Both negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors&#; values change with temperature but impact their use differently.
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As temperature increases, NTC thermistors&#; resistance drops from high to low, allowing current to pass through. In a circuit, they can limit in-rush current by self-heating when current is initially applied and then allow normal current flow since their resistance drops to a negligible amount during steady-state operation. This capability makes NTC thermistors the most commonly used thermistor. They are also the type most commonly used for temperature sensing applications.
In contrast, for PTC thermistors, as the temperature rises, the resistance increases from low to high and blocks the overcurrent. As a result, PTC thermistors are generally used as fuses.
Both NTC and PTC thermistors are highly nonlinear, so even though they can measure quite accurately and repeatably, additional circuitry is required to linearize the output. Another design consideration is the maximum temperature NTC thermistors can measure is less than 130°C.
The different reaction to temperature for NTC and PTC thermistors. Source: Ametherm.
For temperature sensing, the sensor&#;s package dictates its more common applications. For example, due to their high stability and ruggedness, glass probe thermistors are used for:
- Fluid level measurement
- Fluid flow measurements
- Temperature measurements
- Temperature compensation
- Thermal conductivity measurement
In contrast, glass bead thermistors with small size and fast thermal response time are very sensitive to both voltage and current changes. With these characteristics, typical applications include:
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Most thermistors used for measuring temperature are NTC. They are specified relatively tightly in terms of their parameters for that application.
They usually change quite a bit with temperature, of the order of -5% for every degree C, so that they end up being very nonlinear, to the point that for a wide temperature range you need a lot of dynamic range in the measuring circuitry or you lose resolution at the extremes. For example a thermistor may be 10K nominal at 25°C but only 516 ohms at 125°C, and 210K at -40°C.
There are some PTC thermistors designed for measuring temperature, they tend to have more gentle temperature coefficients (such as the silicon resistance types).
There are also RTDs which are (sometimes pure noble) metals which tend to change about +0.4%/°C near room temperature.
PTC thermistors are used when you want to increase the resistance offered with an increase in temperature. NTC is used when you want to reduce the resistance for an increase in temperature.
PTC thermistors are used as resettable fuses in circuits. When the current flow increases, the temperature increases, and the PTC thermistor resistance increases, and the current flow is reduced.
But using NTC or PTC thermistor depends on the circuit you want to design.
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