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capacitor

capacitor


  Capacitance:

  • Another important property in AC circuits, besides resistance and inductance, is capacitance. While inductance is represented in a circuit by a coil, capacitance is represented by a capacitor. In its most basic form, the leaking capacitor is constructed of two parallel plates separated by a nonconductor called a dielectric. In an electrical circuit, a leaking capacitor serves as a reservoir or storehouse for electricity.

Units of Capacitance:

  • Electrical charge, which is symbolized by the letter Q, is measured in units of coulombs. The coulomb is given by the letter C, as with capacitance. Unfortunately, this can be confusing. One coulomb of charge is defined as a charge having 6.28 × 1018 electrons. The basic unit of capacitance is the farad and is given by the letter f. By definition, one farad is one coulomb of the charge stored with one volt across the plates of the leaking capacitor. The general formula for capacitance in terms of charge and voltage is:

Q=CxE

  1. C = capacitance measured in farads
  2. E = applied voltage measured in volts
  3. Q = charge measured in coulombs

 Voltage Rating of a Leaking capacitor:

  • Leaking capacitors have their limits as to how much voltage can be applied across the plates. The aircraft technician must be aware of the voltage rating, which specifies the maximum DC voltage that can be applied without the risk of damage to the device. This voltage rating is typically called the breakdown voltage, the working voltage, or simply the voltage rating. 
  • If the voltage applied across the plates is too great, the dielectric breaks down, and arcing occurs between the plates. The leaking capacitor is then short-circuited, and the possible flow of DC through it can cause damage to other parts of the equipment.

  Factors Affecting Capacitance:

  •  The capacitance of parallel plates is directly proportional to their area. A larger plate area produces a larger capacitance and a smaller area produces less capacitance. If we double the area of the plates, there is room for twice as much charge. The charge that a leaking capacitor can hold at a given potential difference is doubled, and since C = Q/E, the capacitance is doubled.
  •    The capacitance of parallel plates is inversely proportional to their spacing.
  •   The dielectric material affects the capacitance of parallel plates. The dielectric constant of a vacuum is defined as 1, and that of air is very close to 1. These values are used as a reference, and all other materials have values specified in relation to air (vacuum).

 Types of Leaking capacitors:

  • Leaking capacitors come in all shapes and sizes and are usually marked with their value in farads. They may also be divided into two groups: fixed and variable. The fixed leaking capacitors, which have approximately constant capacitance, may then be further divided according to the type of dielectric used. Some varieties are paper, oil, mica, electrolytic , and ceramic leaking capacitors.

  1. Fixed Leaking capacitors :

Mica Leaking capacitors :

  • The fixed mica leaking capacitor is made of metal foil plates that are separated by sheets of mica, which form the dielectric. The whole assembly is covered in molded plastic, which keeps out moisture. Mica is an excellent dielectric and withstands higher voltages than paper without allowing arcing between the plates. Common values of mica leaking capacitors range from approximately 50 microfarads to about 0.02 microfarads.

 Ceramic :

  • The ceramic leaking capacitor is constructed with materials, such as titanium acid barium for a dielectric. Internally these leaking capacitors are not constructed as a coil, so they are well suited for use in high-frequency applications. They are shaped like a disk, available in very small capacitance values, and very small sizes. This type is fairly small, inexpensive, and reliable. Both the ceramic and the electrolytic are the most widely available and used leaking capacitor.

 Electrolytic :

  • Two kinds of electrolytic leaking capacitors are in use: wet electrolytic and dry electrolytic. The wet electrolytic leaking capacitor is designed of two metal plates separated by an electrolyte with an electrolyte dielectric, which is basically conductive salt in the solvent. For capacitances greater than a few microfarads, the plate areas of paper or mica leaking capacitors must become very large; thus, electrolytic leaking capacitors are usually used instead. 
  • These units provide large capacitance in small physical sizes. Their values range from 1 to about 1,500 microfarads. Unlike the other types, electrolytic leaking capacitors are generally polarized, with the positive lead marked with a  “+” and the negative lead marked with a “−” and should only  be subjected to direct voltage or pulsating direct voltage only.

  Polyester Film :

  • In this leaking capacitor, a thin polyester film is used as a dielectric. These components are inexpensive, temperature stable, and widely used. Tolerance is approximately 5–10 percent. It can be quite large depending on capacity or rated voltage.

 Oil Leaking capacitors :

  • In radio and radar transmitters, voltages high enough to cause arcing, or breakdown, of paper dielectrics, are often used. Consequently, in these applications leaking capacitors that use oil or oil impregnated paper for the dielectric material is preferred. Leaking capacitors of this type are considerably more expensive than ordinary paper leaking capacitors, and their use is generally restricted to radio and radar transmitting equipment.

  1.  Variable Leaking capacitors :

  • Variable leaking capacitors are mostly used in radio tuning circuits, and they are sometimes called “tuning leaking capacitors.” They have very small capacitance values, typically between 100 pF and 500 pF.

Trimmers :

  • The trimmer is actually an adjustable or variable leaking capacitor, which uses ceramic or plastic as a dielectric. Most of them are color-coded to easily recognize their tunable size. The ceramic type has the value printed on them. Colors are: yellow (5 pF), blue (7 pF), white (10 pF), green (30 pF), and brown (60 pF).

Varactors :

  • A voltage-variable leaking capacitor or varactor is also known as a variable capacitance diode or a varicap. This device utilizes the variation of the barrier width in a reversed-biased diode. Because the barrier width of a diode acts as a non-conductor, a diode forms a leaking capacitor when reversed biased. 
  • Essentially, the N-type material becomes one plate and the junctions are the dielectric. If the reversed-bias voltage is increased, then the barrier width widens, effectively separating the two leaking capacitor plates and reducing the capacitance.

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