# PTC Thermistor

Model switching type positive temperature coefficient (PTC) thermistor

Sensors

## Description

The PTC Thermistor block represents a switching type PTC thermistor. This type of thermistor has a decreasing resistance with temperature increasing up to the Curie temperature. Above the Curie temperature the resistance increases very rapidly with increasing temperature, as shown in the following plot. The region to the right of the Curie temperature is called the Positive Temperature Coefficient (PTC) regime. To represent a non-switching linear PTC thermistor, use the Thermal Resistor block.

For a switching type PTC thermistor, the resistance R at temperature T is given by

${T}_{c}=\frac{\mathrm{log}\left({R}_{1}\right)-\mathrm{log}\left({R}_{0}\right)+{\alpha }_{0}{T}_{0}-{\alpha }_{1}{T}_{1}}{{\alpha }_{0}-{\alpha }_{1}}$

where:

• Tc is the Curie temperature.

• R0 is the resistance at nominal temperature T0.

• R1 is the resistance at reference temperature T1.

• T0 is the nominal temperature at which the resistance is quoted, usually room temperature. T0 is less than the Curie temperature Tc.

• T1 is the reference temperature, equal or greater than the Curie temperature Tc, which means that at this temperature the PTC regime is in force.

• α0 is the temperature coefficient at nominal temperature T0.

• α1 is the temperature coefficient at reference temperature T1.

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.

• 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.

## Dialog Box and Parameters

### Electrical Tab

Nominal resistance R0 at T0

The nominal resistance of the thermistor at the nominal temperature. Many datasheets quote the nominal resistance at 25°C and list it as R25. The default value is `1000` Ω.

Temperature coefficient alpha0 at T0

The temperature coefficient at the nominal temperature. The value must be less than zero. The default value is `-0.01` 1/K.

Nominal temperature T0

The temperature at which the nominal resistance is measured. The default value is `298.15` K.

Reference resistance R1 at T1

The reference resistance of the thermistor at the reference temperature. The default value is `10000` Ω.

Temperature coefficient alpha1 at T1

The temperature coefficient at the reference temperature. The value must be greater than zero. The default value is `1` 1/K.

Reference temperature T1

The temperature at which the reference resistance is measured. This temperature must be in the PTC regime. The default value is `398.15` K.

### Thermal Tab

Thermal time constant

The time it takes the sensor temperature to reach 63% of the final temperature change when a step change in ambient temperature occurs. The default value is `5` s.

Dissipation factor

The thermal power required to raise the thermistor temperature by one K. The default value is `7.5e-4` W/K.

Initial temperature

The temperature of the thermistor at the start of the simulation. The default value is `298.15` K.

## Ports

The block has the following ports:

`A`

Thermal port

`+`

Positive electrical port

`-`

Negative electrical port