A thermistor is a type of resistor whose resistance varies significantly(more than in standard resistors) with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, selfresetting overcurrent protectors, and selfregulating 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 [usually −90 °C to 130 °C].
Assuming, as a firstorder approximation, that the relationship between resistance and temperature is linear, then:
where
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(smallest possible k), so that their resistance remains nearly constant over a wide temperature range.
Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance α (alpha) or α_{T} is used. It is defined as^{[1]}
For example, for the common PT100 sensor, α = 0.00385 or 0.385 %/°C. This α_{T} coefficient should not be confused with the α parameter below.
Contents
SteinhartHart equation
In practice, the linear approximation (above) works only over a small temperature range. For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The SteinhartHart equation is a widely used thirdorder approximation:
Full article ▸
