Introduction to Mylar Capacitors
Mylar capacitors are a type of electrical component that stores electrical energy in an electric field. They are constructed using a dielectric material called mylar, which is a polyester film, placed between two electrodes. Mylar capacitors are known for their excellent electrical properties, including high insulation resistance, low dissipation factor, and good stability over a wide range of temperatures and frequencies.
What is a Mylar Capacitor?
A mylar capacitor is an electrical component that consists of two conductive plates separated by a thin layer of mylar film. The mylar film acts as a dielectric, which is an insulating material that can store electrical energy in an electric field. When a voltage is applied across the plates, an electric field is created, and the mylar film becomes polarized, allowing it to store electrical charge.
History of Mylar Capacitors
Mylar capacitors were first developed in the 1950s by the DuPont company. The development of mylar film as a dielectric material was a significant breakthrough in capacitor technology, as it offered several advantages over other materials such as paper and ceramic. Mylar film has a high dielectric strength, which means it can withstand high voltages without breaking down. It also has a low dissipation factor, which means it can store and release electrical energy efficiently.
Construction of Mylar Capacitors
Materials Used in Mylar Capacitors
The primary materials used in the construction of mylar capacitors are:
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Mylar film: This is the dielectric material that separates the two conductive plates. Mylar film is a polyester film that has excellent electrical properties, including high insulation resistance and low dissipation factor.
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Aluminum foil: This is the conductive material used for the plates of the capacitor. Aluminum foil is chosen because it is lightweight, highly conductive, and has a low resistivity.
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Leads: These are the metal wires that connect the capacitor to the circuit. The leads are typically made of tinned copper or silver-plated copper.
Manufacturing Process of Mylar Capacitors
The manufacturing process of mylar capacitors involves several steps:
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Mylar film preparation: The mylar film is cut to the desired size and thickness. The film is then metallized on one or both sides with a thin layer of aluminum to create the conductive plates.
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Winding: The metallized mylar film is wound into a cylindrical shape along with the aluminum foil plates. The number of turns and the spacing between the turns determines the capacitance of the capacitor.
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Flattening: The wound capacitor is flattened to create a more compact and stable shape.
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Leads attachment: The leads are attached to the capacitor using a soldering process.
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Encapsulation: The capacitor is encapsulated in a plastic or epoxy resin to protect it from environmental factors such as moisture and dust.
Electrical Properties of Mylar Capacitors
Capacitance
Capacitance is the measure of a capacitor’s ability to store electrical charge. The capacitance of a mylar capacitor depends on several factors, including the area of the conductive plates, the distance between the plates, and the dielectric constant of the mylar film. The capacitance of a mylar capacitor is typically expressed in microfarads (μF) or nanofarads (nF).
| Capacitance Range | Typical Applications |
|---|---|
| 0.001 μF – 0.1 μF | Coupling, decoupling, and filtering in low-frequency circuits |
| 0.1 μF – 1 μF | Timing circuits, energy storage, and filtering in medium-frequency circuits |
| 1 μF – 10 μF | Power supply filtering and energy storage in high-frequency circuits |
Voltage Rating
The voltage rating of a mylar capacitor is the maximum voltage that can be applied across the capacitor without causing damage or breakdown of the dielectric material. Mylar capacitors are available in a wide range of voltage ratings, from a few volts to several kilovolts. The voltage rating of a mylar capacitor depends on the thickness of the mylar film and the distance between the conductive plates.
| Voltage Rating | Typical Applications |
|---|---|
| 50V – 100V | Low-voltage circuits, such as audio and signal processing |
| 100V – 400V | Medium-voltage circuits, such as power supplies and motor controls |
| 400V – 1kV | High-voltage circuits, such as X-ray machines and particle accelerators |
Dissipation Factor
The dissipation factor of a capacitor is a measure of its energy loss due to leakage current and dielectric absorption. A low dissipation factor indicates that the capacitor can store and release electrical energy efficiently, with minimal energy loss. Mylar capacitors have a very low dissipation factor, typically less than 0.5%, which makes them ideal for applications that require high efficiency and low energy loss.
| Dissipation Factor | Quality Grade |
|---|---|
| < 0.1% | High-grade |
| 0.1% – 0.5% | Standard-grade |
| > 0.5% | Low-grade |
Temperature Coefficient
The temperature coefficient of a capacitor is a measure of how its capacitance changes with temperature. Mylar capacitors have a very low temperature coefficient, typically less than 200 ppm/°C, which means their capacitance remains stable over a wide range of temperatures. This makes mylar capacitors suitable for applications that require stable performance in varying temperature conditions.
| Temperature Range | Temperature Coefficient |
|---|---|
| -55°C to +85°C | ± 100 ppm/°C |
| -55°C to +125°C | ± 200 ppm/°C |
Applications of Mylar Capacitors
Audio Equipment
Mylar capacitors are commonly used in audio equipment, such as speakers, amplifiers, and crossover networks. They are chosen for their low dissipation factor, which ensures that the audio signal is not distorted by energy loss in the capacitor. Mylar capacitors are also used for coupling and decoupling in audio circuits to prevent noise and interference.
Power Supplies
Mylar capacitors are used in power supplies for filtering and smoothing the output voltage. They are chosen for their high capacitance and voltage ratings, which allow them to store and release large amounts of electrical energy. Mylar capacitors are also used for input and output filtering to reduce electromagnetic interference (EMI) and radio frequency interference (RFI).
