When designing precision timing circuits or high-voltage filtering applications, capacitor selection is frequently overlooked. Engineers often default to ceramic capacitors for their low cost and compact size, or aluminum electrolytic capacitors for their high capacitance density. However, neither type delivers optimal performance in many demanding circuits. Mylar capacitors — also known as polyester film capacitors — provide distinct benefits: stable capacitance over wide temperature ranges, robust voltage tolerance, long-term reliability with minimal degradation, and consistent high-frequency performance. Despite these advantages, many design engineers remain unfamiliar with Mylar capacitor characteristics and their ideal application scenarios.
What Are Mylar Capacitors?
Mylar capacitors are passive components that use polyester film (PET — polyethylene terephthalate) as the dielectric material. This film is wrapped or layered between two conductive surfaces (typically aluminum foil or metallized film) to create a capacitor.
Mylar is a registered trademark of DuPont for its PET-based polyester film. While Mylar originally describes DuPont’s material, the term “mylar capacitor” is widely adopted within the electronics industry to describe all polyester film capacitors, regardless of raw-material supplier. Mylar-style polyester film has been used in capacitor manufacturing for decades and remains a proven, reliable technology.
Construction and How It Differs from Alternatives
Mylar Construction
A typical mylar capacitor consists of:
- Dielectric layer:thin polyester film, typically 3-20 micrometers thick
- Conductive layers:metallized aluminum coating on the film (metallized type) or separate aluminum foil (non-metallized type)
- Terminals:leads or connection points for circuit integration
- Packaging:wound into a cylindrical form, sometimes encased in plastic or epoxy resin for environmental protection
Comparison with Ceramic
Ceramic capacitors use ceramic materials as the dielectric, delivering higher capacitance density than film capacitors. The main trade-off is capacitance stability under temperature and voltage stress. High-k ceramic dielectrics (Y5V, Z5U) can exhibit capacitance shifts of 20-50 % or more over the operating-temperature range, while polyester-based Mylar capacitors typically shift only 2-5 %.
Comparison with Electrolytic
Electrolytic capacitors rely on a thin metal-oxide dielectric layer together with an electrolyte, allowing very high capacitance within a compact footprint. The key drawback is limited long-term reliability: electrolytic capacitors age over time, gradually losing capacitance and developing higher leakage current. By contrast, Mylar capacitors exhibit minimal performance degradation across decades of service.
Key Electrical Properties
Capacitance Stability Across Temperature
| Temperature | Ceramic X7R | Ceramic Y5V | Mylar |
| -55°C | +10% | Not rated for -55 °C | +2% |
| +25°C | 0% | 0% | 0% |
| +85°C | -10% | -50% | -3% |
| Total Change | ±15 % max | -82 % ~ +22 % max | ±5% typical |
Mylar (polyester-PET) capacitors exhibit a positive temperature coefficient, typically +200 to +600 ppm/°C. Unlike Class-II ceramic dielectrics such as X7R and Y5V, which show large-magnitude non-linear percentage-based capacitance shifts, Mylar delivers relatively linear, low-magnitude capacitance change across its operating temperature range. This makes Mylar well-suited for circuits where capacitance drift would degrade overall performance.
Voltage Handling
Mylar (polyester-PET) film capacitors are available across a broad voltage range, up to several thousand volts for standard product lines:
- Low voltage:16V, 25V, 50V
- Mid voltage:100V, 250V, 400V, 600V
- High voltage:1,000V, 2,000V, 5,000V
For voltages above 5 000 V, designers more commonly select polypropylene film capacitors rather than Mylar-style polyester types. By comparison, high-voltage ceramic capacitors can achieve very high breakdown ratings, but practical capacitance values drop sharply above 500-2 000 V. Aluminum electrolytic capacitors top out at roughly 450-550 V for most standard-range parts. This strong high-voltage capability makes Mylar capacitors well-suited for high-voltage filtering, coupling, and protection circuits.
