Self-regulating heater

Some PTC heating elements are designed to have a sharp change in resistance at a particular temperature. These elements are called self-regulating, because they tend to maintain that temperature, even if the applied voltage[1] or heat load[2] changes. Below that temperature, the element produces a large amount of heating power, which tends to raise the temperature of the heating element. Above that temperature, the element produces little heating power, which tends to allow it to cool.

In some applications, this self-regulating characteristic allows PTC heaters to be used without thermostats or overtemperature protection circuits.[1] One very important use of self-regulating heating elements is to assure the heating element will not become so hot as to damage itself or other parts of the heater. In some applications, where the heating element is directly connected to the item being heated, a self-regulating heater may also provide adequate temperature control of the item to be heated.

However, many applications require control of two temperatures. For example, space heaters use heating elements much hotter than the room being heated. In these applications, a thermostat may be better able to sense and control the temperature of the item being heated. Nevertheless, a self-regulating heating element may still be used to keep the heating element from damaging itself or other parts of the heater.

If the heat required to hold the desired temperature is known, a PTC heating element can be selected to provide the correct amount of heat at the desired temperature. Such a heating element will warm up quickly, because it produces more heat at low temperatures. In contrast, a conventional heating element that produces the correct amount of heat at the desired temperature will produce the same amount of heat at low temperature, resulting in long warm-up times.

A heater with adjustable heat output can be accomplished with a PTC heating element plus a means to adjust the heat load. For example, a space heater with an adjustable speed fan.[3]

If being able to adjust the temperature is more important than holding a fixed temperature, then a PTC material whose resistance changes smoothly with temperature can be used. The temperature such a material tends to hold can be adjusted by changing the voltage applied.

PTC heating elements are more tolerant of small defects, which allows them to be manufactured in more shapes than conventional metal heating elements. Generally, metal elements are made long and thin, in part to assure all the electrical current goes through the entire element. Such metal elements are often in the form of wires or ribbons and wrapped into a coil. If a metal element was made wide, then there would be more than one path for the electricity, and a small defect could cause the element to fail. The defect might cause most of the electrical current to be concentrated in a small portion of the element, which might then overheat and fail. This would then steer the current into another areas, which would also overheat and fail until the entire element had failed. By comparison, if too much current goes through a portion of a PTC heating element, that portion will increase in resistance, redirecting the current more evenly across the element and preventing failure.

One application of a specially shaped heating element is to increase the surface area of the heating element. A large surface area means the element can operate at a lower temperature and still deliver a large amount of heat. The lower temperature may make a heater safer. However, other safety measures can assure the safety of conventional heaters.

Another application of a specially shaped heating element is to closely match the shape of the item being heated, which helps assure the object is maintained close to the same temperature of the heating element.

Although the most commonly available ceramic materials are electrical insulators, some conduct electricity with a positive temperature coefficient. Such PTC ceramic heating elements are often called "stones".[1][4]

Some polymers are suitable as PTC heating materials. These have the useful property that they can be made in the form of inks. Heating elements of complex shape can be easily manufactured using printing techniques. If the inks are printed onto a flexible substrate, then the whole heating element can be flexible.[5]

One type of polymer is a PTC rubber, which is a type of silicone rubber.