Investigation of the pulse load behavior of power wire-wound resistors

A long-term resistance drift of wire-wound resistors or even a failure is frequently caused by short but repeated temperature peaks during pulse load operation. In this paper this aspect is investigated by comparison of two types of power wire-wound resistors with identical resistance value, nominal power, and size, but significantly different thermal behavior under pulse load condition. In order to capture the thermal cause as directly as possible a specific measuring circuit was developed to record the temperature as a function of time in the resistor wire during the dynamic cooling phase immediately after a pulse. In this way a peak temperature of e.g. 405degC was observed in one resistor type whereas the peak temperature in the other one at the same pulse load condition exceeded even 1000degC which is undoubtedly detrimental if periodically repeated. The measuring principle is presented in detail. The measuring accuracy is estimated and its meaning for the interpretation of the obtained results is shown. Based on these results the dependence of the pulse load resistivity of a wire-wound resistor on its interior structure is discussed. It is demonstrated that the pulse load performance can be improved significantly by increasing the thermal capacitance of the resistor wire without increasing the resistor size.