Current-Voltage (I-V) Characteristics
The current - voltage characteristics describes the voltage change as the current flow through a thermistor varies.
I-V curves of NTC thermistors are characterized in that the voltage rises linearly as the current rises. However, the voltage begins to decrease after achieving its peak at a certain current value.
A thermistor begins self-heating when a current is applied, and the amount of heat generated by the thermistor becomes larger as that of current flow increases.
While the heat generation is small, the heat release from the surface and lead wires of the thermistor is large. Thus, the thermistor body temperature will not change and so too its resistance. The current and voltage are directly proportional to each other according to Ohm’s Law.
However, once the amount of heat generation becomes larger than that of heat release, the thermistor body temperature rises, its resistance lowers, and the proportional relationship between current and voltage is lost. Then the voltage gradually decreases after achieving its peak at a certain point.
The chart below shows the I-V characteristics of different thermistor elements. It is important to use a thermistor within the range where a line is straight and self-heating has little effect on the resistance.
Using voltages over the peaks shown on the chart may bring the thermistors into “a runaway mode” where they glow and break down in a short time. Particular attention should be given to voltages that will be applied.