Capacitor Types

Polyester Capacitors

Polyester capacitors, also known as PET (Polyethylene Terephthalate) film capacitors, are among the most widely used general-purpose film capacitors. They offer a good balance of performance, reliability and cost, making them suitable for thousands of electronic applications. Because they are non-polarised, polyester capacitors can be connected in either direction and are commonly used for signal coupling, timing circuits, filtering and general-purpose electronics.

Polyester Capacitors

What Is a Polyester Capacitor?

A polyester capacitor uses a thin polyester plastic film as its dielectric. The conductive electrodes may be made from metal foil or a thin metallised layer deposited directly onto the polyester film.

Many modern polyester capacitors are metallised film types, which offer compact size and self-healing characteristics.

Construction

Component Purpose
Polyester (PET) Film Dielectric.
Metal Foil or Metallised Layer Conductive electrodes.
Lead Wires or SMD Terminals Electrical connection.
Epoxy Resin or Plastic Case Mechanical and environmental protection.

How Polyester Capacitors Work

Polyester capacitors store energy in the electric field created between two conductive electrodes separated by the polyester dielectric.

The dielectric provides good insulation while allowing the capacitor to charge and discharge rapidly with low energy loss.

Typical Specifications

Specification Typical Values
Capacitance 100 pF to 10 µF
Voltage Rating 50 V to 2,000 V
Tolerance ±5%, ±10% or ±20%
Operating Temperature -55°C to +125°C
Polarity Non-polarised

Advantages

  • Low cost.
  • Good electrical stability.
  • High insulation resistance.
  • Non-polarised.
  • Compact size.
  • Long operating life.
  • Available in a wide range of capacitance and voltage ratings.

Disadvantages

  • Higher dielectric losses than polypropylene capacitors.
  • Not as accurate for precision timing applications.
  • Moderate temperature stability.
  • Not ideal for very high-frequency circuits.

Typical Applications

  • Audio coupling circuits.
  • General-purpose filtering.
  • Timing circuits.
  • Oscillators.
  • Power supplies.
  • Control circuits.
  • Industrial electronics.
  • Consumer electronic equipment.
  • Television and radio circuits.
  • Microcontroller projects.

Self-Healing Property

Many polyester capacitors use metallised film construction. If a small dielectric breakdown occurs, the metal surrounding the fault vaporises, isolating the damaged area and allowing the capacitor to continue operating.

This self-healing property improves long-term reliability.

Testing Polyester Capacitors

  1. Inspect the capacitor for cracks or physical damage.
  2. Measure capacitance using a capacitance meter.
  3. Check insulation resistance if necessary.
  4. Measure ESR for larger values where appropriate.
  5. Replace components showing abnormal readings or visible damage.

Common Faults

Fault Possible Cause
Reduced capacitance. Ageing or dielectric damage.
Open circuit. Broken internal connection.
Cracked housing. Mechanical damage.
Insulation failure. Excessive voltage.
Moisture ingress. Damaged encapsulation.

Polyester vs Polypropylene

Feature Polyester (PET) Polypropylene (PP)
Cost Lower. Higher.
Accuracy Good. Excellent.
Temperature Stability Good. Excellent.
Dielectric Losses Moderate. Very Low.
Typical Applications General electronics. Precision, pulse and audio circuits.

Real-World Examples

Equipment Typical Use
Audio Amplifier Signal coupling capacitor.
Television Timing and filtering circuits.
Industrial Controller Signal conditioning.
Power Supply Noise suppression.
Arduino Project General-purpose filtering.

Key Points

  • Polyester capacitors use PET film as the dielectric.
  • They are non-polarised and suitable for AC and DC circuits.
  • They provide a good balance between performance and cost.
  • Many types have self-healing metallised film construction.
  • They are widely used in general-purpose electronic equipment.

Next Lesson

Continue by learning about Polypropylene Capacitors, which offer lower losses, better stability and excellent performance in pulse, audio and precision electronic circuits.

Next Lesson → Polypropylene Capacitors