Electronic Parts

Film Capacitors

Film capacitors use a thin plastic film as the dielectric. Common dielectric materials include polypropylene, polyester, polyphenylene sulfide (PPS), and other specialized films. They are valued for low loss, stable capacitance, good pulse capability, and predictable AC behavior.

Film-and-foil versus metallized film

Two broad constructions are common:

Self-healing

In a metallized film capacitor, a tiny dielectric breakdown can vaporize a small amount of the very thin metal electrode around the fault. This isolates the damaged spot and allows the remainder of the capacitor to continue working. The process is called self-healing. It improves reliability, but repeated stress still ages the component and can reduce capacitance.

Polypropylene

Polypropylene film has very low dielectric loss and good stability. It is widely used in pulse circuits, resonant converters, snubbers, precision AC work, speaker crossovers, motor-run capacitors, and DC-link applications. It is often chosen where substantial AC current or low dissipation factor matters.

Polyester

Polyester film capacitors are compact, economical, and useful for many general-purpose signal, timing, and coupling applications. They usually have greater dielectric loss and temperature dependence than polypropylene, but may provide more capacitance in a given package size.

PPS and precision film types

Polyphenylene sulfide (PPS) and selected precision film constructions can offer good stability in compact packages, including surface-mount forms. Exact temperature coefficient, loss, and voltage behavior depend on the dielectric and series.

Pulse and AC service

Film capacitors are often selected for circuits in which voltage and current are continually changing. Important limits can include:

• RMS current.
• Peak current.
• Pulse repetition rate.
• dV/dt, the rate at which voltage changes.
• Internal heating.
• Maximum case temperature.

Across-the-line safety capacitors

Capacitors connected directly to the AC mains are specially designed and safety-certified. Class X capacitors are generally used across the line, while Class Y capacitors are used where failure could connect mains voltage to accessible or grounded parts. These safety classes specify tested failure behavior and surge performance.

Motor capacitors

Film capacitors are common in AC motor-run service because they can tolerate continuous alternating current. Motor-start capacitors are a different duty: they may use electrolytic construction and are intended only for short starting intervals. A start capacitor must not be substituted for a continuous-duty run capacitor.

Audio and signal use

Film capacitors are popular in audio coupling, tone networks, filters, and loudspeaker crossovers because many film dielectrics offer low loss and stable capacitance. In ordinary audio circuits, choosing the correct capacitance, voltage, tolerance, and circuit impedance is generally more important than exotic claims about capacitor sound.

Typical applications

• Audio coupling and crossovers.
• Precision timing and filtering.
• Snubbers and pulse networks.
• Power-factor correction and motor-run service.
• DC-link energy storage in converters and inverters.
• AC mains interference suppression using approved safety types.

Selection checklist

• Dielectric material.
• Capacitance and tolerance.
• DC and AC voltage requirements.
• RMS and pulse current.
• dV/dt and repetition rate.
• Temperature and expected life.
• Safety approval where connected to the mains.

Further reading

• Vishay — General technical information for film capacitors
• TDK — Film capacitor families and applications
• For mains-connected use, select only a capacitor carrying the required safety classification and approvals.