Polycarbonate Capacitor

The polycarbonate capacitor was a very useful capacitor, but the dielectric is no longer manufactured.

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The polycarbonate capacitor was a very useful form of capacitor. The polycarbonate dielectric was very stable, having a high tolerance and it could operate over a temperature of range of typically -55°C to +125°C without de-rating.

In addition to this the polycarbonate capacitor had a high insulation resistance and low dissipation factor. The dielectric constant meant that polycarbonate capacitors were a reasonable size for their capacitance.

Unfortunately the Bayer Corporation which manufactured the majority of polycarbonate announced in 2000 that it was to discontinue production of the dielectric film used in these capacitors. Despite this there are a very few smaller sources of the dielectric material and some polycarbonate capacitors are still made.

For the vast majority of applications, it is not wise to use polycarbonate capacitors, but instead look at other replacements that fulfil the role.

Polycarbonate dielectric

Polycarbonates are a group of thermoplastic polymers which find uses in many areas of industry as they are easily moulded and thermoformed. They also posses a number of useful features in that they are temperature resistant impact resistant.

Polycarbonate is also used in capacitors as a dielectric. Polycarbonate is very stable, and enables high tolerance capacitors to be made using the dielectric. They can be used over a wide temperature range, and show little sign of ageing.

The basic electrical properties of polycarbonate are summarised below:

Polycarbonate Dielectric Properties
Parameter	Value
Dielectric constant	3.2
Dielectric strength	38 kV / mm
Dissipative factor	0.0007 @ 50Hz 0.001 at 1MHz
Volume resistivity	10^-17 ohm cm
Water absorption	0.16%

Polycarbonate capacitor construction

Polycarbonate dielectric capacitors are typically manufactured in an extended foil format. Metallized electrodes are then used to make the connections. This dielectric is made from a solvent casting process and performs best as a metallized construction. Metallized types feature vapour deposited metal electrodes and give significant size savings, a definite plus in precision applications. In addition, they feature self-healing. Self-healing removes a fault or short circuit by vaporizing the electrode in the region of the short and restores the capacitor to useful life, thereby greatly extending the lifetime of the capacitor.

Polycarbonate capacitor encapsulation

The encapsulation of the polycarbonate capacitor is important, and a variety of different types can be used. Typically the capacitor may be contained within an epoxy moulded encapsulation, but other popular alternatives include a metal enclosure or preformed box assembly.

It is important to choose the encapsulation required for the particular environment in which the capacitor will be used because the polycarbonate dielectric is sensitive to moisture which can be absorbed as seen by the figures given in the electrical properties section. This water absorption in the polycarbonate dielectric will naturally change some of the electrical properties.

Polycarbonate capacitor applications and use

Polycarbonate capacitors have been used in a wide variety of applications because of the superior performance offered. Typically they are used in applications where precision capacitors are needed (less than ±5%). They are generally used in electronics circuits such as filters, as well as for timing and precision coupling applications.

These capacitors can also be used for AC applications and they are sometimes found in switching power supplies. Care must be taken when using them in these applications. Although the dissipation factor is low, the current must be restricted to prevent them from overheating, although they can tolerate temperature better than many other types of capacitor.

Replacements capacitor types

Now that polycarbonate capacitors are no longer available new, alternative types are needed for replacement for service and repair. A variety of types can be used as almost direct replacements:

Polyethylene napthalate (PEN)
Polyimide (PI)
Polyphenylene sulphide (PPS)
Polytetrafluoroethylene (PTFE)

Of the polycarbonate replacement options available, polyphenylene sulphide, PPS is being widely used in many areas as an almost direct replacement.

Polyphenylene sulphide, PPS has many of the same characteristics of polycarbonate and can be often be used as a direct replacement. It available with a variety of MIL standards and as such it is being used in many high specification applications. PPS has been found to have a superior temperature performance both in terms of the temperature range applicable and the temperature coefficient.

It is found that polyphenylene sulphide, PPS and polycarbonate have the almost the same dielectric constant. This means that the size of equivalent capacitors will be virtually the same, making replacement in existing designs much easier. Unfortunately not all capacitors will be able to be made exactly the same size because PPS and polycarbonate are not available in the same thicknesses.

Although polycarbonate capacitors are virtually no longer available, several other suitable alternatives are now available. With capacitor technology continually developing, even better capacitors will become available to replace polycarbonate capacitors in new designs and for repair and service.