Wirewound resistors, ideal at handling overvoltage surges, are often specified for the surge and overload conditions of harsh environments. The core of a wirewound resistor typically consists of fiberglass or a ceramic rod, with metallic wire wrapped around the substrate. While ceramic is more expensive, it acts as a heat sink and provides for an increase in efficiency as the surge event lengthens, whereas fiberglass provides little thermal benefit in overload situations.
There are several ways to increase the surge capability of a wirewound resistor. First, carefully selected longer lengths of larger diameter (lower resistance) wire can be tightly wound on the substrate. This leads to an increased resistance element mass. Additionally, the surge capability can be increased by using two layers of wire and winding them in opposite directions to reduce the resistor's inductance. Not only does this decrease the inductance, but it also increases the resistance wire mass, which in turn improves the resistor's surge handling capability.
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