The long-term stability of fixed resistors

The causes of long-term failure under practical conditions of use or storage of different types of fixed resistors commonly used in electronic equipment have been investigated. Some reported life tests have proceeded without interruption for almost four years. Carbon-composition (grade 2) resistors under load fail by slow thermal degradation of the resistive material. Drift of value may also occur if unloaded resistors of this type are stored in a damp atmosphere. Vitreous-enamelled wire-wound resistors made with fine wire may fail during tropical exposure both unloaded and especially when lightly loaded with direct current. This is owing to electrochemical corrosion taking place at faults in the vitreous coating. High-stability cracked-carbon (grade 1) resistors may fail rapidly under light d.c. load by electrochemical action if moisture condensation occurs and the protective paint or varnish coating is inadequate. Tests for long-term resistor stability are critically discussed.