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Methodology for metrological evaluation of a PTC resettable thermistor
Alcir de Faro Orlando, Marlon Huamani Bellido and Reinaldo Nivaldo da Silva

The objective of this work is the development of a methodology for experimentally characterizing a polymeric thermistor used as an over current and temperature protection device. Uncertainty of measurements of parameters, such as temperature, voltage, current and ohmic resistance is also estimated. The tested thermistor has a Positive Temperature Coefficient (PTC), which means that its ohmic resistance increases with temperature. In order to measure the thermistor characteristics, an electric circuit was mounted and placed in a dry oven; the circuit consists of a stable voltage source and a variable resistance, both in series with the thermistor. A K type thermocouple was attached to it, so that its temperature could be measured and controlled. By gradually decreasing the circulating current through the circuit, the Joule effect could be neglected, and the thermistor resistance is only a function of temperature, which is easier to measure. The acquired dates were used to build several characteristics curves, which were analyzed and compared to the available ones from the manufacturer data. Finally, this work proposes the development of a systematic procedure to obtain the thermistor performance data, so that the manufacturer data can be verified, together with its uncertainty, providing at the same time information not presently available in the market for system design.

Keywords: Uncertainty of measurement, PTC polymeric resettable thermistor, polymer, circuit protection, thermistor characteristic curves, sensor performance evaluation.

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