What Is A Capacitor
Capacitors are simple passive devices which are used to store electricity. The capacitor has the ability or “capacity” to store energy in the form of an electrical charge producing a potential difference (Static Voltage) across its plates, much like a small rechargeable battery.
A capacitor is formed from two conducting plates separated by air or by some form of a good insulating material such as waxed paper, mica, ceramic, plastic or some form of a liquid gel. The insulating layer between a capacitors plates is commonly called the Dielectric.
Due to this insulating layer, the d.c. current can not flow through the capacitor but instead a voltage is produced across the plates in the form of an electrical charge.
If a current i flows, positive change, q, will accumulate on the upper plate. To preserve charge neutrality, a balancing negative charge will be present on the lower plate.
Hence, there will be a potential energy difference (or voltage v) between the plates which is proportional to the charge q.
where A is the area of the plates
d is their separation
Ɛ0 is the permittivity of the insulating layer ( Ɛ0 = 8.85 pF/m for vacuum).
The capacitance is given by the following expression :
The capacitance is measured in Farads (F)
The charge q is hence given by the expression :
The current, i, which is the rate of charge flow is given by :
The conductive metal plates of a capacitor can be either square, circular or rectangular, or they can be of a cylindrical or spherical shape with the general shape, size and construction of a parallel plate capacitor depending on its application and voltage rating.
When used in a direct current or DC circuit, a capacitor charges up to its supply voltage but blocks the flow of current through it because the dielectric. However, when a capacitor is connected to an alternating current or AC circuit, the flow of the current appears to pass straight through the capacitor with little or no resistance.
Types of Capacitors
There are a large variety of different types of capacitor available in the market with their own set of characteristics and applications, from very small delicate trimming capacitors up to large power metal-can type capacitors used in high voltage power correction and smoothing circuits.
Capacitor types are distinguished by the material used as the insulator.
Let us now discuss a few common types of capacitor available.
Dielectric capacitors are usually of the variable type where a continuous variation of capacitance is required for tuning transmitters, receivers and transistor radios.
Variable Capacitor Symbol
Variable Capacitor Symbol
Variable dielectric capacitors are multi-plate air-spaced types that have a set of fixed plates (the stator vanes) and a set of movable plates (the rotor vanes) which move in between the fixed plates.
The position of the moving plates with respect to the fixed plates determines the overall capacitance value.
The capacitance is generally at maximum when the two sets of plates are fully meshed together.
High voltage type tuning capacitors have relatively large spacings or air-gaps between the plates with breakdown voltages reaching many thousands of volts.
Film Capacitors are the most commonly available of all types of capacitors
These capacitors have a relatively large family with the difference being in their dielectric properties which include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metalised paper, Teflon etc.
Film type capacitors are available in capacitance ranges from as small as 5pF to as large as 100uF .
Film Capacitors which use polystyrene, polycarbonate or Teflon as their dielectrics are sometimes called “Plastic capacitors”. The main advantage of plastic film capacitors compared to impregnated-paper types is that they operate well under conditions of high temperature, have smaller tolerances, a very long service life and high reliability.
Ceramic Capacitors or Disc Capacitors are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked them together .
For very low capacitance values a single ceramic disc of about 3-6mm is used.
Ceramic capacitors have a high dielectric constant and are used so that relatively high capacitance can be obtained in a small physical size. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads ( μF ).
They exhibit large non-linear changes in capacitance against temperature and hence, used as de-coupling or by-pass capacitors as they are also non-polarized devices.
Ceramic types of capacitors generally have a 3-digit code printed onto their body to identify their capacitance value in pico-farads. Generally the first two digits indicate the capacitors value and the third digit indicates the number of zero’s to be added. For example, a ceramic disc capacitor with the markings 103 would indicate 10 and 3 zero’s in pico-farads which is equivalent to 10,000 pF or 10nF. Letter codes are sometimes used to indicate their tolerance value such as: J = 5%, K = 10% or M = 20% etc.
Electrolytic Capacitors are generally used when very large capacitance values are required.
Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).
The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for a small physical size as the distance between the plates, d is very small.
Most of the electrolytic types of capacitors are Polarised , which means the DC voltage applied to the capacitor terminals must be of the correct polarity, i.e. positive to the positive terminal and negative to the negative terminal as an incorrect polarisation will break down the insulating oxide layer and permanently damage the capacitor.
Hence, all polarised electrolytic capacitors have their polarity clearly marked with a negative sign to indicate the negative terminal and this polarity must be followed.
Electrolytic Capacitors are generally used in DC power supply circuits due to their large capacitance’s and small size to help reduce the ripple voltage or for coupling and decoupling applications.
One main disadvantage of electrolytic capacitors is their relatively low voltage rating and due to the polarisation of electrolytic capacitors, which means that they must not be used on AC supplies.
The other drawbacks of electrolytic capacitors are large leakage currents, value tolerances, equivalent series resistance and a limited lifetime.
Electrolytic capacitors can be either wet-electrolyte or solid polymer.
They are commonly made of tantalum or aluminum, although other materials may be used.
Supercapacitors are a special subtype of electrolytic capacitors, also called double-layer electrolytic capacitors, with capacitances of hundreds and thousands of farads.
Electrolytic capacitors generally come in two basic forms ;
- Aluminium Electrolytic Capacitors
- Tantalum Electrolytic Capacitors
1. Aluminium Electrolytic Capacitors
There are basically two types of Aluminium Electrolytic Capacitor, the plain foil type and the etched foil type.
The thickness of the aluminium oxide film and high breakdown voltage give these capacitors very high capacitance values for their size.
The foil plates of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material and determines which side of the plate is positive and which side is negative.
The etched foil type differs from the plain foil type in that the aluminium oxide on the anode and cathode foils has been chemically etched to increase its surface area and permittivity.
This gives a smaller sized capacitor than a plain foil type of equivalent value but has the disadvantage of not being able to withstand high DC currents compared to the plain type. Also their tolerance range is quite large at up to 20%.
Typical values of capacitance for an aluminium electrolytic capacitor range from 1uF up to 47,000uF.
Etched foil electrolytic’s are best used in coupling, DC blocking and by-pass circuits while plain foil types are better suited as smoothing capacitors in power supplies.
2. Tantalum Electrolytic Capacitors
Tantalum Electrolytic Capacitors are available in both wet (foil) and dry (solid) electrolytic types with the dry or solid tantalum being the most common.
Solid tantalum capacitors use manganese dioxide as their second terminal and are physically smaller than the equivalent aluminium capacitors.
The dielectric properties of tantalum oxide is also much better than those of aluminium oxide giving a lower leakage currents and better capacitance stability which makes them suitable for use in blocking, by-passing, decoupling, filtering and timing applications.
Also, Tantalum Capacitors although polarised, can tolerate being connected to a reverse voltage much more easily than the aluminium types but are rated at much lower working voltages.
Solid tantalum capacitors are usually used in circuits where the AC voltage is small compared to the DC voltage.
However, some tantalum capacitor types contain two capacitors in-one, connected negative-to-negative to form a “non-polarised” capacitor for use in low voltage AC circuits as a non-polarised device.
Generally, the positive lead is identified on the capacitor body by a polarity mark, with the body of a tantalum bead capacitor being an oval geometrical shape.
Typical values of capacitance range from 47nF to 470uF.