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A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive. As heaters, in the automotive industry, to provide cabin heating (in addition to heating provided by a heat pump or the waste heat of an internal combustion engine), or to heat diesel fuel in cold conditions before engine injection.
The Steinhart–Hart equation is a model relating the varying electrical resistance of a semiconductor to its varying temperatures.The equation is = + + (), where is the temperature (in kelvins),
A big drawback is that it is low-impedance: that is, the power used to heat the thermistor comes from the circuit being measured. If the circuit being measured can support the heating current, then it is possible to make a post-measurement calculation to correct the effect, as the impedance of the heating element is known.
An NTC thermistor's resistance is low at high temperatures. When the circuit is closed, the thermistor's resistance limits the initial current. After some time, current flow heats the thermistor, and its resistance changes to a lower value, allowing current to flow uninterrupted. It is inherently impossible for 100% of supply voltage to appear ...
The temperature coefficient of a PTC heating element generally is a function of temperature. The Steinhart–Hart equation is often used to approximate this function. In some applications where the heater is used only in a narrow temperature range, a simple linear equation may be adequate.
Figure 4. These circuits are equivalent: (A) A resistor at nonzero temperature with internal thermal noise; (B) Its Thévenin equivalent circuit: a noiseless resistor in series with a noise voltage source; (C) Its Norton equivalent circuit: a noiseless resistance in parallel with a noise current source.
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Another type of thermal switch is a PTC (Positive Temperature Coefficient) thermistor; these thermistors have a "cutting off" temperature at which the resistance suddenly rises rapidly, limiting the current through the circuit. When used in conjunction with a thermistor relay, the PTC will switch off an electrical system at a desired temperature.