Motor Controls
Mylar capacitors are used in motor control circuits for starting and running capacitors. Starting capacitors provide a high starting torque to the motor, while running capacitors improve the power factor and efficiency of the motor. Mylar capacitors are chosen for their high voltage ratings and low dissipation factor, which ensures reliable and efficient operation of the motor.
Medical Equipment
Mylar capacitors are used in various medical equipment, such as X-ray machines, defibrillators, and pacemakers. They are chosen for their high reliability, low leakage current, and stable performance over a wide range of temperatures. Mylar capacitors are also used for noise suppression and filtering in medical electronics to ensure accurate and safe operation.
Advantages of Mylar Capacitors
High Insulation Resistance
Mylar capacitors have a very high insulation resistance, typically greater than 100,000 megohms. This means that they have very low leakage current, which is the current that flows through the dielectric material when a voltage is applied. Low leakage current is important for applications that require long-term energy storage, such as backup power supplies and timing circuits.
Low Dissipation Factor
Mylar capacitors have a very low dissipation factor, typically less than 0.5%. This means that they have very low energy loss due to leakage current and dielectric absorption. Low dissipation factor is important for applications that require high efficiency and low heat generation, such as audio equipment and power supplies.
Good Stability
Mylar capacitors have good stability over a wide range of temperatures and frequencies. Their capacitance remains stable over a temperature range of -55°C to +125°C, and their dissipation factor remains low over a frequency range of 50 Hz to 100 kHz. This makes mylar capacitors suitable for applications that require consistent performance in varying environmental conditions.
High Voltage Rating
Mylar capacitors are available in a wide range of voltage ratings, from a few volts to several kilovolts. This makes them suitable for a variety of applications, from low-voltage electronics to high-voltage power systems. The high voltage rating of mylar capacitors is due to the high dielectric strength of the mylar film, which can withstand high electric fields without breaking down.
Low Cost
Mylar capacitors are relatively inexpensive compared to other types of capacitors, such as ceramic and tantalum capacitors. This is due to the low cost of the materials used in their construction, such as mylar film and aluminum foil. The low cost of mylar capacitors makes them a popular choice for high-volume production and cost-sensitive applications.
Disadvantages of Mylar Capacitors
Limited Temperature Range
Although mylar capacitors have good stability over a wide range of temperatures, their operating temperature range is limited compared to other types of capacitors, such as ceramic and mica capacitors. Mylar capacitors are typically rated for a temperature range of -55°C to +125°C, which may not be sufficient for some high-temperature applications.
Polarized
Some mylar capacitors are polarized, which means that they have a specific positive and negative terminal that must be connected correctly in the circuit. Reversing the polarity of a polarized mylar capacitor can cause permanent damage and failure of the capacitor. Non-polarized mylar capacitors are available, but they have lower capacitance and voltage ratings compared to polarized mylar capacitors.
Larger Size
Mylar capacitors are generally larger in size compared to other types of capacitors with similar capacitance and voltage ratings. This is due to the construction of mylar capacitors, which requires a larger surface area of the dielectric material to achieve the desired capacitance. The larger size of mylar capacitors may be a disadvantage in applications where space is limited, such as portable electronics and miniaturized circuits.
Frequently Asked Questions (FAQ)
1. What is the difference between mylar and polyester capacitors?
Mylar and polyester capacitors are essentially the same thing. Mylar is a trademark of DuPont for a specific type of polyester film, but the term “mylar capacitor” is often used interchangeably with “polyester capacitor” to refer to capacitors that use polyester film as the dielectric material.
2. Can mylar capacitors be used in AC circuits?
Yes, mylar capacitors can be used in AC circuits. In fact, they are commonly used for filtering and coupling in AC circuits, such as audio equipment and power supplies. However, the voltage rating of the capacitor must be sufficient to withstand the peak voltage of the AC signal.
3. How do I know if a mylar capacitor is polarized or non-polarized?
Polarized mylar capacitors are usually marked with a stripe or a plus sign (+) on the positive terminal. Non-polarized mylar capacitors do not have any polarity markings and can be connected in either direction in the circuit.
4. What is the shelf life of mylar capacitors?
Mylar capacitors have a very long shelf life, typically more than 10 years, if stored in a cool and dry environment. However, the actual shelf life may vary depending on the specific type and grade of the capacitor, as well as the storage conditions.
5. Can mylar capacitors be used in high-frequency applications?
Mylar capacitors are generally not recommended for high-frequency applications above 1 MHz, due to their relatively high parasitic inductance and resistance. For high-frequency applications, ceramic or film capacitors with lower parasitic inductance and resistance are typically used.
Conclusion
Mylar capacitors are a versatile and reliable type of capacitor that offers several advantages over other types of capacitors. They have high insulation resistance, low dissipation factor, good stability, high voltage rating, and low cost. These properties make mylar capacitors suitable for a wide range of applications, from audio equipment to power supplies to medical devices.
However, mylar capacitors also have some limitations, such as a limited temperature range, polarization, and larger size compared to other types of capacitors. These limitations should be considered when selecting mylar capacitors for specific applications.
Overall, mylar capacitors are a proven and trusted technology that has been used in electronics for several decades. They continue to be a popular choice for many applications due to their excellent performance and cost-effectiveness.
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