Thermistor design and measurement considerations

Edited by the Engineeringtalk editorial team Feb 28, 2005

NTC thermistors have many advantages over other temperature sensor types, such as variety of electrical characteristics, options for physical configurations and ease of electrical interfacing.

NTC thermistors have many advantages over other temperature sensor types, such as variety of electrical characteristics, options for physical configurations and ease of electrical interfacing.

To make best use of the inherent useful features of thermistors, developers of thermistor-based solutions for temperature sensing applications need to be aware of certain design and measurement considerations.

It is important for temperature sensing system designers to work closely with thermistor suppliers to define the optimum thermistor sensor configuration for an application.

Although thermistor customers and suppliers may both have a good understanding of thermistor issues, there can be some fundamental differences in outlook between them.

Thermistor suppliers tend to think in terms of resistance of the thermistor as the most important parameter.

Technical datasheets from thermistor suppliers tend to refer to resistance as the main parameter and concentrate on accurately measuring thermistor resistance at fixed temperature points for thermistor device characterisation.

Thermistor system designers tend to think of temperature as the most important parameter since the function of most thermistor systems is to obtain an estimate of temperature based on electrical measurements.

Although thermistor characteristics are defined in terms of resistance versus temperature, system designers usually need to work with instrumentation that requires a voltage input, so that the relationship between the voltage across a thermistor in an interface circuit and the temperature of the thermistor is the most important relationship for them.

Thermistor manufacturers perform resistance versus temperature testing for thermistors in ideal measurement environments.

The temperature measurement is provided by a suitable temperature controlled system, such as a liquid bath.

The thermal mass of the liquid in the bath is generally greater than one thousand times the thermal mass of the thermistor device being measured.

This reduces the possibility of the thermistor disturbing the temperature of the measurement environment by thermal shunting.

The resistance of the thermistor is usually measured with a high quality multimeter that can be configured to optimise measurement parameters for various resistance ranges.

Power levels in the thermistor can be maintained at suitable levels to minimise self-heating (Joule effect) during measurement.

The thermistor test systems can also be adapted to allow for the time response or settling times for thermistor devices with large thermal mass.

The measurement of thermistors in applications are often made in conditions that are not ideal.

For example: the temperature environment that is being monitored may fluctuate rapidly; the measuring instrumentation may consist of basic components and may not be able to implement suitable current limitation in the thermistor; or measurement sampling times may not be suitable for the thermal time response of a thermistor device.

Some of these issues may have to be overcome in the system design to obtain an electrical measurement that will allow the temperature to be determined with accuracy.

Further discussion on factors that affect the measured value of a thermistor and guidelines for interfacing thermistors to measurement systems can be obtained in the NTC thermistor theory section of the Betatherm website which is available to registered users.