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The silver mica capacitor provides close tolerance values with low levels of value change with temperature.
In view of its performance it was the capacitor of choice for many RF circuits like oscillators and filters where close tolerance and low drift were needed.
Nowadays other types like ceramic capacitors are able to provide a similar level of performance in a smaller volume and at much lower cost. As a result the silver mica capacitor is not widely used. However it will be encountered when repairing older radios and other equipment.
What is a silver mica capacitor
Silver mica capacitors, as the name implies us a mica dielectric which is then silvered to provide the plates or electrodes of the capacitor.
Mica is a shiny naturally occurring silicate mineral which has a layered structure. There are several different forms of mica, but they all have very similar properties.
It has a relatively high dielectric constant compared to many other naturally occurring layered minerals. It is typically between 5 and 7. Mica is also very stable mechanically and chemically making it a good base for electronic capacitors. The chemical stability means that the long term performance of the silver mica capacitor is particularly good.
Natural mica has to be carefully selected because some samples contain impurities that would degrade the performance of the silver mica capacitor. Also the sheets of mica used in capacitor manufacture are relatively thin ranging from about 0.025 to 0.1 mm.
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:
|Mica Dielectric Parameters|
|Dielectric strength||10 000 volts per mil|
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. This property is useful in ensuring oscillators (and filters) do not drift unduly with temperature.
- 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. This is one property of the capacitors that is useful in making stable low noise oscillators, and selective filters.
Although silver mica capacitors have a high tolerance and low temperature co-efficient they are known to jump in value on occasions. When this occurs, it can cause an oscillator to jump in frequency, or a filter to change its properties.
Silver mica capacitor structure
Very early mica capacitors in the 1920s were made by physically clamping sheets if mica with copper electrodes between them. These were left open to the atmosphere. The performance of these early mica capacitors was very not surprisingly very poor, but this was improved when silver electrodes were used instead..
Current silver mica capacitors use silver electrodes that are plated directly onto the mica. Several layers are used to gain the required level of capacitance. Also the area of the electrodes has a major effect on capacitance.
Once the basic silver mica capacitor has been made, it is housed in a ceramic encapsulation which reduces ingress of moisture and other contaminants.
Those silver mica capacitors used for valve or vacuum tube radios were generally encapsulated in wax. This did tend to melt relatively easily leaving small ‘blobs’ of wax at the bottom of the radio.
Silver mica capacitor applications
Silver 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 particular for radio frequency applications:
- Filters: The close tolerance levels of the silver mica capacitor enables the exact performance to be calculated and predicted. Also the low loss levels of the silver mica capacitor enable high Q filters to be realised.
- RF oscillators: Silver mica capacitors can fund uses within RF oscillators. In these applications their low loss levels enable the Q of the tuned circuit to be improved. This yields improved stability and lower phase noise levels. Also their low temperature coefficient enables even free running oscillators to achieve good stability levels and low drift.
- Coupling and decoupling: The cost of silver mica capacitors generally means that they are not used for coupling and decoupling, but they can withstand high voltages and this may have meant they have been used in these applications occasionally.
Silver mica capacitor summary
The table below provides some of the salient features about some of the more widely used metal film capacitors that can be taken into consideration when designing circuits or replacing old components.
|Silver Mica Capacitor Summary|
|Value range||Few pF to ~4700pF|
|Working voltage||100V to 1kV|
|Temperature coefficient||30 – 75ppm / °C|
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