Capacitor Basics


  Capacitors are simple passive devices that store charge on their plates when connected to a voltage source

  Capacitors are components that have the ability or "capacity" to store energy. The form of charge creates a potential difference (static voltage) across its plates, much like a small rechargeable battery.

  There are many different kinds of capacitors available from very small capacitor beads for resonant circuits to large power factor correction capacitors, but they all do the same thing, they store charge.

  In its basic form, a capacitor consists of two or more parallel conductive (metal) plates that are not connected or touching each other, but are connected by air or some form of good insulating material such as waxed paper, mica, ceramic, plastic or Some form of liquid gel for electrical insulation, as used in electrolytic capacitors. The insulating layer between the capacitor plates is often called the dielectric.

  typical capacitor

  Due to this insulating layer, DC current does not flow through the capacitor as it blocks it, instead allowing voltage to exist on the board in the form of charge.

  Conductive metal plate capacitors can be square, round or rectangular, and cylindrical or spherical, having the general shape, size and construction of parallel plate capacitors, depending on their application and voltage rating.

  In direct current, or DC circuits, a capacitor charges up to its supply voltage but blocks current flow through it because the capacitor's dielectric is non-conductive and essentially an insulator. However, when a capacitor is connected to AC current or an AC circuit, the flow of current appears to be directly through the capacitor with little or no resistance.

  There are two types of charges, positive charges in the form of protons and negative charges in the form of electrons. When a DC voltage is placed across a capacitor, positive (+ve) charge rapidly accumulates on one plate, while a corresponding opposite negative (-ve) charge accumulates on the other plate. For every particle of +ve charge that reaches one plate, a charge of the same sign will leave the -ve plate.

  The plates then remain charge neutral, and by this charge a potential difference is established between the two plates. Once the capacitor reaches its steady state condition, current cannot flow through the capacitor itself and around the circuit due to the insulating properties of the dielectric used to separate the plates.

  The flow of electrons to the upper plate of the capacitor is called the capacitor charging current, which continues to flow until the voltage across the two plates (and thus the capacitor) equals the applied voltage Vc. At this point the capacitor is said to be "fully charged with electrons".

  The magnitude or rate of the charging current reaches a maximum value when the plates are fully discharged (initial condition) and becomes zero when the potential difference between the capacitive plates charged to the capacitor plates is equal to the supply voltage.

  The amount of potential difference that exists across the capacitor depends on how much charge the capacitor has deposited on the board. The work is done by the supply voltage and how much capacitance the capacitor has, which will be explained below.

  A parallel plate capacitor is the simplest capacitor. It can be constructed using two metal or foil plates at a parallel distance from each other, with a capacitance value in farads fixed by the surface area of ​​the conducting plates and the separation distance between them. Changing either of these two values ​​changes its capacitance value, which forms the basis of variable capacitor operation.

  Also, because capacitors store the energy of electrons in the form of charge. The larger the plates and/or the smaller their spacing, the more charge a capacitor can hold for any given voltage across its plates. In other words, bigger boards, smaller distances, bigger capacitance.

  By applying a voltage to the capacitor and measuring the charge on the plate, the ratio of the charge Q to the voltage V will give the capacitance value of the capacitor, so it is given as follows: C = Q / V This equation can also be rearranged to give the familiar formula. The charge on the plate is: Q = C x V

  Although we have said that the charge is stored on the plates of the capacitor, it is more accurate to say that the energy within the charge is stored in the "electrostatic field" between the two plates. When current flows into the capacitor, it charges up, so the electrostatic field becomes stronger as it stores more energy between the plates.

  Likewise, when current flows out of a capacitor, discharging, the potential difference between the two plates decreases and the electrostatic field decreases as energy is removed from the plates.

  The property of a capacitor to store charge on its plates in the form of an electrostatic field is known as the capacitance of the capacitor. Not only that, but capacitance is also a property of a capacitor, which resists changes in voltage.

  Capacitance of a parallel plate capacitor

  The capacitance of a parallel plate capacitor is proportional to the area, A in meters2 and inversely proportional to the distance or spacing of the two plates, d (i.e. the thickness of the dielectric in meters between the two conducting plates.

  The generalized equation for the capacitance of a parallel plate capacitor is: C = ε(A/d) where ε represents the absolute permittivity of the dielectric material used. The dielectric constant of vacuum, ε o is also known as the "permittivity of free space", with a constant of 8.84x10 -12 Farads per meter.

  To make the math a little easier, the permittivity ε o of this free space, can be written as: 1 /(4πx9×10 9 ), which can also be given as a constant in units of picofarads (pF) per meter: 8.84 for the available space value. Note that the resulting capacitance values ​​will be in picofarads rather than farads.

  Usually, the conductive plates of a capacitor are separated by some kind of insulating material or gel, rather than a perfect vacuum. When calculating the capacitance of a capacitor, we can consider the dielectric constant of air, especially dry air, since they are very close to the same value of vacuum.

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