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The current and voltage across the thermistor depend on the particular circuit configuration. As a simple example, if the voltage across the thermistor is held fixed, then by Ohm's law we have = /, and the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor:
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),
Differential Equations: Applied to model and analyze the behavior of circuits over time. Used in the study of filters, oscillators, and transient responses of circuits. Complex Numbers and Complex Analysis: Important for circuit analysis and impedance calculations. Used in signal processing and to solve problems involving sinusoidal signals.
For example, a PTC heating element with a sharp change in resistance at a particular temperature can be fitted with a constant voltage source and a variable-speed fan. With the fan at a low setting, the heating element draws only a small amount of current, resulting in a low heat output.
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Such relationship may take the form of a graph, where numerical values of a circuit variable are plotted versus frequency or component value (the most common example would be a plot of the magnitude of a transfer function vs. frequency). Symbolic circuit analysis is concerned with obtaining those relationships in symbolic form, i.e., in the ...
For example, resistors, capacitors, and inductors are linear, while diodes and transistors are nonlinear. An I–V curve which is a straight line through the origin with positive slope represents a linear or ohmic resistor, the most common type of resistance encountered in circuits.
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.