Leakage Current
Mylar capacitors feature excellent low-leakage performance, with typical leakage current in the nanoamp range, comparable to or better than many ceramic capacitors. Leakage increases as temperature rises, though at a far lower rate than seen with aluminum electrolytic capacitors.
Frequency Response
Mylar capacitors work reliably from DC up to several megahertz. Above approximately 10 MHz, parasitic inductance from internal winding, leads and package dominates, degrading performance. For circuits operating at higher MHz frequencies, smaller-footprint film-capacitor constructions or dedicated high-frequency film-dielectric types are preferred.
Temperature Stability and Long-Term Reliability
Environmental Stability
Unlike electrolytic capacitors, which exhibit predictable aging, properly operated Mylar capacitors show minimal capacitance shift and negligible leakage-current rise over 20-30-year service periods under normal operating conditions. This makes them well-suited for mission-critical or long-life applications where component replacement is impractical.
When properly derated for voltage and temperature, a Mylar capacitor can deliver decades-long stable performance. However, PET polyester film is susceptible to hydrolysis under sustained high‑humidity and high-temperature conditions, which can gradually degrade performance. This reliability profile is especially valuable for aerospace, military, and industrial systems designed for multi-decade service life.
Moisture Resistance
The polyester-film dielectric provides good resistance against moisture penetration compared with many multi-layer ceramic capacitors. Certain Class-II ceramic devices are prone to moisture-related performance drift under high-humidity conditions. Properly packaged Mylar capacitors can retain stable performance within tropical, marine, and industrial environments, avoiding the rapid degradation observed in some ceramic types. Note that while the PET film itself resists moisture, capacitor packaging and terminations may still allow humidity ingress, potentially triggering PET hydrolysis over long exposure.
Self-Healing in Metallized Film
In metallized Mylar capacitors, when a microscopic defect (contaminant or thin-film weak spot) in the dielectric causes local voltage breakdown, the nearby metallized-aluminum layer vaporizes. This action electrically isolates the damaged spot and restores dielectric integrity. This self-healing effect enhances overall reliability and lowers early-life (infant-mortality) failure rates.
Advantages of Mylar Capacitors
Stable performance across temperature: Typically ±2-5 % capacitance change from −55 °C to +125 °C, compared with 20-50 % drift for high-k Class-II ceramic dielectrics such as Y5V and Z5U.
Excellent voltage handling: Standard Mylar polyester-film parts are offered from 16 V up to several thousand volts. For voltages above 5 000 V, polypropylene film capacitors are more commonly selected. Mylar delivers strong high-voltage capability relative to aluminum electrolytic capacitors, while high-voltage ceramic options suffer from sharply reduced practical capacitance values at elevated ratings.
Long-term reliability: Minimal performance degradation under normal operating conditions, making Mylar well-suited for lifetime-critical applications. Note that PET-based Mylar is still susceptible to hydrolysis under sustained high-humidity and high-temperature exposure.
Non-polarized: No orientation concerns during assembly, unlike polarized aluminum electrolytic capacitors.
Environmental resistance: Good moisture- and heat-related performance compared with many Class-II ceramic capacitors under harsh operating conditions.
Good frequency response: Suitable for applications ranging from DC up to several megahertz.
Limitations and Practical Considerations
Lower capacitance density: Mylar capacitors require larger physical size compared to electrolytic or multi-layer ceramic capacitors of equal capacitance. Very-high-capacitance values (such as 100 µF) are rarely available in Mylar-style polyester-film formats.
Mid-range cost: Mylar capacitors carry a higher per-microfarad cost than commodity ceramic and aluminum electrolytic parts at large capacitance values. They are most cost-competitive within small-to-mid capacitance ranges (roughly 1 nF up to several microfarads).
