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What are Fixed Capacitors?

By W. Joyner
Updated May 17, 2024
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A fixed capacitor is a part of an electronic device or electrical appliance that helps maintain a constant charge and energy output. It helps store energy and moderates its flow. In general there are two broad types of capacitors, either fixed or variable. Capacitors in the “fixed” category tend to be most common in the timing circuits of small appliances and electronics used by individuals, in homes, and in offices. They provide a more or less constant flow of energy to the device, which allows for uninterrupted use. In almost all cases they’re built with shut-off capabilities to protect against power surges or overflow energy situations. From an engineering perspective their mechanics can be somewhat complex, and there are a couple of different varieties and specifications depending on the setting. In general, though, the concept is consistent; these components filter electric energy and control its flow to a mainframe or internal processing center of a device, which then translates it into a useful task.

Capacitor Basics

Practically every electronic device uses a capacitor in some manner, and the best way to characterize them is usually in terms of how they process incoming energy. Fixed capacitors are those that maintain a constant and unchanging value of what’s known as “capacitance,” or the ability to hold an electrical charge. Variable capacitors are characterized by the fact that their value of capacitance can be adjusted or varied.

The history of capacitors can be traced to the 18th century. Peter van Musschenbroek of the University of Leyden in the Netherlands developed what came to be known as the Leyden jar, an early form of a capacitor. The American innovator and later president Benjamin Franklin is credited with producing the first flat capacitor. Both of these early models were fixed. The main characteristic of a capacitor that is specifically fixed is its ability to keep a charge constant no matter the circuit-level fluctuations.

Common Uses

This sort of capacitor is perhaps most often found in timing circuits. Though frequently used in tandem with a resistor to create a timer, fixed capacitors are also used to supply a continuous flow of level current. This helps to avoid spikes and surges that might occur in the power supply of an electrical circuit.

Varieties and Materials

There are various types of capacitors that can be described or grouped as “fixed,” and in most cases they’re organized according to the dielectric material they’re made of. Basically, a dielectric is a material that does not conduct electricity. The dielectric is used in the fixed capacitor to insulate or separate the materials that do conduct electricity.

The capacitor is constructed with the dielectric sandwiched between the two conducting plates. In this way, each plate is capable of being charged with electrical current and has the ability to hold the charge. The difference in the charge levels of the conductive plates allows an electric field to exist in the dielectric.

A variety of materials are available for use as dielectrics, including paper, plastic, and ceramic. In some cases it’s also possible to use air as the insulating layer between the conducting plates, and such is the theory behind vacuum tubes.

Rating Differences and Measuring Standards

The capacitance rating of a capacitor that is fixed is affected by the thickness of the dielectric. Additionally, the type of material used for the conducting plates is of vital importance, because some materials have a far greater rate of conductivity than others.

Capacitance usually is measured in terms of farads or microfarads. Capacitors are available in a wide variety of shapes, sizes and, most importantly, capacitance ratings. In some applications, fixed models are linked together in series to form what is known as a fixed capacitor bank.

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