Polycarbonate capacitors

- an overview, tutorial about the basics of the polycarbonate capacitor or polycarbonate film capacitor: its construction, properties and general data and information.

The polycarbonate capacitor has been available for many years. The polycarbonate dielectric material is very stable having a high tolerance and can operate over a temperature of range of typically -55°C to +125°C without de-rating. Additionally the insulation resistance and dissipation factor are good and the dielectric constant means that polycarbonate capacitors are a reasonable size for their capacitance.

While polycarbonate capacitors have been widely used within many electronics circuits and found favour with many electronics design centres, they are not as widely used these days. The Bayer Corporation which manufactures the majority of polycarbonate announced in 2000 that it was to discontinue production of the dielectric film used in these capacitors.

Although many saw this as the end of polycarbonate capacitors, there are still some smaller sources of the dielectric material and some capacitors are still made.

However many are cautious about using polycarbonate capacitors in new electronics designs as there are fewer suppliers, and relying on a single source for the long term supply of an electronics component is not wise.

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 (virtually bullet-roof). They can also be used for vandal-proof glazing.

Polycarbonate is also used in capacitors as a dielectric. Polycarbonate is very stable, offering the possibility of high tolerance capacitors that can be used over a wide temperature range, and shows little sign of ageing.

The basic electrical properties of polycarbonate are summarised below:

Parameter Value
Dielectric constant 3.2
Dissipative factor 0.0007 @ 50Hz
0.001 at 1MHz
Volume resistivity 10-17 ohm cm
Dielectric strength 38 kV / mm
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.

Polycarbonate capacitors can also be used for AC applications. 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.

Polycarbonate capacitor replacements

With polycarbonate capacitors being less widely available these days since the Bayer Corporation ceased production of polycarbonate in a form suitable for use as a dielectric, a number of alternative types of capacitor have been sought, especially for use in some military applications where capacitors to a given standard need to be used. A variety of types can be used as almost direct replacements:

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

Of these 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 has gaining 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.

By Ian Poole

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