Both negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors’ values change with temperature but impact their use differently.
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:
- Thermodilution cardiac catheters
- Biomedical assemblies
- Fluid flow
- Fluid level measurement
- Gas analysis
- Small surface is temperature measurements
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