Thermistor Specifications / Specs

- correctly specifying a thermistor enables the required performance to be obtained. It is essential the correct specifications are used for any given application.

There are several important thermistor specifications that need to be considered for any given thermistor application. Although resistance is the prime spec, there are others that need to be considered.

There are a number of different thermistor specifications, some are more important than others, but there are a number of specs that are applicable to all thermistor applications.

Basic thermistor specifications

Some of the more important thermistor specifications are summarised below:

  • Resistance:   This is one of the key parameters for any thermistor. They can be obtained with a variety of resistance figures ranging from Ohms up to many kilo-Ohms. As the resistance varies with temperature the temperature must also be stated. This is often given at 25°C, and is often known as the R25 value. For more specialist applications other temperatures may be used. Also beware, sometimes temperatures may be quoted in absolute temperatures, i.e. °K.
  • Tolerance on resistance value:   As with any resistor, there is a tolerance of the standard resistance. This is taken as the R25 value, or the value at the temperature for which the resistance is given. Values of ±2%, ±3% and ±5% are normally available.
  • B value / constant :   Also referred to as the β value, this thermistor specification is a simple approximation for the relationship between the resistance and temperature for an NTC thermistor. To obtain the value, two temperatures are used to obtain the value for β. This is a very useful parameter where relatively small temperature differences are likely to be encountered. The two temperature values are added to the B value, as they are an integral part of the specification.
  • Tolerance on B value / constant :   As the name indicates this is the tolerance on the value of β.
  • Time constant:   No body can instantly raise its temperature from one value to another. It follows and asymptotic curve. Also the larger the body, the longer it takes for the temperature to rise. Accordingly the time constant of the device is an important thermistor specification for some applications.

    For measurement purposes the temperature required for the τ measurement; i.e. the time to reach the resistance for 63.2% of the temperature difference is:

    Tτ = 0.632 x (t2 - t1)

  • Thermal dissipation factor δ :   This is an important feature of the thermistor because all thermistors need to pass some current for the operation of the circuit in which they are included. This causes self-heating of the thermistor.

    This thermistor specification defines the relationship between the applied wattage and the thermistor self-heating. If too much current is passed through the thermistor, then it will offset the operation of the thermistor. Accordingly this specification governs the current that can be passed through the device. The dissipation factor, δ is expressed in terms of mW/°C.

    δ = P / ΔT


    Where
        P = power dissipated in watts
        ΔT = the rise in temperature in °C

    A particular value of δ will correspond to the level of power needed to raise the thermistor temperature by 1°C. The dissipation factor depends upon a number of factors and as a result the thermistor specification for dissipation factor, δ is really only useful as a guide rather than an exact figure.
  • Maximum power dissipation:   This is the maximum power dissipation for the device. Normally the power dissipation is kept low to prevent self-heating, but under some circumstances there may be reasons to dissipate more power. The specification for maximum power dissipation should not be exceeded if damage is not to result. For greatest reliability the device should be operated well inside its maximum power dissipation - often only 50 - 66% of the specification.
  • Operating temperature range:   This is the temperature range for which the thermistor is designed to operate. Materials, construction and other similar factors limit the range over which the device can operate. Accordingly, for reliability as well as performance, the thermistor should not be operated outside its specified temperature range.

By Ian Poole


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