Insight - How Polyester Capacitor works

Working of capacitors often gets restricted due to losses such as leakage loses or surge shocks. While leakage loses can be reduced through proper insulation through an epoxy or ceramic coating, damage due to short circuits can be avoided only when the metal plates aren’t eroded due to high current. At surge voltages, while capacitors such as ceramic break down and lose their ability to store charge metalized film capacitors can repair themselves.Whenever high voltage is across the capacitor, instead of breaking down, the metal on the film gets perforated. The high voltage in turn also heats the dielectric film which expands and fills the part where perforation occurred. This increases the longevity of the capacitor, thus improving the working of the circuit.

Based on the type of dielectric used, metal film capacitors are either of polypropylene or of polyester type. This article covers the polyester metallic film capacitor which is also known as Polythene –Teraphthalate (PET) capacitor. These capacitors are highly heat resistant and can work at temperatures close to 150 °C.

Outer Structure

The shell of the capacitor is composed of an epoxy resin capsule which is firmly coated around the capacitor. The upper surface of the epoxy shell has the voltage and capacitance printed over it. It has a pair of connecting leads at both sides as shown in the image below.

Fig. 1: Outer Structure of A Polyester Capacitor

Resin coating is extended by a few millimeters on the sides, in order to protect the junction where the connecting leads are attached to the capacitor.

Fig. 2: Side View of Polyester Capacitor

Internal Structure

The epoxy layer is fairly thin and can be removed when tweaked from both sides. Right under the epoxy resin coating is the thoroughly coiled structure of the metal film. The image below shows the internal structure of the polyester capacitor.

Outer Structure of A Polyester Capacitor

Fig. 3: Closer View of A Typical Polyester Capacitor

Under the epoxy shell, the core of the capacitor appears similar to a rectangular piece of metal. However, the film is extremely thin and can be easily uncoiled. Once the film starts to uncoil, metal layers are fairly visible on it.

Dielectric Layer

Fabrication of these capacitors involves a thin film of dielectric which is metalized by sandwiching it between thin metal coatings from both sides. These metal films are opposite in polarity keeping the net charge zero and thus forming a capacitor that has metal plates and the dielectric rolled to form of a dense coil.