Self-Regulating PTC Heating Systems: A New Approach for Electric Heating Appliances

Self-regulating ceramic heating systems that operate without conventional thermostatic controls are now being designed into heated appliances. The result is longer life, lower cost, and more streamlined appliance packaging. The development appropriately comes at a time when end of life safety is receiving increased attention, cost reductions in heater-thermostat systems are producing diminishing returns, and the volume of heated portable appliances for personal care and food warming is growing rapidly. The Semiconducting barium titanate heaters undergo a crystalline phase change and resultant resistivity increase in the vicinity of the ferro-electric curie temperature. This characteristic provides high initial power at which point the sharply increasing heater resistance reduces power to the required steady-state level. Positive temperature coefficient (PTC) barium titanate has been used since the 1950´s for low voltage sensors, but was only made available for appliance heaters by the recent development of line voltage materials for a different segment of industry¿color television. One third of all color sets now use 120-V PTC degaussing elements. The paper discusses the complex thermal design considerations involved in the development of a PTC heater for 10-200 W appliances and reviews the application of a PTC heater to a food warmer.