Polyester Capacitor

- although not as widely used these days, the polyester capacitor often called the Mylar capacitor can provide an ideal solution to many capacitor requirements.

The polyester uses a polyester film for its dielectric which sits between the two capacitor plates.

It is the properties of this dielectric that gives the polyester capacitor its unique properties.

These capacitors also are known by the trade name Mylar capacitors as this is a trade name for the polyester dielectric film used.

An example of a leaded polester capacitor.
Polyester capacitor example

Polyester capacitor properties

Polyester film capacitors are not as widely used these days as they are only available as leaded components.

They tended to be used where cost was a consideration because they did not offer a high tolerance. Many polyester film capacitors have a tolerance of only 5% or 10%, which is adequate for many applications but for all applications as much better tolerance capacitors are available at a lower cost these days.

These capacitors became a standard type for many DC applications.

Polyester capacitor construction

The polyester capacitor may be seen in two main varieties, namely a metallised film capacitor and also a foil version.

Polyester capacitors use layers of metal and polyester or Mylar dielectric to make a wide range of capacitance values but normally within a relatively small package and for low voltages.

The use of "rolled" film layers lead to high dissipation and capacitance vs. temperature problems. Also operation at high frequencies as a result of the construction, etc was poor and as a result they were not used for high frequency or high current applications.

Polyester has a high dielectric constant and it is able to deliver a low cost product within a small outline and it is able to operate at high temperatures up to 125°C for example, albeit with a reduced voltage rating - it may be around half the rated voltage at these extended temperatures.

By Ian Poole

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