Thermistors

ThermistorsPrepared by

M JUNAIDZUBAIR

MUGHALATHASHAMMUZAFAR

Dept of Mechanical EngineeringUNIVERSITY OF LAHORE

OverviewOverview

• 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.

Classification

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

ClassificationClassification

• 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

Thermistor constructionThermistor 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.

Working principleWorking 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.

PTC working principlePTC 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

PTC working principlePTC working principle

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

NTC working principle NTC working principle

• Resistance of NTC thermis-tors decreases proportionally with increases in temperature.

• Thermistor resistance-temperature relationship can be approximated by,

NTC working principleNTC 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

SpecificationSpecification

• 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

ApplicationsApplications

• 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.

Application of PTC ThermistorsApplication 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’.

Application of NTC ThermistorApplication 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.

Application of NTC ThermistorApplication 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

Advantages of ThermistorsAdvantages 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

LimitationsLimitations

• 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

Selection, cost, buying infoSelection, 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.

ReferencesReferences

• 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

Thank You !Thank You !