Thermistors are an exceptional type of resistor with high temperature sensitivity compared to standard resistors. The resistance of a thermistor decreases with a rise in temperature - this is the basic principle of thermistors. In most metals, resistance increases as the temperature goes up, but thermistors react the opposite way. This unique characteristic allows the thermistor circuit to detect small temperature changes that cannot be observed with RTDs or thermocouples. Because the resistance of a thermistor is temperature-dependent, it can be used to measure body temperature when connected to an electrical circuit. The high sensitivity of thermistors to temperature changes makes them useful in accurately controlling and calibrating temperature measurements.

There are several uses of thermistors, including as a temperature probe, inrush current limiter, self-regulating overcurrent protection device, and self-regulating heating element.

Two main types of thermistors are Positive Temperature Coefficient Thermistors (PTC) and Negative Temperature Coefficient Thermistors (NTC).

PTC thermistors exhibit an increase in resistance as temperature rises, and this relationship can be expressed by a linear equation involving a change in resistance, temperature, and temperature coefficient. PTC thermistors can be used to replace fuses in circuits, provide overload protection, serve as timers in the magnetic coil circuit of maximum CRT displays, and as inexpensive heaters in the car industry to warm up diesel engine compartments or diesel in cold climates prior to engine injection.

NTC thermistors can be made from an extruded disk or a cast semiconductor chip such as a sintered metal oxide. Increasing the temperature of the semiconductor increases the number of electrons that can move and carry charge, resulting in more current that can be conducted. NTC thermistors exhibit a linear relationship between the resistance of a material and temperature over a small range of temperature changes. There are various semiconductor thermistors that range from 0.01 Kelvin to 2000 Kelvin, and current can be measured using an ammeter with compensation needed for large temperature changes. The formula for NTC thermistors involves current, carrier density, cross-sectional area, carrier charge velocity, and electron charge.

The construction of thermistors involves sintering metallic oxide mixtures such as cobalt, uranium, and manganese. These devices are made from semiconductor materials, and their bends can have diameters as small as a few millimeters.

Thermistors exhibit high non-linear resistance characteristics that vary with temperature. PTC thermistors can serve as heating elements in temperature-controlled ovens, while NTC thermistors can function as inrush limiting devices in power circuits. The temperature of a thermistor can be adjusted externally by modifying the ambient temperature and through internal heating resulting from current flowing through the device.

Inrush current is a large instantaneous current that pulls the device when it is first activated.

The resistance-temperature characteristics equation expresses the relationship between thermistor resistance and absolute temperature. It includes constants that depend on the thermistor material, which typically ranges from 3500 to 4000. While the thermistor performance is non-linear, a linear approximation of the resistance-temperature curve is possible. The resistance of a thermistor changes according to a certain formula, and it has a negative coefficient of temperature resistance of around 0.05/[].

The Steinhart equation accurately approximates the individual thermistor curve, and it includes curve fitting constants A, B, and C. These constants can be found through selecting three data points from the published data curve and solving three systems of equations.

The thermistor voltage drop increases with increasing current until it reaches a peak value beyond which the voltage drop increases as the current increases. At this point, the thermistor exhibits a negative resistance characteristic. If a small voltage is applied to the thermistor, the resulting small current does not produce enough heat to raise the temperature above ambient temperature.

The current time characteristics curve shows the time delay to reach maximum current as a function of the applied voltage. A finite time is required for the thermistor to heat and the current to build up to a maximum steady state value when the heating effect described earlier occurs in a thermistor network. This characteristic provides a simple and accurate means of achieving time delays from milliseconds to minutes.

The use of thermistors is widespread in many home appliances that require temperature control and measurement. Surprisingly, thermistors are found in many devices around the house, including digital thermometers, protective devices, and even microwave or boiler systems. Thermistors are commonly used in 3D printers to maintain the precise temperature control required by their print materials.

Practical Problems:

1) What are the practical applications of thermistors? Answer: Temperature sensor, Fire alarm, Digital thermometers.

2) What is the sensitivity of thermistors? Answer: 10 []/[].

3) What is the resistance range of thermistors? Answer: 0.5 ῼ to 0.75 Mῼ.

4) If the resistance of a thermistor is 200ῼ at 30[], what is the resistance at 40[]? Answer: 100 []ῼ.

5) What are the different applications of thermistors? Answer: Vacuum measurement, CO₂ measurement in air, Temperature compensations.

Related Content:

• Williams R.J., Thompson R.C. A device that continuously records body temperature through the ear canal. Science. 23 July 1948; 108(2795): 90-91.

• Benjamin JM Jr., Horvat S. M. Measurement of internal body temperature with a thermistor. Science. June 10, 1949; 109(2841): 592-593.

• Drummer LF, Jr, Fastie WG. A simple resistance thermometer for measuring blood temperature. Science. 17 January 1947; 105(2716): 73–75.

• Team Wavelength. Thermistor Basics.