Capacitor Types

Polypropylene Capacitors

Polypropylene capacitors use polypropylene (PP) plastic film as their dielectric. They are recognised for their extremely low dielectric losses, excellent capacitance stability, high insulation resistance and outstanding performance at both low and high frequencies. These characteristics make polypropylene capacitors the preferred choice for precision timing circuits, audio equipment, pulse circuits, motor run applications and power electronics.

What Is a Polypropylene Capacitor?

A polypropylene capacitor consists of two conductive electrodes separated by a thin polypropylene film dielectric. The electrodes may be made from metal foil or from a thin metallised coating deposited directly onto the polypropylene film.

Most modern polypropylene capacitors are metallised film types that combine compact size with excellent long-term reliability and self-healing capability.

Construction

Component	Purpose
Polypropylene Film	Dielectric.
Metal Foil or Metallised Film	Conductive electrodes.
Lead Wires or SMD Terminals	Electrical connection.
Plastic or Epoxy Case	Mechanical and environmental protection.

How Polypropylene Capacitors Work

When a voltage is applied across the capacitor, electrical charge is stored between the conductive electrodes. The polypropylene dielectric provides excellent insulation and very low dielectric absorption, allowing accurate charging and discharging.

Because electrical losses are extremely low, polypropylene capacitors are ideal for circuits where efficiency, accuracy and stability are important.

Typical Specifications

Specification	Typical Values
Capacitance	100 pF to 100 µF
Voltage Rating	63 V to several kilovolts
Tolerance	±1%, ±2%, ±5% or ±10%
Operating Temperature	-55°C to +105°C (some types higher)
Polarity	Non-polarised

Advantages

Very low dielectric losses.
Excellent capacitance stability.
Very low dielectric absorption.
High insulation resistance.
Excellent pulse current capability.
Suitable for AC and DC circuits.
Long operating life.
Many types are self-healing.

Disadvantages

Larger than ceramic capacitors of similar capacitance.
Generally more expensive than polyester capacitors.
Maximum capacitance is lower than electrolytic capacitors.
Can be physically larger at high voltage ratings.

Self-Healing Construction

Most metallised polypropylene capacitors have self-healing properties. If a small dielectric breakdown occurs, the thin metallised layer surrounding the fault vaporises, isolating the damaged area and allowing the capacitor to continue operating.

This feature significantly improves reliability in power electronic and industrial applications.

Typical Applications

Audio crossover networks.
Signal coupling.
Precision timing circuits.
Pulse circuits.
Snubber circuits.
Motor run capacitors.
Power factor correction.
IGBT and MOSFET protection.
Switch-mode power supplies.
Solar inverters.

Testing Polypropylene Capacitors

Inspect for cracks or swelling.
Measure capacitance using a capacitance meter or LCR meter.
Check insulation resistance where required.
Measure ESR if appropriate.
Replace capacitors with abnormal readings or physical damage.

Common Faults

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

Polypropylene vs Polyester

Feature	Polypropylene (PP)	Polyester (PET)
Dielectric Losses	Very Low	Moderate
Temperature Stability	Excellent	Good
Accuracy	Excellent	Good
Pulse Performance	Excellent	Moderate
Cost	Higher	Lower
Typical Use	Precision and power electronics.	General-purpose electronics.

Real-World Examples

Equipment	Typical Use
Hi-Fi Audio Amplifier	Audio crossover and signal coupling.
Motor Run Circuit	Continuous AC operation.
IGBT Inverter	Snubber capacitor.
Solar Inverter	DC-link filtering and pulse circuits.
Tesla Coil	High-voltage pulse capacitor.
Class D Amplifier	Output filter network.

Key Points

Polypropylene capacitors use PP film as the dielectric.
They are non-polarised and suitable for AC and DC applications.
They provide excellent stability, low losses and high insulation resistance.
They are ideal for audio, pulse, timing and power electronic circuits.
Metallised polypropylene capacitors often include self-healing construction for improved reliability.

Next Lesson

Continue by learning about Supercapacitors, which can store much more energy than traditional capacitors and are used for backup power and energy storage applications.

Next Lesson → Supercapacitors