Silver Mica Capacitor

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

Silver mica capacitors have been widely used as high performance capacitors over the years. Although silver mica capacitors are not as widely used, these days, nevertheless they are still available and used in a variety of applications where their particular properties are needed.

Silver mica capacitorSilver mica capacitors are able to provide very high levels of accuracy, stability and low loss. As a result silver mica capacitors found many uses in radio frequency applications, particular for oscillator and filter circuits where their stability, accuracy and low loss (leading to high Q) were needed. Although not as widely used these days, they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required.

Two main reason for the decline in the use of silver mica capacitors is their size, resulting from the materials used and their construction. The cost of silver mica capacitors is higher than many other types that can often be used these days.


Silver mica capacitor properties

The reason for the continued use of silver mica capacitors is the fact that they can offer very high levels of performance, better in many areas than any other type of capacitor. However in many applications, other more modern technologies provide levels of performance that meets the needs for that particular requirement.

The particular properties of the silver mica capacitor are summarised below:

  • High accuracy:   Silver mica capacitors can be obtained with tolerance figures of +/- 1%. This is much better than virtually every other form of capacitor available today.
  • Temperature co-efficient:   The temperature co-efficient of silver mica capacitors is much better than most other types of capacitor. The temperature coefficient is positive and is normally in the region 35 to 75 ppm / C, with +50 ppm / C being an average value
  • Value range:   Values for silver mica capacitors are normally in the range between a few picofarads up to two or possibly three thousand picofarads.
  • Low capacitance variation with voltage :   Silver mica capacitors exhibit very little voltage dependence.
  • High Q :   Silver mica capacitors have very high levels of Q and conversely small power factors. These are both almost independent of frequency.

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


Mica dielectric

The mica dielectric obviously forms the basis for silver mica capacitors. Its properties govern the performance of the silver mica capacitor. It was also one of the first dielectric materials to be used for capacitors in the early days or wireless because of its combination of stability and general physical and mechanical attributes.

Silver mica capacitorAlthough there are several different forms of mica, they all have very similar properties. They are fundamentally very stable both mechanically and chemically, enabling the capacitors manufactured with mica to exhibit similar properties. The material has a dielectric constant ranging from around 5 to 7.

It is also found that the crystalline structure of mica has binding forces that are different in different planes. In one plane they are strong, but weak in the perpendicular plane. This gives it a layered structure and enables it to be spilt along the lines of the weak bond into very thin flat sheets. The sheets used in capacitor manufacture are from less than about 0.025 to 0.1 mm.

Natural mica has to be carefully selected because some samples do contain impurities including, iron, sodium, ferric oxide, and lithium. This introduces some variability into any mica that might be used for capacitor manufacture and therefore it must be carefully inspected and classified. This is one of the reasons why silver mica capacitors are more expensive than other types which have less manual intervention.

Mica is chemically very stable and chemically inert. Mica does not react with oil, water, many acids alkalis, and solvents. As a result of this, ageing does not occur to any major degree, and the variations of water vapour in the atmosphere do not cause undue variations in the overall capacitor performance.

Although more costly than other dielectrics, mica is an ideal form of dielectric for very high performance capacitors such as silver mica capacitors. A summary of the properties of mica are given below:


Parameter Value
Dielectric constant 6
Dielectric strength 10 000 volts per mil


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.

By Ian Poole


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