Reliable Precision Wirewound Resistor Design

The precision wirebound resistor has undergone fairly extensive design changes within the last four or five years. The old "open bobbin" style resistor has been completely replaced by the molded thermosetting plastic enclosed resistor. The new design is capable of performing at extremely low and high temperatures, and will withstand rugged environmental testing, such as salt water immersion, humidity testing with polarizing voltage, and repeated temperature cycling. Improvements in design and in wire insulation have made it possible to operate these new units at higher internal temperatures, thus increasing their power handling capability. The protection afforded by the encapsulated housing makes the use of fine wire feasible in military resistors with consequent increase in maximum resistance value. This paper compares the old and new resistors, and discusses some of the factors which influenced the new precision resistor de- sign.