Polyester Capacitor: Mylar PET

Polyester capacitors also known as Mylar or PET capacitors are used as leaded capacitors in many applications.

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Polyester capacitors are used mainly in their leaded form. They are not produced as high tolerance capacitors, but they have a low ESR and are able to tolerate large spikes with a high rise time.

The capacitor is robust and can be used in many applications where some other forms may not be suitable.

The polyester capacitor is referred to by other names apart from polyester. The trade name for the material used as the dielectric is Mylar, and they may also be referred to as PET capacitors because the synthetic polyester material used goes by this abbreviation.

Polyester dielectric

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

This is 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 referred to polyethylene terephthalate, PET. It is also sold under the Mylar trade name. As a result, some capacitors are also sold as Mylar capacitors.

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. Often the abbreviation PET may be seen as a type mark in the literature and other places describing the type of capacitor.

Polyester Capacitor Dielectric Properties
Polyester Property	Value / Details
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.

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.
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.
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.

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, Mylar 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
Filtering, where high tolerance levels are not required.
General coupling and decoupling applications and DC blocking.
Power supplies where the very high capacitance levels of electrolytic capacitors are not needed.
Audio applications

Polyester capacitors are often regarded as good general purpose capacitors. Although they cannot provide a high level of tolerance and accuracy, they can be used in many other applications offering a good levels of volumetric capacitance.