Polypropylene Capacitor: PP Metallized Film

- an overview, information or tutorial about the basics of the polypropylene capacitor: its construction, metallized polypropylene capacitors; film capacitors; technical information, properties and applications.

Polypropylene capacitors form one of the many varieties of film capacitor that can be obtained.

Available chiefly as leaded components, their use is not as wide as it was when leaded components were used almost exclusively. Nevertheless polypropylene capacitors can be used to good effect in a number of applications.

Polypropylene

Polypropylene is often abbreviated to PP, and indeed the polypropylene capacitors may be referred to as PP capacitors.

The material has many several advantages and as a result it is widely used in many applications: everything from packaging and labelling to textiles, ropes, stationery, reusable containers and many more items. Polypropylene is even used in some high quality loudspeakers where its properties are ideal to create the required performance.

An example of a polypropylene PP film capacitor (1000pF 630V) showing the casing and size
Polypropylene PP film capacitor example

Polypropylene is what is termed an addition polymer which is made from the monomer propylene. It is rugged and unusually resistant to many chemical solvents, bases and acids, making it ideal for many of its uses.

Many of the key electrical characteristics of the material that are applicable to its use in polypropylene capacitors is given in the table below.


Summary of Properties of Polypropylene Capacitor Dielectric
Property value
Dielectric constant (@ 1MHz) 2.2
Dissipation factor (@ 1 kHz (tanδ in 5) 0.05
Insulation resistance (MΩ x µF 100 000
Dielectric absorption (%) 0.05
Capacitance drift (ΔC/C %) 0.5
Maximum temperature (°C) 100
Temperature coefficient (ppm / °C) +200 ±100

Polypropylene capacitor construction

The polypropylene of PP capacitor is a form of what is termed a film capacitor.

These capacitors use a thin plastic film as the dielectric - in this case polypropylene.

The film is made very thin and to the required thickness using a sophisticated drawing process.

The polypropylene film may then be metallised and in this way the plates are formed across the dielectric. Alternatively the film may be left untreated and separate leaves of a conductor placed between layers of the film to form the electrodes.

Either way multiple layers of the film are needed to give the required capacitance, and the plates are connected to the external leads.

Finally the assembly is mounted into a case which protects it from damage and other environmental factors.

Polypropylene capacitors

Polypropylene capacitors are widely available from many sources and are ideal for many applications, although there are some limitations that need to be noted. Some of the main points to note about polypropylene capacitors are summarised below:

  • Tolerance:   These capacitors are often used where high degrees of tolerance are required. 1% tolerance capacitors are available.
  • Stability:   Polypropylene capacitors exhibit very little change of capacitance with time and voltage applied making them ideal for applications where a stable level of capacitance is needed. It is found that the temperature coefficient is low, negative and virtually linear.
  • ESR:   PP capacitors have a low ESR, equivalent series resistance and also most types have a low inductance level as well.
  • Leaded components:   PP capacitors are only available as leaded components. Surface mount versions are not available and various forms of ceramic capacitor are most widely used.
  • Capacitance range:   Polypropylene capacitors are available over a wide range of values. Typically they are available in values from around 100pF up to 10 nF and more
  • Voltage range:   PP capacitors are able to withstand high voltages. Often even small capacitors will be seen with operating voltages of 630V or even 1kV, and 100V versions are common.
  • Temperature:   These capacitors may be operated up to 100°C and sometimes just over with suitable de-rating - although the manufacturer's specification must be consulted before designing in a specific component.

Polypropylene capacitor applications

PP capacitors find uses in many areas of electronics. Although only available in leaded versions they are still widely used.

A polypropylene PP film capacitor (1000pF 630V) showing the casing and size
Polypropylene capacitor example

They are seen in many applications:

  • High power / High AC voltage circuit applications.
  • Circuits with high peak current levels.
  • High frequency resonant circuits.
  • Precision timing circuits.
  • Lighting ballast systems.
  • Switching power supplies.
  • Sample and hold circuits.
  • Premium audio applications where many enthusiast believe they offer better performance and hence a better sound quality.
  • High frequency pulse discharge circuits
  • Energy storage circuits - their high resistance enables low levels of self discharge.

Polypropylene capacitors are available for many applications where leaded capacitors can be used. This limits their use to low volume of specialist applications. Despite this, they are still widely available from may distributors.

By Ian Poole


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