Chloe Zhang
Start capacitors are used to provide an initial jolt of electricity to get a motor running. They are typically non-polarized electrolytic types and are usually encased in plastic. The use of plastic films in capacitors began during World War II when organic chemists developed thinner polymer films as a replacement for paper. Plastic film capacitors have several advantages over paper capacitors, including reduced defects, improved volumetric efficiency, and long-term stability of electrical parameters. The plastic casing also serves as a dielectric spacer, increasing the capacitor's charge capacity and allowing for standardized terminal spacings. Additionally, most start capacitors have a distinct physical design, often featuring round cases made of black phenolic or Bakelite materials.
| Characteristics | Values |
|---|---|
| Plastic film capacitors have fewer defects than paper sheets used in paper capacitors | Allows the manufacture of plastic film capacitors with only a single layer of plastic film |
| Plastic film capacitors are smaller in physical size than paper capacitors | Same capacitance value and dielectric strength as comparable paper capacitors |
| Plastic is much less hygroscopic than paper | Reduces the deleterious effects of imperfect sealing |
| Plastic is subject to fewer chemical changes over long periods | Long-term stability of electrical parameters |
| Plastic film capacitors are packaged in axial, radial, and SMD styles | Radial capacitors are available potted in plastic cases, or dipped in epoxy resin to protect the capacitor body |
| Plastic film capacitors can withstand transient heat of reflow soldering | Available in surface-mounted device (SMD) packages |
| Plastic film capacitors have electrodes of metallized aluminum or zinc | Applied directly to the surface of the plastic film, or as a separate metallic foil |
| Plastic film capacitors are constructed with terminal spacings at standardized distances | Starting with 2.5 mm pitch and increasing in 2.5 mm steps |
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What You'll Learn
Plastic film capacitors are smaller in size than paper capacitors
The introduction of plastic films during the Second World War led to the development of thinner polymer films, which replaced the traditional wax-impregnated paper used in capacitors. The use of thinner plastics, such as polystyrene, polyethylene terephthalate (PET), and polypropylene, significantly reduced the dimensions of metallized polyester film capacitors. This reduction in size is further enhanced by the “stacked” configuration of film capacitors, where the metallized films are wound around a large core and then sawn into smaller segments, creating multi-layer capacitors (MLPs).
Additionally, plastic film capacitors offer high stability, long shelf life, low equivalent series resistance, and a high ability to absorb power surges. The roughness of metallized paper surfaces can cause air-filled bubbles between the dielectric and metallization, reducing the breakdown voltage. In contrast, plastic film capacitors are less prone to environmental defects and have better long-term stability of their electrical parameters due to the lower hygroscopic nature of plastic compared to paper.
The smaller size of plastic film capacitors also reduces the overall cost of the circuit. The cost of a printed circuit board is calculated per square millimeter, so a smaller capacitor footprint results in cost savings. This, combined with their superior electrical properties and reliability, has led to plastic film capacitors almost completely replacing paper capacitors in low-power DC electronic equipment.
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Plastic films have fewer defects than paper sheets
Plastic film capacitors are widely used in electronic equipment and are favoured over paper capacitors due to their fewer defects. Plastic films used in capacitors are drawn to an extremely thin thickness and provided with electrodes, which can be metallized aluminium or zinc applied directly to the surface of the plastic film, or a separate metallic foil.
Plastic films have considerably fewer defects than paper sheets, which is a significant advantage in capacitor fabrication. This allows for the manufacture of plastic film capacitors with only a single layer of plastic film, while paper capacitors require a double layer of paper. Plastic film capacitors are physically smaller, with the same capacitance value and dielectric strength as comparable paper capacitors.
The introduction of plastic films in capacitors began during the Second World War, when organic chemists developed new plastic materials. The capacitor industry started to replace paper with thinner polymer films. Plastic is much less hygroscopic than paper, reducing the negative effects of imperfect sealing. Additionally, most plastics are subject to fewer chemical changes over long periods, providing long-term stability of their electrical parameters.
Paper capacitors were used extensively in older capacitors and are still used today, particularly in low-power DC electronics. Paper capacitors are made by sandwiching a strip of wax-impregnated paper between strips of metal, and rolling the result into a cylinder. Paper capacitors offer high voltage performance, but plastic film capacitors are more commonly used due to their superior defect resistance and smaller physical size.
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Plastic is less hygroscopic than paper
Plastic film capacitors are commonly used in electronic equipment. They are constructed by placing electrodes made of metallized aluminium or zinc directly on the surface of the plastic film or using a separate metallic foil. Two of these conductive layers are wound into a cylinder-shaped winding, which is usually flattened to reduce mounting space requirements on a printed circuit board.
