Thermistor

Negative temperature coefficient thermistor using B-parameter equation

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Libraries:
Simscape / Electrical / Sensors & Transducers

Description

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

$$R=R_0{e}^{B(1/T-1/T_0)}$$

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:

$$Q=K_d{t}_c\frac{dT}{dt}$$

where:

• Q is the net heat flow into port A.

• K_d is the Dissipation factor parameter value.

• t_c 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 Simscape Ideal 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 A_nom.

4. Set the Heat transfer coefficient parameter of the Convective Heat Transfer block to K_d/A_nom.

Variables

To set the priority and initial target values for the block variables before simulation, use the Initial Targets section in the block dialog box or Property Inspector. For more information, see Set Priority and Initial Target for Block Variables.

Use nominal values to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources. One of these sources is the Nominal Values section in the block dialog box or Property Inspector. For more information, see System Scaling by Nominal Values.

Examples

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.

• Open Model

Ports

Conserving

A — Thermal port

thermal

Thermal port.

+ — Positive port

electrical

Electrical conserving port associated with the thermistor positive port.

- — Negative port

electrical

Electrical conserving port associated with the thermistor negative port.

Parameters

Electrical

Nominal resistance R0 at reference temperature T0 — Nominal resistance at reference temperature

1000 Ohm (default)

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

Characteristic temperature constant B — Characteristic temperature constant

3500 K (default)

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

Reference temperature T0 — Reference temperature

298.15 K (default)

Temperature at which the nominal resistance was measured.

Thermal

Thermal time constant — Thermal time constant

5.0 s (default)

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

Dissipation factor — Dissipation factor

0.75e-3 W/K (default)

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

Extended Capabilities

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

Version History

Introduced in R2008a