Control device and safety circuit for heating pads with ptc heater

Abstract

Heating pads using positive temperature coefficient (PTC) resistance material are subject to fire risk if one of the conductor wires between which the PTC material extends should break and produce an electric arc. Protection by fuse and a fuse-blowing circuit responsive to fire detection must allow for an immense inrush of current when the cold pad is turned on. A heat setting control using a microprocessor can reduce the fuse rating by chopping the a.c. heating current for a short start-up period following with full-on feed until the heat setting is reached. The presence of a microprocessor allows response to a safety circuit that detects a break in a heater feed or return conductor before much excess heat develops, so that the microprocessor can turn off the heater switch. That response is so quick that it can be confirmed by repeated detection after very short pauses before the heater switch (a triac) is turned off. The safety circuit producing the fault detection signal may be external to the microprocessor chip, or most of it can be built into the microprocessor chip, which then receives two inputs from a smaller circuit connected to the heating pad. A second triac can be used to shut off the heater if the heater switch malfunctions by locking in its 'on' position.