Self-regulating heater
A self-regulating heater is an electrical heater which strives to keep a constant temperature regardless of how the ambient conditions changes. It is the material in the heater itself that regulates the temperature. The heater requires no regulating electronics, temperature sensors, overheat protection etc. The temperature the heater will hold is decided when the material and the heater is produced. It can be fine tuned by changing the voltage to the heater. Self-regulating heaters are made from materials with strong Positive temperature coefficient (PTC) characteristics, i.e. the resistivity of the material increases rapidly with increasing temperature. Such materials include ceramic PTC stones.[1] and PTC rubber.[2]
Ceramic type
A ceramic PTC heater is made of small ceramic pieces pressed between two metal plates. The constraint compartment causes them to get a strong PTC effect. The typical temperature of ceramic stones is 140–250 °C (284–482 °F). Ceramic stones actually have Negative temperature coefficient (NTC) properties at low temperatures which makes them slow to heat up.
Rubber type
A rubber PTC heater is made from a special type of rubber which conducts electricity, but only up to a temperature which is defined at the production of the PTC rubber. Typical design temperatures are between 0–80 °C (32–176 °F). The resistivity of the rubber increases exponentially with temperature for all temperatures up to the design temperature. Hence, it has strong PTC properties for all temperatures and heats up rapidly. Above this temperature the rubber is an electrical isolator and cease to produce heat. This makes the heater self-limiting. The rubber foil is thin and flexible and can be formed to any shape and size.
Operation
When a voltage is applied to a PTC heater electrical current flows through the material and the resistivity of the material causes it to heat up (P(T)=U∧2÷R(T)). As it heats up the resistivity increases rapidly and the power (heat) produced decreases. At the same time heat is being transferred from the heater to the object it is attached to, and its surrounding. Eventually the amount of heat produced is in equilibrium with the amount of heat conducted and radiated away from the heater. The heater reaches its equilibrium temperature and remains there. The exponential PTC properties of the material assures that the equilibrium temperature is virtually insensitive to changes in the ambient temperatures.