Capacitor and their usage

What do we mean by the word capacitor?

Capacitors are electric device used for storage electrical charges. This device is like a condenser. It is an important device in any electrical or electronic and in telecommunication industries. E.g. in radio, television receiver and in transmitter circuit.

Types of capacitor

There are different types of Capacitors available in the market place and each one has its own set of characteristics and applications from small delicate trimming capacitors up to large power metal can type capacitors used in high voltage power correction and smoothing circuits. Like resistors, there are also variable types of capacitors which allow us to vary their capacitance value for use in radio or "frequency tuning" type circuits. Either way, capacitors play an important part in electronic circuits so here are a few of the more "Common" types of capacitors available.

1.Dielectric
 Dielectric Capacitors are usually of the variable type such as used for tuning transmitters, receivers and transistor radios. They have a set of fixed plates and a set of moving plates that mesh with the fixed plates and the position of the moving plates with respect to the fixed plates determines the overall capacitance.

The capacitance is generally at maximum when the plates are fully meshed. High voltage type tuning capacitors have relatively large spacing's or air-gaps between the plates with breakdown voltages reaching many thousands of volts.

Variable Capacitor Symbols

2. Film Capacitors
Film Capacitors are the most commonly available of all types of capacitors, consisting of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester polystyrene,polypropylene,polycarbon-ate, metalized paper etc.

 Film type capacitors are available in capacitance ranges from 5pF to 100uF depending upon the actual type of capacitor and its voltage rating. Film capacitors also come in an assortment of shapes and case styles which include:

Rectangular & Round film capacitors are the rectangular metalized film and cylindrical film & foil types are shown below

Cylindrical  Type

3. Ceramic Capacitors
Ceramic Capacitors or Disc Capacitors as they are generally called, are made by coating two sides of a small ceramic disc with silver and are then stacked together to make a capacitor.

For very low capacitance values, a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance can be obtained in a small physical size.

They are large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads but their voltage ratings are generally quite low.

For example, 10³ would indicate 10 x 10³pF which is equivalent to 10,000 pF or 0.01μF.
4. Electrolytic Capacitors
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 majority of electrolytic types of capacitors are polarized, that is the voltage applied to the capacitor terminals must be of the correct polarity as an incorrect polarization will break down the insulating oxide layer and permanent damage may result.

    Electrolytic Capacitors are generally used in DC power supply circuits to help reduce the ripple voltage or for coupling and decoupling applications. Electrolyte's generally come in two basic forms; Aluminium Electrolytic and Tantalum Electrolytic capacitors.

Electrolytic Capacitor

Also tantalum capacitors although polarized, can tolerate being connected to a reverse voltage much more easily than the Aluminium types but are rated at much lower working voltages. Typical values of capacitance range from 47nF to 470uF

Aluminium & Tantalum Electrolytic Capacitor

Capacitor Characteristics
The characteristics associated with the humble capacitor so here are just a few of the more important ones.
1. Working Voltage, (Vn)
The Working Voltage (Wvdc, Wvac) is the maximum continuous voltage that can be applied to the capacitor without failure during its working life.

DC and AC values are usually not the same as the AC value refers to the r.m.s. value. Common working DC voltages are 10V, 16V, 25V, 35V, 63V, 100V, 160V, 250V, 400V and 1000V and are printed onto the body of the capacitor. 2. Tolerance, (±%)
As with resistors, Capacitors also have a tolerance rating expressed as a plus-or-minus value either in Picofarads (±pF) for low value capacitors generally less than 10pF or as a percentage (±%) for higher value capacitors generally higher than 10pF. Capacitors are rated according to how near their actual values are to the rated capacitance with coloured bands or letters used to indicated the actual tolerance. The most common tolerance for capacitors is 5% or 10% but some electrolytic capacitors are rated as high as 20%.

3. Leakage Current
The dielectric used inside the capacitor is not a perfect insulator resulting in a very small current flowing or "leaking" through the dielectric when applied to a constant supply voltage. This small current flow in the region of micro amps (μA) is called the Leakage Current. This leakage current is a result of electrons physically making their way through the dielectric medium, around its edges or across the leads. The "leakage current" of a capacitor is sometimes called the "insulation resistance" and can be found using Ohm's law.

4. Working Temperature, (T)
Changes in temperature around the capacitor affect the value of the capacitance because of changes in the dielectric. If the air or surrounding temperature becomes to hot or to cold the capacitance value of the capacitor may change so much as to affect the correct operation of the circuit. The normal working range for most capacitors is -30°C to +125°C with nominal voltage ratings given for a working temperature of no more than +70°C. Generally electrolyte's can not be used below about -10°C, as

the electrolyte jelly freezes.
6. Polarization
Polarization generally refers to the Electrolytic type capacitors but mainly the Aluminium Electrolyte's, with regards to their connection. The majority are polarized types, that is the voltage connected to the capacitor terminals must have the correct polarity, i.e. +ve to +ve and -ve to -ve. Incorrect polarization can cause the oxide layer inside the capacitor to break down resulting in very large currents flowing through the device.

