Resumen de Thermistor circuit theory
Thermistors (themally sensitive resistors), i.e., resistors made from solid semiconducting matherial and having a large negative temperature coefficient, are now widely used as components of measuring and control circuits. The calculation of electric circuits containing thermistors is complicated by two circumstances: the thermistor is a non-linear element and it has thermal inertia.
The lecture gives de outlines of methods for the calculation of thermistor circuits which have been published in recent years in the Transactions of Chalmers University of Technology (A Study of Thermistor Circuits: No. 1/1954, No. 2/1955, No. 3/1957, No. 4/1959.) A characteristic feature of these methods is that the thermistor temperature has been consistently replaced as a variable by the heat power, dissipated from the thermistor to the surroundings. To a given thermistor temperature generally corresponds a certain dissipated power. The latter quantity has the advantage, however, that it can be determined by purely electrical means.
If we want to solve a static problem, we must know the static characteristics of the thermistor. if we want to solve a dynamic problem, we must also know the quantity which takes care of the thermal inertia i.e., the thermal time constant. The investigation shows that in most cases all data required can be obtained through a small number of electric measurements.
Small signal equivalent circuits are given for directly as well as indirectly heated thermistors. The large signal behaviour is studied by semi-graphical means.
The theory is supported by extensive measurements on different types of thermistors.
