Film Capacitors
Film capacitors use a thin plastic film as their dielectric material. They are well known for their excellent stability, low electrical losses, long service life and reliable performance over a wide range of temperatures and frequencies. Because of these characteristics, film capacitors are widely used in audio equipment, power electronics, timing circuits, motor applications and high-voltage systems.
What Is a Film Capacitor?
A film capacitor consists of two conductive electrodes separated by a thin plastic film dielectric. Depending on the design, the electrodes may be constructed from metal foil or from a very thin metal layer deposited directly onto the plastic film (metallised film).
Film capacitors are non-polarised, allowing them to be connected in either direction.
Construction
| Component | Purpose |
|---|---|
| Plastic Film Dielectric | Stores electrical energy. |
| Metal Foil or Metallised Film | Conductive electrodes. |
| Leads or SMD Terminals | Electrical connection. |
| Epoxy or Plastic Case | Mechanical and environmental protection. |
Common Dielectric Materials
| Material | Characteristics | Typical Applications |
|---|---|---|
| Polyester (PET) | General-purpose, economical. | Coupling, decoupling, timing circuits. |
| Polypropylene (PP) | Very low losses, excellent stability. | Audio, pulse, high-frequency and precision circuits. |
| Polyphenylene Sulfide (PPS) | Excellent temperature stability. | Precision electronics. |
| Polystyrene | Very high accuracy and low dielectric absorption. | Measurement and laboratory equipment. |
Foil vs Metallised Film
| Metal Foil | Metallised Film |
|---|---|
| Higher pulse current capability. | Smaller size. |
| Very robust construction. | Self-healing properties. |
| Generally larger. | Lower cost for many applications. |
| Used in demanding pulse circuits. | Common in general-purpose electronics. |
Self-Healing Property
Many metallised film capacitors have a self-healing characteristic. If a small dielectric breakdown occurs, the thin metal coating around the fault vaporises, isolating the damaged area while allowing the capacitor to continue operating.
This improves reliability and extends the service life in many applications.
Typical Specifications
| Specification | Typical Values |
|---|---|
| Capacitance | 100 pF to several hundred µF |
| Voltage Rating | 50 V to several kilovolts |
| Tolerance | ±1% to ±20% |
| Operating Temperature | -55°C to +125°C (type dependent) |
| Polarity | Non-polarised |
Advantages
- Excellent long-term stability.
- Very low ESR and dielectric losses.
- High insulation resistance.
- Non-polarised.
- Excellent frequency response.
- Long operating life.
- Many types offer self-healing capability.
Disadvantages
- Larger than ceramic capacitors of similar capacitance.
- Generally larger than electrolytic capacitors for high capacitance values.
- High-performance types may be more expensive.
Applications
- Audio crossover networks.
- Signal coupling.
- Timing circuits.
- Snubber circuits.
- Motor run capacitors.
- Power factor correction.
- Switch-mode power supplies.
- Pulse circuits.
- EMI suppression.
- High-voltage equipment.
Testing Film Capacitors
- Inspect for cracks or swelling.
- Measure capacitance using a capacitance meter.
- Check insulation resistance if required.
- Measure ESR when appropriate.
- Replace any capacitor with abnormal readings or visible damage.
Common Faults
| Fault | Possible Cause |
|---|---|
| Reduced capacitance. | Ageing or dielectric damage. |
| Open circuit. | Broken internal connection. |
| Insulation breakdown. | Over-voltage. |
| Cracked housing. | Mechanical damage. |
| Moisture ingress. | Damaged casing or poor sealing. |
Film vs Ceramic vs Electrolytic
| Feature | Film | Ceramic | Electrolytic |
|---|---|---|---|
| Polarity | No | No | Yes |
| Stability | Excellent | Good to excellent | Moderate |
| Capacitance | Medium | Low to medium | High |
| High-Frequency Performance | Excellent | Excellent | Limited |
| Typical Use | Audio, timing, pulse circuits. | Filtering, bypassing. | Power supply filtering. |
Real-World Examples
| Equipment | Typical Application |
|---|---|
| Audio Amplifier | Signal coupling and crossover networks. |
| Motor Run Circuit | Continuous AC operation. |
| SMPS | Snubber and EMI suppression circuits. |
| Solar Inverter | Pulse and DC-link filtering. |
| Oscillator Circuit | Frequency-determining network. |
Key Points
- Film capacitors use a plastic film dielectric.
- They are non-polarised and suitable for AC and DC circuits.
- They offer excellent stability and very low losses.
- Metallised film capacitors often have self-healing properties.
- They are widely used in audio, timing, pulse and power electronic applications.