A capacitor (C) is an electronic component consisting of two conducting parallel plates separated by an insulating material called the dielectric. A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated conductors.

An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.

The properties of capacitors in a circuit may determine the resonant frequency and quality factor of a resonant circuit, power dissipation and operating frequency in a digital logic circuit, energy capacity in a high-power system, and many other important system characteristics.

The current that an ideal capacitor "passes" through it depends on the rate of change of the voltage applied across it. The direction of the current will be in such a way as to counteract the change of voltage.[1]

Factors affecting capacitance[]

  • The charge held depends on the applied voltage
  • The capacitance increases as the total area of the opposing surfaces of the plates increases, because a larger plate area can hold a greater charge.
  • The capacitance increases as the distance between the plates decreases because the electric field then becomes more concentrated
  • The capacitance depends upon the dielectric material

Three uses of the capacitor[]

These devices are used in a variety of diffrent applications but you will see them most widely used in electrical engineering as blockers, filters, ansd spark suppressors.

  • Since there is an insulator, the device can be used to block the passage of DC current. However, alternating or AC current of certain frequencies will pass through.
  • A capacitor can be used as a rectifier or as a filter, so that pure DC is obtained as output.
  • Minimizes the effect of sparking and extends the life of relay contacts as a spark suppressor.

In fact, in theory real capacitors have non-zero DC leakage current, non-zero lead resistance and inductance, memory behavior etc.

Cap Check[]

Capacitor test

Capacitor Testing using digital multimeter. A Shirted cap equals zero while an open cap equals infinity.

Bad capacitor1

Red are the bubbled top of the can that is defective and the cap must be replaced. The Blue flat can is good.

Testing a capacitor using a digital multimeter is a process. First, drain or discharge the capacitor by shorting both capacitor leads together with a screwdriver or inserting a large resistor in its path. All capacitors should be discharged before doing a test or soldering one into circuitry. Next, if its an electrolytic capacitor connect negative lead to negative side of component with the multimeter "off". Turn the multimeter on and read the capacitance on the LCD display. In normal operation, the meter will read near to zero (acts as a short) and then gradually increase in resistance towards infinite (acts as an open) resistance. If you have a cap that is shorted then the meter will always read near "0" volts and if its open it will read "Infinity". Lastly, if the polarity of the meter leads are reversed, the capacitor will not charge properly using the multimeter and result in an inaccurate test. A visual inspection should also be performed to ensure the capacitor cans on top are flat and not leaky, bubbled, or swollen.

Charge Stored[]


The capability of a capacitor to store charge of a voltage


Capacitive Reactance[]


Capacitive Reactance (Xc) is equal to to 1 over 2 pi times Frequency (f) times Capacitance (C).

Xc = 1/2pifC

Phase Angle[]

For capacitor without resistance, Voltage and current have a phase difference of 90o

For capacitor with resistance, Voltage and current have a phase difference of θ

Tan θ = = 1 / 2πf CRC

When phase angle change, frequency also change . Therefore, capacitor can be used for frequency shifting

f = 1 / 2π Tanθ CR

Since frequency is one over time . Therefore, Time

t = 2π Tanθ CRC

When Tanθ = 1 or θ = 45o

t = 2π CRC = 0.3 CRC

will the time to charge or discharge capacitor to halved a voltage . Therefore, capacitor can be used as Timer.


  1. E. J. Mastascusa . "Capacitors." Bucknell University . 2008.


See also[]


  • Electronics_Capacitors_1965_US_Air_Force_Training_Film

    Electronics Capacitors 1965 US Air Force Training Film

  • MAKE_presents_The_Capacitor

    MAKE presents The Capacitor