Influence of the contact material on the performance of temperature-dependent switching controllers in household appliance

Due to their design, the switching behavior of temperature-dependent electro-mechanical switching controllers is completely different to electromagnetically driven devices, especially with regard to opening operation. Usually the expansion of a medium (e.g. liquid, bimetallic strip) is used to initiate the switching operation and thus the break operation is slow. As a result, sticking or welding of contacts can occur, leading to troublesome exceeded temperature or reduced service life. Electrical tests with various contact materials were carried out in a model switch designed to meet the switching conditions in switching controllers. It could be shown that silver/tin oxide containing merely 2% by weight of metal oxide (tin oxide+additives) has significant lower opening forces. Tests in a real power controller could be clearly improved by the application of this special silver/tin oxide contact material. The way this doped tin oxide works is tried to be explained by means of metallographic investigation.