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.
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