Capacitors and Capacitance Basics

- tutorial about the basics of capacitance and capacitors - how they work and what they are.

Capacitance is one of the most important effects used in electronics. Along with this the associated components - capacitors are widely used, the second most widely used component.

Capacitors find uses in virtually every form of electronics circuit from analogue circuits including amplifiers and power supplies through to oscillators, integrators and many more. Capacitors are also used in logic circuits, primarily for providing decoupling to prevent spikes and ripple on the supply lines which could cause spurious triggering of the circuits.

What is capacitance

Capacitance is the ability to store electric charge. In its simplest form a capacitor consists of two parallel plates or electrodes that are separated from each other by an insulating dielectric. It is found that when a battery or any other voltage source is connected to the two plates as shown a current flows for a short time as it charges up. It is found that one plate of the capacitor receives an excess of electrons, while the other has too few. In this way the capacitor plate or electrode with the excess of electrons becomes negatively charged, while the capacitor electrode becomes positively charged.

Charge stored between two plates of a capacitor

Charge stored between two plates of a capacitor

If the battery is removed the capacitor will retain its charge. However if a resistor is placed across the plates, a current will flow in the resistor until the capacitor becomes discharged.

Units of capacitance

It is necessary to quantify a capacitor in terms of its ability to store charge. The basic unit of capacitance is the Farad, named after Michael Faraday.

The definition of A Farad is: A capacitor has a capacitance of one Farad when a potential difference of one volt will charge it with one coulomb of electricity (i.e. one Amp for one second).

In view of the fact that a capacitor with a capacitance of one Farad is too large for most electronics applications, components with much smaller values of capacitance are normally used. Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):


Prefix Multiplier  
µ 10-6 (millionth) 1000000µF = 1F
n 10-9 (thousand-millionth) 1000nF = 1µF
p 10-12 (million-millionth) 1000pF = 1nF

Capacitor charge discharge cycle

It is also possible to look at the voltage across the capacitor as well as looking at the charge. After all it is easier to measure the voltage on it using a simple meter. When the capacitor is discharged there is no voltage across it. Similarly, one it is fully charged no current is flowing from the voltage source and therefore it has the same voltage across it as the source.

In reality there will always be some resistance in the circuit, and therefore the capacitor will be connected to the voltage source through a resistor. This means that it will take a finite time for the capacitor to charge up, and the rise in voltage does not take place instantly. It is found that the rate at which the voltage rises is much faster at first than after it has been charging for some while. Eventually it reaches a point when it is virtually fully charged and almost no current flows. In theory the capacitor never becomes fully charged as the curve is asymptotic. However in reality it reaches a point where it can be considered to be fully charged or discharged and no current flows.

Similarly the capacitor will always discharge through a resistance. As the charge on the capacitor falls, so the voltage across the plates is reduced. This means that the current will be reduced, and in turn the rate at which the charge is reduced falls. This means that the voltage across the capacitor falls in an exponential fashion, gradually approaching zero.

The rate at which the voltage rises or decays is dependent upon the resistance in the circuit. The greater the resistance the smaller the amount of charge which is transferred and the longer it takes for the capacitor to charge or discharge.

Capacitor charge and discharge

Voltage on a capacitor charging and discharging

So far the case when a battery has been connected to charge the capacitor and disconnected and a resistor applied to charge it up have been considered. If an alternating waveform, which by its nature is continually changing is applied to the capacitor, then it will be in a continual state of charging and discharging. For this to happen a current must be flowing in the circuit. In this way a capacitor will allow an alternating current to flow, but it will block a direct current. As such capacitors are used for coupling an AC signal between two circuits which are at different steady state potentials.


Phase

In an electric circuit it is found that the voltage and current are not exactly in phase. Because current flows through the capacitor when there is a change in voltage the current leads the voltage by 90 degrees. The maximum rate of change in voltage takes place when the voltage is midway between the two peaks. This is when the maximum current flows. The minimum rate of change of voltage occurs at either peak and hence the current is at a minimum.


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