Download - Thermistors (Sensors)

Thermistors (sensors)Prepared by

KASHINATH DAS

Dept. of Applied electronics & Instrumentation engineeringHALDIA INSTITUTE OF TECHNOLOGY

Roll no.- 09/EI/60 University registration no.-

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.

Sample configuration in application Sample configuration in application (PTC Thermistor)(PTC Thermistor)

• There are very few commercial applications involving PTC thermistors that are based upon the resistance-temperature characteristic.

• Most PTC thermistor applications arebased upon either the steady state self-heated condition (voltage-current char-acteristic) or upon the dynamic self-heatedcondition (current-time characteristic) or a combination of both.


• The dramatic rise in resistance of a PTC at and above the transition temperature makes it ideal forover current protection.

• For all currents below the desired limiting current, the power dissipated in the thermistoris not sufficient to self-heat the device to its transition temperature.

• Should an over-current condition occur, the thermistor will self-heat beyond the transition temperature and its resistance rises dramatically.This causes the current in the overall circuit to be reduced.


• 3 more applications are listed , in that they all rely on the dynamic operation (Current-Time Characteristic) of a self-heated PTC thermistor. In each case, current is allowed to pass through a series circuit for a prescribed amount of time before the thermistor self-heats into a high resistance condition.

Time Delay circuit


Motor starting Degaussing

Sample configuration in applicationSample configuration in application(NTC Thermistor)(NTC Thermistor)

• NTC thermistor is a versatile component that can be used in a wide variety of applications where the measured is temperature dependent.

• Thermistor applications are grouped according to one of the three fundamental electrical characteristics;

The current-time characteristics The voltage-current characteristic The resistance-temperature characteristic


• Application based on Current-Time characteristic

Time delay, surge suppression, inrush current limiting and sequential switching represent some of the earliest, high volume uses of thermistors. These thermistor applications are all based upon the current-time characteristic.

• Application based on Resistance -temperature characteristic

Applications that are based upon the resistance temperature characteristics include temperature measurement, control, and compensation.


• Linear Voltage Divider

The simplest thermistor network used in many applications is the voltage divider circuit

The output voltage is taken across the fixed resistor.

This has the advantages of providing an increasing output voltage for increasing temperatures and allows the loading effect of any external measurement circuitry to be included into the computations for the resistor, R .

The loading will not affect the output voltage as temperature varies


• Linear Voltage DividerThe output voltage as a function of temperature is as follows

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

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

http://www.efunda.com/

http://www.thermometrics.com/assets/images/ptcnotes.pdf

http://www.dataacquisitionweb.com/sensors/ntc_thermistors

Thank You !Thank You !

KASHINATH DAS

Download - Thermistors (Sensors)

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