Thermistors
Post on 03-Jan-2016
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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 !
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