NTC Thermistors

An NTC thermistor is a thermally sensitive resistor whose resistance exhibits a large, precise and predictable decrease in resistance, as the core temperature of the element increases over the operating temperature range.

Characteristics of NTC Thermistors

Unlike RTDs (Resistance Temperature Detectors), which are made from metals, NTC thermistors are generally made of ceramics or polymers. Difference materials used result in different temperature responses, as well as other characteristics.

Temperature Response

While most NTC thermistors are typically suitable for use within a temperature range between -55°C (-67°F) and 200°C (392°F), where they give their most precise readings, there are special families of NTC thermistors that can be used at temperatures approaching absolute zero (-273.15°C (-459.67°F) as well as those specifically designed for use above 150° C (302°F).

Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). As their name indicates, the resistance of an NTC thermistor will decrease with temperature rise and the resistance of a PTC thermistor will increase with temperature rise.

Both PTC as well as NTC thermistors can be checked by using an analogue or digital multimeter. Keep the analogue or digital multimeter in resistance mode. Connect the multimeter leads to the thermistor leads. Polarity is not an issue here. Now heat the thermistor by exposing to a known temperature source. You can see the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. This will happen only for a healthy thermistor.

For a faulty thermistor, following observations are possible: The change in reading will not be smooth or there will not be any change. A shorted thermistor will always read zero while an open thermistor will always read infinity.

Using an ohmmeter to test the thermocouple will determine if the thermocouple sensor is defective or not.

If a thermistor goes out of calibration, shorts, or is open, then the HVAC system cannot adjust operational speed according to the changing load condition. A simple test meter can be used for testing of the thermistor.

Using the following 10K-ohms chart, and based on the thermistors associated location temperature, then ohm out the thermistor quickly and compare the ohms value from the chart to the actual ohms reading on the meter.