The Effect of Moisture on Molded Composition Resistors

Moisture problems have perennially plagued the use of the electrical and electronic products. Carbon composition resistors represent a widely used component which must withstand the effects of moisture. Continual improvements in construction and materials have made this possible. To illustrate and magnify the changes that occur in commercial composition resistors with moisture exposure, molded resistors were prepared without insulating jackets or impregnant. Moisture changes are demonstrated to be largely reversible by drying and to be cumulative on extended or repeated moisture exposure. A good correlation was obtained between changes in length and resistance during moisture exposure. An impregnating wax is shown to reduce greatly the moisture susceptibility of the model resistors. Some comparative results obtained with different types of resin binder are also shown, indicating the significant contribution of this phase.