A Negative Temperature Coefficient thermally sensitive resistor, or NTC thermistor, exhibits a sharp decrease in resistance as temperature rises. Thermistors are ceramic semiconductors manufactured by sintering metal oxide compounds at high temperatures. Controlling the thermistor material composition makes it possible to adjust the material’s resistivity to determine the thermistor’s size and resistance so that the finished product is optimized for a range of applications. This characteristic decline in resistance over temperature is what makes this device ideal as a highly sensitive temperature sensor — the device can be formulated so that the most sensitivity occurs at different temperatures. An application with an ice cube maker would likely leverage the dark blue curve, since any small change in temperature below the 0 °C range yields a massive change in resistance. On the other hand, a coffee pot would require a thermistor with sensitivity in the range of temperatures that coffee is brewed; around 90 °C to 96 °C. Engineers can get an idea of this change in resistance over temperature from either the alpha coefficient, the beta relationship, a R-T lookup table, or the steinhart-hart coefficients. One of the more commonly used parameters is the beta value. This value uses the change rate in resistance between two given temperatures to provide a basic approximation of the resistance versus temperature response of an NTC thermistor. A smaller beta value will correspond to a lower sensitivity within the temperature span, while a larger beta value will have a higher sensitivity within the specified temperature differential. Discrete NTC thermistors can either come in a surface mount, bare chip with silver or gold metallized ceramic. Lead-in devices can either be in a radial or axial lead configuration. Glass encapsulated thermistors offer the most ruggedization with operating temperatures up to 300 °C and much higher long-term stability at high temperatures. Epoxy or polyimide encapsulated discrete NTC thermistors have operating temperatures up to 125 °C but are a cost-effective option.
