Capacitors come in many specific categories, such as starting capacitors, running capacitors, and load capacitors. For more information on capacitors, please refer to the previous two articles. To enhance your understanding of capacitors, this article will introduce film capacitors. If you are interested in capacitor-related knowledge, please continue reading.
I. Film Capacitors
Film capacitors are capacitors constructed by overlapping metal foil as electrodes with plastic films such as polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), or polycarbonate (PC) films at both ends and then winding them into a cylindrical shape. Depending on the type of plastic film, they are called polyethylene terephthalate capacitors (also known as Mylar capacitors), polypropylene capacitors (also known as PP capacitors), polystyrene capacitors (also known as PS capacitors), and PC capacitors. Film capacitors are high-performance capacitors due to their many excellent characteristics. Their main characteristics are as follows: non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and very low dielectric loss. Based on these advantages, film capacitors are widely used in analog circuits. Especially in signal coupling sections, capacitors with good frequency characteristics and extremely low dielectric loss are essential to ensure that signals are not significantly distorted during transmission. Among all plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most outstanding characteristics; naturally, these two types of capacitors are also more expensive. However, in order to improve sound quality, audio equipment uses increasingly higher-grade materials for its components, and price is no longer the most important consideration. Therefore, PP capacitors and PS capacitors are being used more and more frequently in audio equipment. Readers often see certain brands of equipment claiming to use a certain number of PP or PS capacitors from certain famous brands as a standard for sound quality, and this is the reason why.
II. Why do the capacitance values of film capacitors and electrolytic capacitors decrease?
For plug-in capacitors, film capacitors and electrolytic capacitors are the most commonly used. However, some capacitors in circuits are prone to problems, specifically a rapid decrease in capacitance. Why does this happen? Why do the capacitances of film capacitors and electrolytic capacitors degrade?
1. Reasons for the degradation of electrolytic capacitors.
Electrolytic capacitors are the easiest to degrade in capacitance, and the prices of the same type of electrolytic capacitor vary greatly between different manufacturers. Many low-quality electrolytic capacitors have a very short lifespan, and their capacitance degrades very easily, causing them to malfunction.
The capacitance of an electrolytic capacitor is controlled by the electrolyte. Because the electrolyte is an organic substance, it can deteriorate and dry out. In particular, some electrolytic capacitors of relatively poor quality undergo chemical decomposition at the metal electrodes, resulting in a smaller electrolyte volume and thus a smaller capacitance.
Capacitance decay of electrolytic capacitors is a common phenomenon. Generally, the capacitance decay of higher quality capacitors is slower, while that of lower quality capacitors is more likely to decay very quickly.
2. Reasons for the decay of film capacitor capacity.
The market situation for film capacitors and electrolytic capacitors is similar. Brand-name capacitors that prioritize quality generally have very little capacitance decay. However, there are a large number of inferior film capacitors on the market that engage in price wars. These capacitors use poor materials and cut corners in the production process, resulting in rapid capacitance decay.
One of the reasons for the degradation of film capacitors: humid air.
During the film winding process, a large amount of humid air is present. Under normal circumstances, the film capacitor winding workshop must be dust-free and fully enclosed, and air conditioning is required to maintain the humidity and temperature inside the workshop. However, most small factories cannot achieve this production environment.
Meanwhile, in the hot pressing process, if vacuum hot pressing is used, the humid air inside the film capacitor core can be expelled. However, most small manufacturers do not use vacuum hot pressing because the equipment is more expensive, the production speed is slow, and the electricity consumption and production costs are high.
If there is a lot of humid air inside the film capacitor core, an air ionization reaction will occur during operation, producing a large amount of ozone. Ozone is a strong oxidant, which will damage part of the film, reduce the area of the capacitor plates, and decrease the capacitance.
Reason 2 for capacitance decay of thin film capacitors: inferior film.
Thin film raw materials also vary between large and small manufacturers. Although the thin film materials produced by small manufacturers are extremely cheap, they are of poor quality. For example, the vapor deposition may be uneven, with some parts of the film not being deposited, or there may be dust or burrs on the film. These defects may cause the film capacitor to break down during use. Although the film capacitor can heal itself, it will lead to a decrease in the capacitance of the film capacitor.
Recommendation: To buy film capacitors with low capacitance decay, try to avoid products without a brand name and neutral markings. There are far too many inferior products like these. Opt for branded film capacitors, as they are generally of better quality and will decay much more slowly.