The majority of electrolytic capacitors have their -ve terminal clearly marked with a black stripe or black arrows down the side to prevent any incorrect connection. Some electrolyte's have their metal can connected to the negative terminal but high voltage types.

Types of connection in  capacitors.
1. parallel and series connection.

Capacitance and Charge
We saw in the previous tutorials that a Capacitor consists of two parallel conductive plates (usually a metal) which are prevented from touching each other (separated) by an insulating material called the "dielectric". We also saw that when a voltage is applied to these plates an electrical current flows charging up one plate with a positive charge with respect to the supply voltage and the other plate with an equal and opposite negative charge. Then, a capacitor has the ability of being able to store an electrical charge Q (units in Coulombs) of electrons.

When a capacitor is charged there is a potential difference between its plates, and the larger the area of the plates and/or the smaller the distance between them (known as separation) the greater will be the charge that the capacitor can hold. The Capacitors ability to store this electrical charge (Q) between its plates is proportional to the applied voltage, V for a capacitor of known capacitance in Farads, capacitance C is always positive. The greater the applied voltage the greater will be the charge on the plates. Likewise, the smaller the applied voltage the smaller the charge. Therefore, the actual charge Q on the plates of the capacitor can be calculated as:

Capacitor Charge

where A is the area of the plates in square metres, d is the distance between them and ε (epsilon) is the value of the dielectric constant.

Parallel Plate Capacitor

The capacitance of a parallel plate capacitor is proportional to the area A and inversely proportional to the distance, d between the plates. The capacitance can be increased by inserting a dielectric which has a relative permittivity or dielectric constant greater than that of air with typical values of epsilon ε being: Air = 1, Paper = 2.5, Glass = 5, Mica = 7 etc.

Charging & Discharging a Capacitor
Consider the following circuit.

Assume that the capacitor is fully discharged and the switch connected to the capacitor has just been moved to position A. The voltage across the 100uf capacitor is zero at this point and a charging current i begins to flow charging up the capacitor until the voltage across the plates is equal to the 12v supply voltage. The charging current stops flowing and the capacitor is said to be "fully-charged".
Then, Vc = Vs = 12v. Once the capacitor is "fully-charged" in theory it will maintain its state of voltage charge even when the supply voltage has been disconnected as they act as a sort of temporary storage device.

However, while this may be true of an "ideal" capacitor, a real capacitor will slowly discharge itself over a long period of time due to the internal leakage currents flowing through the dielectric. This is an important point to remember as large value capacitors connected across high voltage supplies can still maintain a significant amount of charge even when the supply voltage is switched OFF.
If the switch was disconnected at this point, the capacitor would maintain its charge indefinitely, but due to internal leakage currents flowing across its dielectric the capacitor would very slowly begin to discharge itself as the electrons passed through the dielectric.

The time taken for the capacitor to discharge down to 37% of its supply voltage is known as its Time Constant.  If the switch is now moved from position A to position B, the fully charged capacitor would start to discharge through the lamp now connected across it, illuminating the lamp until the capacitor was fully discharged as the element of the lamp has a resistive value. The brightness of the lamp and the duration of illumination would ultimately depend upon the capacitance value of the capacitor and the resistance of the lamp (t = CxR). The larger the value of the capacitor the brighter and longer will be the illumination of the lamp as it could store more charge.

Example No1.
Calculate the then the charge on the capacitor is 1.2 millicoulombs.


The Farad
We now know that the ability of a capacitor to store a charge gives it its capacitance value C, which has the unit of the Farad, F. But the farad is a extremely large unit on its own making it impractical to use so submultiples or fractions of the standard Farad unit are used instead. The prefixes used in charge in the above capacitor circuit.

electronic formulas and component values are the micro (μ), nano (n) and the pico (p). For example;
Sub-units of the Farad
Convert the following capacitances a) 22nF to uF, b) 0.2uF to nF, c) 550pF to uF a)  22nF = 0.022uF
b)  0.2uF = 200nF
c)  550pF = 0.00055uF
Energy When a capacitor charges up from the power supply connected to it, energy in Joules is stored in the capacitor and is given by the formula;

REFERENCES

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Terrell Croft "Library of Practical Electricity"(1917)

 J. Ho, T. R. Jow, S. Boggs, Historical Introduction to Capacitor Technology

 Adam Marcus Namisnyk (23 June 2003). "A Survey of Electrochemical Supercapacitor Technology" (PDF). Retrieved 2011-06-24.

 WIMA, Characteristics of Metallized Film Capacitors in Comparison with Other Dielectrics [1]

Film Capacitors, TDK Epcos, General technical information

 AVX, Dielectric Comparison Chart

 Holystone, Capacitor Dielectric Comparison, Technical Note 3

  P. Bettacchi, D. Montanari, D. Zanarini, D. Orioli, G. Rondelli, A. Sanua, KEMET Electronics Power Film Capacitors for Industrial Applications

 S. P. Murarka, Moshe Eizenberg, A. K. Sinha (2003) (in German), Interlayer dielectrics for semiconductor technologies, Academic Press, pp. 338–339, ISBN 9780125112215

  General technical information of (RFI/EMI)Noise suppression capacitors on AC mains