Capacitor

In electronics a capacitor is a device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator. There are two terminals on a capacitor for application of connection to an electrical circuit. Although similar to a battery in some respects, a capacitor uses two plates to store electricity in an electrical field. A battery uses a chemical reaction to release electrical current.

Capacitance formula:

    q
C = -
    V

Where: q - charges on the plates V - voltage between the plates

The capacitor plates store the electrons. If the size of the plates is increased, the capacitance increases due to the increased surface area of the plates to store electrons. If the plates are moved farther apart, the capacitance goes down, because the electric field strength between them goes down as the distance increases. Large plates very close together have greater capacitance, however, the voltage rating is less when the plates are more closely spaced. Increasing the space between the plates increases the voltage rating, and decreases capacitance. This can be offset by increasing the surface area of the plates. The reason that the voltage rating is limited is due to arching that can take place between the plates.

Polarized Capacitor

A Polarised Capacitor’s plates are polarity sensitive and are normally electrolytic. The positive lead is shown on the schematic with a "+" symbol. The negative lead is generally not shown on the schematic, but may be marked on the capacitor with a bar or "-" symbol. Polarized capacitors are the electrolytic type or tandalium type. Polarized capacitors have large leakage current if the voltage is inverted.

The reason an electrolytic capacitor is polarized is due to the fact that the aluminum oxide layer is held in place by the electric field, and when reverse-biased, it dissolves into the electrolyte. Connecting an electrolytic capacitor backwards

The Farad

The farad (symbol: F) is the SI derived unit of electrical capacitance. It is named after the English physicist Michael Faraday. The Farad is the capacitance which stores a one-coulomb charge across a potential difference of one volt.

There are various increments of the unit of a farad from larger to smaller.

• 1 mF (millifarad, one thousandth (10−3) of a farad) = 1000 μF = 1000000 nF
• 1 μF (microfarad, one millionth (10−6) of a farad) = 0.000 001 F = 1000 nF = 1000000 pF
• 1 nF (nanofarad, one billionth (10−9) of a farad) = 0.001 μF = 1000 pF
• 1 pF (picofarad, one trillionth (10−12) of a farad)

There is some confusion in various means of representation of these units. For example the mFd is the same as uF, which is also the same as the symbol "µ" as seen in "µF". Although the "mfd" represents "milliFarad" while "uF" represents the smaller "microFarad," some older capacitors show "mF" where it should be "µF".

Sometimes you see the millifarad marked as mF and sometimes mFd. The proper values are usually specified in farads (F), microfarads (μF), nanofarads (nF) and picofarads (pF). A value of 0.1 pF is about the smallest available in capacitors for general use in electronic design, since smaller ones would be dominated by the parasitic capacitances of other components, wiring or printed circuit boards. Capacitance values of 1 pF or lower can be achieved by twisting two short lengths of insulated wire together.

Capacitor in Series or Parallel

When capacitors are connected in parallel, the total capacitance is the sum of both capacitors. When you connect two or more capacitors together in Parallel you achieve the sum of the total plate area of each capacitor, so think of it as one big capacitor.

When capacitors are connected in series, the total capacitance is half the sum of both capacitors, therefore you reduce capacitance when you connect them in series. However, you do get twice the voltage rating, therefore two 16v 200μF rated capacitors now act as a single 32v rated capacitor with 100μF capacitance.