Fig 5. PTC Thermistor Current Voltage Characterisitics
Figure 6 shows two I/V characteristics of one and the same PTC thermistor for two different ambient temperatures T1 and T2, with T1 < T2. At the higher temperature the PTC thermistor has a higher resistance value although the conditions are otherwise the same. Therefore, it carries less current. The curve for T2 is thus below that for T1. The breakdown voltage, too, depends on the ambient temperature. If the latter is higher, the PTC thermistor reaches the critical temperature where breakdown occurs on lower power or operating voltage. VBD2 is therefore lower than VBD1.