Thermistors

1. Thermistors Prepared by M JUNAID ZUBAIR MUGHAL ATHASHAM MUZAFAR Dept of Mechanical Engineering UNIVERSITY OF LAHORE

2. Overview • A Thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. • Thermistors can measure temperatures across the range of -40 ~ 150 ±0.35 °C • Typical operation resistances are in the kW range, although the actual resistance may range from few W to several MW.

3. Classification • Thermistors can take various shapes; rod, disc, washer, bead

4. Classification • Thermistors come in two varieties; NTC, negative thermal coefficient, and PTC, positive thermal coefficient. • The resistance of NTC thermistors decreases proportionally with increases in temperature. • PTC thermistors have increasing resistance with increasing temperature

5. Thermistor construction • NTC Thermistors are most commonly made from the oxides of metals such as manganese, cobalt, nickel and copper. The metals are oxidized through a chemical reaction, ground to a fine powder, then compressed and subject to very high heat. Some NTC thermistors are crystallized from semiconducting material such as silicon and germanium. • PTC Thermistors are generally made by introducing small quantities of semiconducting material into a polycrystalline ceramic. When temperature reaches a critical point, the semiconducting material forms a barrier to the flow of electricity and resistance climbs very quickly.

6. Working principle • Electrical resistance of a metal depends on the temperature. • The basic principle of thermistors is that; change in temperature changes its resistance, this change can be converted to electrical signal.

7. PTC working principle • Resistance of these types of thermistors increases with the rise in temperature. • Due to the special Resistance- Temperature-characteristic, there is no additional temperature regulation or safety device necessary while reaching high heat-power level when using the low resistance area

8. PTC working principle • The PTC-heating element regulates the power sensitively according to the required temperature. The power input depends on the requested heat output.

9. NTC working principle • Resistance of NTC thermis- tors decreases proportionally with increases in temperature. • Thermistor resistance-temperature relationship can be approximated by,

10. NTC working principle • where:T is temperature (in Kelvin),  TRef is the reference temperature, usually at room temp. (25 °C; 77 °F; 298.15 K), R is the resistance of the thermistor (W),  RRef is the resistance at TRef,  b is a calibration constant depending on the thermistor material, usually between 3,000 and 5,000 K.  • Courtesy: http://www.dataacquisitionweb.com/sensors/ntc_thermistors

11. Specification • Major specifications to be considered while using a thermistor. • Resistance temperature curve: it varies from thermistor to thermistor and the specifications are provided by the manufacturer. • Nominal resistance value • Resistance tolerance: The specifications for this is provided by the manufacturer • Beta tolerance : This depends on the material being used for the thermistor

12. Applications • Thermistor is a versatile component and used in various applications where temperature is a factor to be considered. • Depending on type of application and specific output,either PTC or NTC thermistors are used. • The application part is broadly divided into PTC thermistor application and NTC thermistor application.

13. Application of PTC Thermistors • They are used as resettable fuses. • They are used in time delay circuits. • PTC Thermistors are used in motor starting circuits. • They are also used in Degaussing circuitry. • The PTC Thermistor can provide a combination of heater and thermostat in one device • They are used as ‘liquid level’ and ‘flow sensors’.

14. Application of NTC Thermistor • General industrial applications • Industrial process controls • Plastic laminating equipment • Fiber processing & manufacturing • Hot mold equipment (thermoplastics) • Solar energy equipment • Automotive and Transportation Application • Emission controls • Engine temperatures • Aircraft Temperatures.

15. Application of NTC Thermistor • Medical Applications • Fever Thermometers • Fluid temperature • Dialysis Equipment • Consumer/Household Applications • Burglar alarm • Refrigeration and air conditioning • Fire detection • Oven temperature control

16. Advantages of Thermistors • Thermistors have high sensitivity, better then that offered by thermocouples, RTD’s. • High accuracy, ~±0.02 °C (±0.36°F) • They offer a wide range of high resistance values. • They have a small size. • Thermistors have a faster response time then that of RTD’s

17. Limitations • Limited temperature range, typically -100 ~ 150 °C (-148 ~ 302 °F). • Nonlinear resistance-temperature relationship, unlike RTDs which have a very linear relationship. • Errors can result from self excitation currents being dissipated by the thermistors. • They get de-calibrated on exposure to higher temperatures

18. Selection, cost, buying info • Based on the application, the type of Thermistor is decided. • Thermistors can cost from $0.5 and above. • Some online stores for buying thermistors.

19. References • www.efunda.com (Introduction to Thermistors) • www.thermometrics.com/assets/images/ntcnotes.pdf • www .thermometrics.com/assets/images/ptcnotes.pdf • http://www.dataacquisitionweb.com/sensors/ntc_thermistors • http://www.ptc-ceramics.com/principle.htm

20. Thank You !