As a circuit protection field application engineer, one of the most common questions I am asked is, “What is the resistance of a PPTC when it is tripped?” The short answer is, “That depends.” PPTCs (polymer positive temperature coefficient devices) are constant power devices in the tripped condition. In other words, in the “untripped” condition, the resistance is constant. But in the tripped condition, the resistance will vary as the voltage varies in order to maintain constant power dissipation, as shown along the curve in Figure 1. Each PPTC device has a power dissipation (Pd) specification. Keep in mind that this is a typical number, not a minimum or maximum. Our customers normally see a lot of variation in the device’s power dissipation, depending on things that may act as a heat sink: e.g. circuit board thickness, trace thickness, etc. There are so many variables in the field that we do not specify an absolute minimum or maximum. Keep reading…