Long-term stability of temperature-sensitve resistors

During the life of a temperature-sensitive resistor (t.s.r.), two processes are at work producing permanent changes in body resistance. One gives an increase in resistance, and is thought to be crack propagation; with constant current loading, it can result in failure by fracture. The other process gives a decrease in resistance, and is thought to be continued reaction of the constituent oxides; with constant current loading, it can result in resistance stabilization. With normal current loadings, the approach to failure with life of an unprotected rod can proceed at a rate such that the `cold¿ resistance is nearly proportional to the square root of lifetime. Operational lifetime of a rod is defined as the time required for the `cold¿ resistance to increase by 15% with constant current loading. The corresponding current rating decreases logarithmically with the required operational lifetime. On this basis, the current rating of a particular rod should be reduced by at least 50%. This need for a reduction in rating is in accordance with the results of conventional life tests. There seems to be little justification for the popular assumption that the rod form of t.s.r. is intrinsically a much inferior component to the bead. This view is an unfortunate consequence of the relatively very different rating standards adopted for the two items. Continuous operation of a typical rod at a current twice the rated value reduces its life by a factor of 20 to about 100h. On the other hand, continuous operation of a bead at the original rating gives a life of more than 10000 h. Suitable derating of the rod would eliminate the apparent difference between the two forms.