From this formula it is possible to calculate thermoresistor resistance size at any temperature of T2 (in the range of working temperatures), knowing value of a constant of B and resistance of a sample at some temperature of T
Thermoresistors are widely used everywhere, and we meet them every day: systems of fire safety, system of measurement and regulation of temperature, thermal control, the scheme of temperature compensation, measurement of power of VCh are founded on them. Also thermoresistors find application in industrial electronics and the household equipment, in medicine, meteorology, in chemical and other industries.
Size B is defined experimentally by measurement of resistance of the thermoresistor at two temperatures of T1 and T Taking the logarithm the previous expression, it is easy to receive, where, and. Dimension of B – degrees Celsius or Calvin. B – it is coefficient of temperature sensitivity. If to define thermoresistor TKS α as it is usually accepted: that of follows that
Further we will provide some words about physical features of the thermoresistors (more true than semiconductor materials, on the basis of which are made thermoresistors) having positive temperature coefficient of resistance in some interval of temperatures. Such thermoresistors are called often pozistor.
, we will receive. This expression in lg R coordinates also represents the straight line equation that considerably facilitates definition of an interval of temperatures in which the formula with a necessary accuracy approximates the valid dependence of RT (T). By results of measurements of RT and T build the schedule of dependence. If through the points received experimentally it is possible to draw a straight line, consider that in this interval of temperatures expression for RT is fair.
The most admissible power of dispersion of Pmax in W which is not causing irreversible changes of characteristics of the thermoresistor. Naturally, at thermoresistor loading Pmax power its temperature should not exceed tmax.
1 A thermal capacity of C in J on 1oC, equal to the amount of heat (energy) necessary for temperature increase of the thermoresistor on 1oC. It is possible to prove that τ, H and C are connected among themselves by the following :.
When the thermoresistive element is received, it protect special varnishes, and in some cases place in the glass or metal case. At measurement of resistance it is necessary to maintain thermoresistor temperature with high precision (0,05-0,1oC) as resistance is function of temperature.
Thermoresistors the unpackaged uninsulated are intended for measurement and regulation of temperature in electric chains of the constant, pulsing and alternating current with a frequency up to 400 Hz, and also for temperature compensation of elements of the electric circuits having positive temperature coefficient of resistance.
The thermoresistor is a device which resistance strongly changes with change of temperature. It is the resistive device possessing high TCR (temperature coefficient of resistance) in the wide range of temperatures. Distinguish thermoresistors with negative TKS which resistance falls with increase of temperature, often called by thermistors, and thermoresistors with positive TKS which resistance increases with increase of temperature. Such thermoresistors are called as pozistor. Thermoresistors of both types make of semiconductor materials, the range of change of their TKS – (-6,5; +7% / C.
Temperature dependence of resistance is the main characteristic of thermoresistors substantially defining other characteristics of these products. Naturally, it is similar to temperature dependence of specific resistance of the semiconductor of which this thermoresistor is made.
It is possible to tell some words about barium titanate. At low temperatures this substance represents dielectric with the prevailing spontaneous polarization, the potential barrier between crystals is small. Temperature corresponding to Curie's point for TiBa is very important. When heating to this temperature spontaneous polarization disappears, height of a barrier increases and, therefore, resistance strongly increases.