Silver Mica capacitors

- an overview, information or tutorial about the basics of the silver mica capacitor: its construction, properties and the uses of the silver mica capacitor particularly in RF circuits.

Silver mica capacitors are not widely used these days except for specialist applications. However silver mica capacitors used to be very widely used some years ago. They are able to provide very high levels of accuracy and stability and as a result they found many uses in radio frequency applications. However they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required. In view of this one of their major uses is within the tuned elements of circuits like oscillators, or within filters.

Silver mica capacitor properties

Values for silver mica capacitors are normally in the range between a few picofarads up to two or possibly three thousand picofarads.

Silver mica capacitors have a very high tolerance and a good temperature coefficient. The temperature coefficient is positive and is normally in the region 35 to 75 ppm / C, with +50 ppm / C being an average value.

Although silver mica capacitors have a high tolerance and low temperature co-efficient they are known to jump in value on occasions.

Construction

For silver mica capacitors the silver electrodes are now plated directly on to the mica dielectric, although originally thin sheets of silver foil were placed between the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole silver mica capacitor assembly is encapsulated to provide protection. Today a ceramic encapsulation is used, although early versions, used in some valve or vacuum tube radios can be seen to have a form of wax encapsulation. This was effective for the day in protecting the capacitor from moisture, but when warmed, the wax melted, and often these capacitors had little wax on them from the warm environment of a vacuum tube or valve radio.