Thermistor

Negative temperature coefficient thermistor using B-parameter equation

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  • Thermistor block

Description

The Thermistor block represents an NTC thermistor using the B-parameter equation. The resistance at temperature T is

R=R0eB(1/T−1/T0)

where:

  • R0 is the nominal resistance at the reference temperature T0.

  • B is the characteristic temperature constant.

The following equation describes the thermal behavior of the block:

Q=KdtcdTdt

where:

  • Q is the net heat flow into port A.

  • Kd is the Dissipation factor parameter value.

  • tc is the Thermal time constant parameter value.

  • dT/dt is the rate of change of the temperature.

To model the thermistor in free space:

  1. Connect the thermistor to the B port of a Simscape™Convective Heat Transfer block.

  2. Connect the A port of the Convective Heat Transfer block to a SimscapeIdeal Temperature Source block whose temperature is set to the ambient temperature.

  3. Set the Area parameter of the Convective Heat Transfer block to an approximate area Anom.

  4. Set the Heat transfer coefficient parameter of the Convective Heat Transfer block to Kd/Anom.

Variables

Use the Variables section of the block interface to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables.

Ports

Conserving

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Thermal port.

Electrical conserving port associated with the thermistor positive port.

Electrical conserving port associated with the thermistor negative port.

Parameters

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Electrical

Nominal resistance of the thermistor at the reference temperature. Many datasheets quote the nominal resistance at 25°C and list it as R25.

Coefficient B in the equation that describes resistance as a function of temperature.

Temperature at which the nominal resistance was measured.

Thermal

Time it takes the sensor temperature to reach 63% of the final temperature change when a step change in ambient temperature occurs.

Thermal power required to raise the thermistor temperature by one K.

Model Examples

Thermistor-Controlled Fan

Thermistor-Controlled Fan

How fundamental thermal, mechanical and electrical components can be used to model a thermistor-controlled fan. The heat-generating device starts producing 2 watts at time zero, and then at 40 seconds this increases to 20 watts. The thermistor therefore heats up, and its resistance decreases thereby increasing the voltage across the PWM reference pins. This increases the PWM frequency which in turn increases average motor current, and the fan speeds up. The additional fan speed increases the convective cooling of the device, moderating the temperature increase of the device.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

See Also

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Introduced in R2008a

Simscape Electrical Documentation

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