Plastic film capacitors were introduced during World War II when organic chemists developed plastic materials. The capacitor industry started replacing paper with thinner polymer films. Plastic film capacitors have several advantages over paper capacitors. One of the key advantages is that plastic is much less hygroscopic than paper.
Hygroscopic materials absorb moisture from the atmosphere. Paper, being highly hygroscopic, can absorb a significant amount of moisture, which can affect the performance and longevity of capacitors. On the other hand, plastic has low hygroscopic properties, making it less susceptible to moisture absorption. This reduces the adverse effects of imperfect sealing, which can be an issue with paper capacitors.
The low hygroscopic nature of plastic helps maintain the integrity of the capacitor over time. It prevents the degradation of the capacitor's electrical properties, ensuring its reliability and consistency in performance. Additionally, plastic's resistance to moisture absorption contributes to the long-term stability of the capacitor's electrical parameters. This stability is crucial for the consistent operation of electronic devices that utilise capacitors.
The use of plastic in capacitors also offers improved volumetric efficiency. Plastic film capacitors are significantly smaller in physical size compared to paper capacitors while offering the same capacitance value and dielectric strength. This size advantage makes plastic film capacitors more versatile and easier to integrate into compact electronic designs.
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Plastic film capacitors are cheaper to manufacture
Additionally, plastic film capacitors have considerably fewer defects than paper capacitors. This is due to plastic being much less hygroscopic than paper, reducing the negative consequences of imperfect sealing. Furthermore, most plastics undergo fewer chemical changes over time, providing long-term stability for the electrical parameters of the capacitor. The increased stability and reduced defects of plastic film capacitors result in lower costs associated with replacement and maintenance.
The physical characteristics of plastic films also contribute to the lower manufacturing costs. Plastic films can be drawn to an extremely thin thickness, allowing for more compact capacitor designs. This improved volumetric efficiency means that more capacitors can be produced within a given space, reducing the cost per unit. Furthermore, plastic film capacitors can be constructed with standardised terminal spacings, facilitating automated insertion during manufacturing. This automation further reduces production costs by decreasing the labour required for assembly.
The versatility of plastic films also contributes to the cost-effectiveness of plastic film capacitors. Plastic films can be utilised in various packaging styles, including axial, radial, and SMD. This adaptability allows manufacturers to cater to diverse industry standards and specifications, increasing the applicability of plastic film capacitors across different electronic equipment.
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Plastic film capacitors have long-term stability
Plastic film capacitors have been widely used since the development of plastic materials during World War II. They are made by sandwiching a thin plastic film between two conductive layers, which are usually metallized aluminium or zinc. These conductive layers form the electrodes of the capacitor, with the plastic film acting as the dielectric.
Plastic film capacitors offer several advantages over other types of capacitors, particularly in terms of long-term stability. Firstly, plastic films have fewer defects than paper sheets, which were previously used in paper capacitors. This means that plastic film capacitors can be manufactured with a single layer of plastic film, resulting in a smaller physical size while maintaining the same capacitance value and dielectric strength as comparable paper capacitors.
Additionally, plastic is less hygroscopic than paper, reducing the negative effects of imperfect sealing. Plastic is also less susceptible to chemical changes over time, providing long-term stability of electrical parameters. This stability is further enhanced by the ability to choose different plastic film materials for the dielectric layer, allowing for the selection of materials with desirable electrical characteristics such as wide temperature ranges and the ability to withstand high voltages.
The long-term stability of plastic film capacitors is also evident in their low failure rates and long service lives. They have low ESR (Equivalent Series Resistance) and low self-inductance, resulting in very low dissipation factors. They can also withstand high voltages and power surges, further contributing to their reliability and stability.
Overall, the use of plastic films in capacitors provides long-term stability through improved defect rates, reduced chemical changes, and enhanced electrical characteristics. These advantages have led to the widespread adoption of plastic film capacitors in various applications, including electronic equipment and high-performance industries such as aerospace and military technology.
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Frequently asked questions
Plastic film capacitors are cheaper to manufacture than paper capacitors, as they require only a single layer of plastic film, whereas paper capacitors need two layers of paper. Plastic film capacitors are also smaller in size, have fewer defects, and are more stable over long periods.
A start capacitor provides an initial jolt of electricity to get a motor running. They are used in AC condensing units and have a black plastic case.
A capacitor is a two-terminal, electrical component that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. They are one of the most fundamental passive components used in circuits.
A faulty start capacitor will usually exhibit a ruptured pressure relief blister or a blown-off top with partially or fully ejected insides.
Start capacitors typically have a capacitance of 50-1200 uf and voltages of 110/125, 165, 220/250, or 330 VAC. They usually have a frequency rating of 50 or 60 Hz and physically have a round case with two terminals per connection post.
