varistor protection circuits - a safe-guard for semi conductors

varistor protection circuits ... elektor march 1984 varistor protection circuits . . Varistors, conventionally classified as "non- linear resistors", are produced from silicon- carbide, zinc oxide (zincite), or titanium oxide. Granules of these materials are sintered at high temperatures in a vitreous ceramic. One outstanding feature of voltage dependent resistors (VDRs) is that their resistance/voltage characteristic (figure la) is symmetrical, that is, independent of po- larity. This is due to the fact that although any single contact in the resistor mass may rectify, random distribution of large numbers of contacts in series and parallel results in equal numbers of contacts rec- tifying in opposite directions. This makes them eminently suitable for AC applications where protection diodes cannot be used. The operation of a varistor is best under- stood by considering it as two zener diodes connected back-to-back. Below a certain voltage the current is small because the resistanc...