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Thermistor Calculation Examples
A representative and typical NTC Resistance/Temperature curve is shown in Fig. 1. This curve displays a non linear drop in resistance as the thermistor body temperature is increased. The graph in Fig. 2 shows a comparison of an NTC thermistor versus a platinum RTD. While the platinum RTD curve is more linear, it is much less sensitive to temperature change. The phenomena of delaying current through a thermistor can be readily seen in Fig. 3. When a voltage is applied to the thermistor, its initial resistance is high, therefore limiting current flow. As the thermistor body temperature rises from self heating, the resistance drops while the current continues to rise. This cycle continues until a point of current saturation is reached, limited by the applied voltage and the initial circuit resistance. A larger body thermistor will have a greater mass and consequently a larger dissipation constant which in turn will slow down the time required to reach the steady state condition. This can be seen by the series of thermistors in the graph with increased mass and associated longer response time to reach saturation. BETA: The characteristic of a thermistor in terms of its Resistance vs Temperature will generally follow the absolute value of its temperature within limits and with a small degree of error. This can be described by the following equation.
where R0T1 is the zero power resistance at absolute temperature T1 where R0T2 is the zero power resistance at absolute temperature T2 e = 2.71828... ß is a constant whose value is determined by the composition of the thermistor material. EXAMPLE : Find the Beta of a thermistor at R25/50 whose values are R25 (T1)= 10000 ohms and R50 (T2) = 3300 ohms
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General Information:Thermistors are generally described as thermally
sensitive resistors whose characteristics exhibit large changes in resistance with
a small change of body temperature. This change of resistance with temperature can result
in a negative coefficient of resistance; where the resistance decreases with an increase
in temperature (NTC thermistor). When the resistance increases with an increase in
temperature, the result is a positive temperature coefficient or a PTC thermistor. Most
metals have a positive temperature coefficient. The product line discussed here is the
Sensor NTC thermistor.
