Polyester Capacitor: Mylar PET

- 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. Also the capacitors may be referred to as PET because the synthetic polyester material used goes by this abbreviation.

Polyester dielectric

Polyester capacitors are based around the use of a polyester dielectric.

The polyester dielectric based on a category of polymers that contain chemical esters their main polymer chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate, PET.

A polyester PET film capacitor showing the casing and size
Polyester capacitor example

Polyesters include both naturally occurring and synthetic materials. The naturally occurring esters include the cutin of plant cuticles. Synthetic polyesters include materials obtained through step-growth polymerization and include materials such as polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not.

As the polyester material used in polyester capacitors is normally polyethylene terephthalate, these capacitors may also be referred to by the abbreviation PET for the dielectric material used.

Summary of Properties of Polyester Capacitor Dielectric
Property value
Dielectric constant (@ 1MHz) 3.2
Dissipation factor (@ 1 kHz (tanδ in 5) 0.5
Insulation resistance (MΩ x µF 25 000
Dielectric absorption (%) 0.2
Capacitance drift (ΔC/C %) 1.5
Maximum temperature (°C) 125
Temperature coefficient (ppm / °C) +400 ±200

Polyester capacitor properties

As with any capacitor, polyester capacitors have their own characteristics. This means that they are suited to some applications more than others.

A polyester PET film capacitor showing the casing and size
Polyester capacitor

Some of their key characteristics are summarised below:

  • Leaded versions only:   Polyester capacitors are only available in leaded versions, and are not available in surface mount packages.
  • High dielectric strength:   The polyester dielectric used in these capacitors also referred to as PET offers a high dielectric strength. This means that high voltage capacitors can be made relatively small.
  • Operating temperature:   As a result of the properties of the polyester dielectric, these capacitors are often able to operate up to temperatures of 125°C, although with a voltage de-rating.
  • ESR:   Polyester capacitors are able to provide a low level of equivalent series resistance.
  • High dV/dt:   The construction and dielectric of polyester, PET capacitors means that they are able to be used in applications where sharp, fast rise time spikes are present as they are able to accommodate high dV/dt figures.
  • 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.

An example of an older style leaded polyester capacitor.
Older style polyester capacitor example

Polyester capacitor construction

The polyester capacitor or PET 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.

Polyester capacitor applications

Polyester capacitors or PET capacitors are used in many areas where leaded capacitors can be accommodated.

Some typical polyester capacitor applications include:

  • Circuits where the capacitor needs to handle high peak current levels
  • General coupling and decoupling applications and DC blocking.
  • Filtering, where high tolerance levels are not required.
  • Audio applications
  • Power supplies where the very high capacitance elvels of electrolytic capacitors are not needed.

Polyester capacitors are often regarded as general purpose capacitors. They provide a good level volume efficiency when compared to other types of film capacitor, and this comes at a moderate level of cost.

By Ian Poole

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