Physics

How Capacitors Work



Tweet
John Traveler's image for:
"How Capacitors Work"
Caption: 
Location: 
Image by: 
©  

Next to resistors and inductors,  the capacitor is one of the simplest of electronic components to understand. The capacitor is referred to as a static electrical charge device because it is comprised of two plates upon which either a surplus or deficiency of electrons are amassed, hence a static electrical charge.

The plate exhibiting the greatest number of electrons is said to have the highest negative charge and the opposite plate with fewer or no electrons the more positive charge. The plates of the capacitor are separated by an insulator material and the ratio of the charge on either plate to the potential difference between the plates is referred to as the property of capacitance. The unit of measure for capacitance is the “farad” named in honor of English physicists Michael Faraday who made monumental contributions to human understanding of electricity and magnetism in the 19th century.

In addition to the relative square surface area of the plates of a capacitor, the material used as an insulator can also effect the value of capacitance of a device. This effect is known as the “dialectic constant.” The dialectic constant of a capacitor contributes to a property known as capacitive resistance and there are two variations of capacitors associated with this property. Capacitors with a non-electrolytic dielectric can be charged in either direction, while capacitors with an electrolytic dielectric can only be charged in one direction. In other words, one plate of an electrolytic capacitor will always hold the positive charge, and the other a negative  one. Also, electrolytic capacitors exhibit higher capacitance than non-electrolytic ones for the same plate surface area. (Incidentally, a footnote with respect to electrolytic capacitors might be appropriate here: when electrolytic capacitors are installed backwards in a D/C circuit they tend to break down and disintegrate, in other words blow up.)

In D/C  electronic circuit applications a capacitor most often functions like a battery to provide damping or as an insulator to isolate different D/C potentials. Power supplies which rectify A/C currents to produce D/C currents commonly use large electrolytic capacitors to dampen A/C ripple and provide a pure constant voltage. Essentially, in a damping application, the capacitor in a D/C circuit is charged to the desired voltage potential. If the voltage begins to drop below that potential, the capacitor begins to discharge maintaining a constant current flow.  Most often a second capacitor is used to shunt the higher frequency peak voltage component of the A/C ripple voltage to ground.        

Capacitors in D/C circuits were once referred to as “condensers” as they could be charged up to a D/C potential and then discharged to provide a highly "condensed" instantaneous current. For instance, older automobiles ( pre-electronic ignition) used such condensers to build a 6 or12 volt D/C charge while the points are in the open position. When the points are mechanically closed, The capacitor discharges with high current through the primary winding of the induction coil to ground. This high current in the primary of the induction coil produces a high voltage (30-60KV) in the secondary winding of the coil, which when felt at the spark plug jumps the gap producing the spark that ignites the gas air mixture in the cylinder.

Understanding how capacitors work in A/C electronic circuit applications is a little more complicated, because in addition to the property of capacitive resistance, the property of capacitive reactance must be taken into account. Reactance is defined as the non-resistive opposition to current flow in an A/C circuit. For a given value of capacitance, the reactance of a capacitor changes depending on the frequency of the A/C signal applied. For instance, a capacitor with a high value of capacitance may look like a short to a lower frequency A/C signal while it appears as an open to a higher frequency A/C signal. In other words, if an appropriate capacitance value is chosen, a capacitor can be used either to block or promote current flow at a given frequency. This property of capacitors is very useful in filtering out signals of different frequencies.

In A/C circuits a range of filtering techniques, usually including combinations of capacitors and inductors are used to filter specific frequencies, or  as band-pass and band-reject filters. With respect to radio frequency receivers, all of these techniques are used to modulate and demodulate intelligence on a transmitter/receiver carrier frequency. On older radios, a variable capacitor with an air dialectic was used to adjust the receiver to the carrier frequency. Newer digital receivers use semiconductor switches to switch different values of capacitance in and out of a circuit accomplishing the same thing.

Today’s electronic circuits are full of capacitors exploiting both the D/C as well as A/C properties  of the devices. They are most often found in conjunction with coils and chokes to design frequency sensitive circuit applications. We end with a safety warning about capacitors, particularly large electrolytic ones. These devices can develop a high voltage charge just from air molecules rubbing up against their contacts, and therefore represent a significant electrical shock hazard. In fact, capacitive discharge is a technique incorporated in defibrillating machines used to shock  the heart into a normal rhythm or even start or stop it. As such, capacitors are electronic devises which demand cautionary respect when handling them. 

Sources:

The information provided in this article is based on the authors formal education, training and 38 years of experience in the field as an electronics engineer. No part of this article, in part or in whole has been borrowed, copied or otherwise references any other material not common knowledge to its author or any other person of competent authority in the field of electronics. This disclaimer has been added in response to Helium.com censorship of this article requiring inclusion of source references. Anyone having question in regard to this author regarding further stipulation of expertise should contact the author via e-mail at the address given on the authors Helium bio page. This author does not respond to emails delivered via Helium's internal e-mail facility.         

Tweet
More about this author: John Traveler

From Around the Web




ARTICLE SOURCES AND CITATIONS