Resettable Circuit Protection Options for Electric Motors, Transformers and Power Supplies

The latest generation of PPTC (polymeric positive temperature coefficient) devices includes components that are rated for line voltages of 120 VAC and 240 VAC and can be used in parallel for increased current-carrying capacity. Their low-cost, resettable functionality and latching attributes make them a reliable, cost-effective circuit protection solution for power supplies, transformers, and small- and medium-sized electric motors.

Traditionally, single-use fuses have been used to protect electronic circuits from overcurrent events.  With this technology, when a wiring fault or part failure creates a condition in which excessive currents can flow, the fuse blows, breaking the electrical connection and preventing more widespread damage or fire hazards. 

The problem with this approach is that a failure in one system component can disable other components downstream and throughout the system. Now, the fuse must be accessed and replaced on all the affected components before the system can be made operational again. Resettable PPTC devices offer a practical alternative to fuse technology and help protect valuable electronic systems, reduce warranty and service costs, and enhance user satisfaction.

This paper describes the PPTC device’s principle of operation and presents test data that compares the performance of the PPTC device to thermal fuses and bimetal breakers. Circuit diagrams illustrate how the device can be installed in motor and transformer circuits to help protect against damage due to overcurrent and overtemperature events. Design considerations for both intermittent- and continuous-operation motor circuits are also presented.