Measurement NTC thermistors
(23)What is a thermistor
A thermistor is a type of a resistor made of semiconductor material, with resistance that is strongly dependent on temperature. Currently, there are many types of thermistors available, offered in various housings and with parameters depending on their application. The smaller the dimensions of a thermistor, the greater the range of resistance changes and the lower the power losses are likely to be. SMD thermistors or thermistors in housings of the size of a match head, typically have a very wide range of resistance changes. Large-size thermistors offer a narrow range of resistance variations and are often fitted with heat sinks or aluminium housings. Many of today's thermistors are almost identical in terms of appearance to a typical SMD resistor.
NTC measuring thermistors, NTC protective thermistors and other thermistors
The basic division of thermistors results from the temperature coefficient and their purpose in the device.
NTC thermistors (Negative Temperature Coefficient) feature a negative temperature coefficient. They have high resistance at low temperatures and low resistance after the thermistor heats up. PTC (Positive Temperature Coefficient) thermistors work the other way round, at low temperatures they have small resistance, which increases with the temperature of the thermistor.
Measuring thermistors act as temperature-to-voltage converters. Due to this application, manufacturers strive to ensure the repeatability of their parameters, small manufacturing tolerances and linearity of resistance to temperature changes. Usually, they will also have small dimensions and thus small thermal inertia. Protective thermistors act as surge current limiters in electrical circuits at the moment of power-on. These are NTC thermistors, which are connected in series to a power circuit. They have relatively large dimensions and a special design, which allows heat to be dissipated to the environment.
What are NTC thermistors and others used for?
Measuring thermistors are used primarily for temperature measurement in the range from approximately -40°C to +125°C. Although there are other special designs with extended range, most of the popular thermistors fall within the one specified above. Measuring thermistors are also used in temperature compensation circuits of electronic components.NTC protective thermistors are used to limit the rate of current increase in sensitive electrical circuits. In the past, they were used in circuits connected to lamp filaments, where they increased the life of lamps by protecting the cold filament when the power was switched on.
NTC thermistors and other thermistors – basic parameters
The basic parameters characterising a thermistor are: nominal resistance at 25°C, temperature coefficient, temperature coefficient of resistance, permissible power loss, nominal voltage, tolerance, type of housing.
Choosing the right thermistor for specific applications
Thermistors are manufactured to meet different values of current and voltage, not only for incorporation into a circuit supplied with safe voltage. Hence the variety of their housings and the wide range of dimensions. Some thermistors, for example, those used in the automotive industry, have the appearance of a screw fitted into the engine housing. The choice of the thermistor will primarily depend on the target application. Measuring thermistors and protective thermistors are chosen based on a different approach.
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