NTC Thermistor (Negative Temperature Coefficient)
It is a semiconductor sensor whose resistance falls as the temperature rises; a typical 10 kΩ NTC shows 10 kΩ at 25 °C and roughly 1 kΩ around 85 °C. It has two functions: measuring the board or heatsink temperature, and being placed in series with the power input to limit the capacitor charging current at first energisation.
When used for measurement the NTC forms a divider with a fixed resistor; the driver reads the voltage in the middle and converts it to temperature. It is common on power boards because it is cheap, small and fast to respond. The resistance-temperature relationship is not linear; the conversion is done with a table or a curve.
When used for inrush limiting, the NTC has a high resistance while cold and cuts the peak current the capacitors draw at the first instant; as current flows it heats up, its resistance falls and it remains as a small loss in the path.
- Its limit: A warmed-up NTC still has a low resistance immediately after a brief interruption; it provides no protection when power is applied and cut repeatedly.
- Placement: The measuring NTC must be positioned next to the component that actually produces the heat, preferably close to the heatsink; a sensor in a far corner of the board sees the real temperature late.
Because the driver's thermal protection relies on this measurement, the sensor position directly determines how early the protection engages.
Context where this term is used: DC Motor Driver Selection Guide