \Delta T = change in temperature k = first-order temperature coefficient of resistance
Thermistors can be classified into two types, depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a k as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.
Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance \alpha_T (alpha sub T) is used. It is defined as[2]
\alpha_T = \frac{1}{R(T)} \frac{dR}{dT}.
This \alpha_T coefficient should not be confused with the a parameter below.
Steinhart–Hart equation
In practice, the linear approximation (above) works only over a small
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically ?90 °C to 130 °C.[1]
Basic operation
Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:
:
\Delta R=k\Delta T \,
where
\Delta R = change in resistance \Delta T = change in temperature k = first-order temperature coefficient of resistance
Thermistors can be classified into two types, depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a k as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.
Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance \alpha_T (alpha sub T) is used. It is defined as[2]
\alpha_T = \frac{1}{R(T)} \frac{dR}{dT}.
This \alpha_T coefficient should not be confused with the a parameter below.
Steinhart–Hart equation
In practice, the linear approximation (above) works only over a small