Frequency limitations: Performance degrades at very high frequencies (>10 MHz) due to internal and lead-based parasitic inductance. For GHz-range circuits, specialized high-frequency ceramic or other dedicated dielectric types are preferred.
Availability: Non-standard capacitance values and specialized package options often come with longer lead times compared to widely-available commodity ceramic capacitors.
Typical Applications for Mylar Capacitors
Timing and oscillator circuits: Precision timing requires stable capacitance. Mylar’s temperature stability supports circuits requiring predictable oscillation frequency across operating temperature ranges.
Signal coupling: Audio-signal and general-purpose coupling circuits take advantage of Mylar’s linear frequency response and stable phase characteristics. Mylar is less common for high-current power-supply bypass duties, where ceramic capacitors are usually preferred.
High-voltage filtering: High-voltage power supplies, power distribution, and surge protection circuits benefit from mylar’s voltage-handling capability.
Precision instrumentation: Medical devices, test equipment, and measurement systems benefit from mylar’s stability and low-drift characteristics.
Power electronics: Some power supply and inverter applications use mylar for filtering and energy storage where voltage handling and temperature stability are critical.
Conclusion
Mylar (polyester-PET film) capacitors deliver stable capacitance performance, good voltage-withstand capability, strong long-term reliability, and solid environmental robustness. Although physically larger and more costly per-microfarad compared with ceramic capacitors, Mylar offers favorable cost-performance for small-to-mid capacitance applications (roughly 1 nF to several microfarads, 50 V-600 V) where stability and reliability are key requirements.
Designers must select between Mylar, ceramic, and electrolytic parts according to project-specific requirements. Mylar shines for temperature-stable timing circuits, high-voltage duties, and long-service-life systems. Ceramic capacitors excel at miniaturization and low-cost high-volume builds. Aluminum electrolytic capacitors are preferred for high-capacitance energy-storage tasks. Recognizing each component technology’s trade-offs supports thoughtful, well-informed component selection.
Frequently Asked Questions
Q: When should I use mylar instead of ceramic capacitors? Use Mylar when temperature stability is critical (timing circuits, precision measurement), when high-voltage ratings are needed where practical ceramic capacitance becomes very small, or when minimal performance degradation under normal operating conditions over long service life is required. Choose ceramic when miniaturization and low cost are the highest priorities.
Q: Can mylar capacitors replace electrolytic capacitors in power supplies? Mylar-style polyester-film capacitors are rarely available above several microfarads, so they cannot directly replace electrolytic parts in the 10-1 000 µF range. For small-value filtering tasks where long-term reliability and temperature stability are important, Mylar can be an option. For large-value bulk energy-storage applications, aluminum electrolytic capacitors remain more cost-effective thanks to their high capacitance density.
Q: What is the operating temperature range for mylar capacitors? Most Mylar polyester-film capacitors are rated for −55 °C to +125 °C. Typical capacitance shift across this range is ±2-5 %, which compares favorably against high-k Class-II ceramic dielectrics such as Y5V and Z5U. Specialized product variants can support higher temperature limits.
Q: What is the self-healing property in metallized mylar capacitors? If a microscopic defect in the film experiences voltage breakdown, the metallized layer vaporizes at that point, isolating the defect and restoring function. This improves reliability compared to non-metallized film.
Q: What frequency range are mylar capacitors suitable for? Mylar film capacitors perform well from DC to several MHz. Beyond 10-100 MHz, depending on package size, specialized high-frequency capacitors (ceramic, polypropylene) are more appropriate.
Q: Why are mylar capacitors not more common in modern designs? Ceramic dominance is due to cost and miniaturization advantages. However, mylar remains the preferred choice for applications valuing stability, environmental resistance, and long-term reliability over cost per microfarad.
Technical guidance in this article reflects general mylar capacitor properties and applications. Specific requirements depend on your operating conditions, frequency range, temperature extremes, and reliability requirements. Always verify component performance against datasheet specifications and supplier documentation before design commitment.
