Capacitor is a two terminal passive electrical device that is formed with two conductive plates separated by any dielectric material. A capacitor can store electric charges by electrostatic force in itself. Capacitance is the property of a capacitor that means ability to store electric charges in it.
In PCBs and electronic schematic diagrams we can find various symbols of capacitors. Schematic symbols may be different according to its type. Some symbols of capacitors shown below:
Unit of Capacitance:
In S. I. unit measuring unit of a capacitance is ‘Farad’ and it is presented by english letter F, For example 20F, 50F, 100F etc. But Farad is a big unit of capacitance. It is not compatible to calculate in electronic circuits, because most of the capacitors used in electronic circuits are in microfarad range. So microfarad is the usual unit of capacitance which is represented by µF.
If we apply 1 volt potential difference across to the terminals of a capacitor and it can store 1 coulomb of electric charge in itself then we can say the capacitance of the capacitor is 1 Farad.
Polarity and Connecting Technique:
In PCBs where we find a symbol of capacitor, we can connect a capacitor there. A non-polar capacitor has no polarity between its terminals so it can be connected in any dirrection with circuit. Incase of polar capacitor such as electrolytic or tantalum it is necessary to identify correct polarity of capacitor and connect in correct direction with PCBs otherwise it may burst or damage during operation.
Identifying Polarity: Incase of electrolytic capacitor a negative sign is marked on its body near to negative terminal. The negative terminal is shorter than the positive terminel also. Incase of tantalum capacitor a positive sign is marked on its body near to positive terminal. All these cases may clear from the figure below:
Connecting With Correct Polarity: In some PCBs we can find some symbols of circle which half is filled of color. This symbol represents that a capacitor have to be connected here and the color filled side is considered as negative terminal. Other type of symbol also shown in figure below. Here one plate of capacitor’s symbol is like curve, this curvature terminal considered as negative terminal.
Measuring the Value of Capacitance:
Manufacturing companies indicate the numeric value on capacitors’ body, for example 22µF, 47µF etc. Incase of small ceramic or mylar capacitor the capacitance value may be indicated by EIA code. We can measure capacitance by using a digital multimeter also. Place the selector knob to capacitor sign then connect two probes of multimeter with capacitor’s terminal, capacitance value will show on multimemer.
Voltage Rating of a Capacitor:
Voltage rating of a capacitor means that the maximum amount of voltage that can be applied across to capacitor’s terminals so that the capacitor can work with safe and long life. If we apply voltage more than voltage rating the capacitor may damage or reduce its working life. Every type of capacitor has its own voltage rating. Capacitor manufacturing companies indicate voltage ratings of capacitor on its body. When we see a electrolytic capacitor, we can find some value written on it, for example 22µF, 25 Volt. Here 22µF is the capacitance and 25 Volt is the voltage rating of capacitor.
Identifying a Faulty Capacitor:
Sometime we able to find some faulty capacitors among new capacitors which are bought from market just now and never used in any circuit. So it is necessary to identify the fault of capacitor before using it in any circuit, otherwise the circuit will malfunction in operation. Most common faults are short circuit and open circuit fault. All these faults can be tested by using a analog ohm meter easily.
Property of good capacitor: When we connect a ohm meter between two terminals of a capacitor, it shows zero ohm resistance initially because a charging current flows through the meter. After this the resistance will increase slowly and finally it will be infinite ohm because the charging current decreased slowly to zero. These phenomenon is considered as property of a good capacitor.
Short Circuit or Constant Resistance Fault: Incase of short circuit fault a capacitor shows constant zero ohmic resistance between its two terminals. Capacitors which show some amount of constant resistance are called constant resistance faulty capacitors. So we should avoid all these faulty capacitors to use.
Open Circuit Fault: Incase of open circuit fault a capacitor shows constant infinite ohmic resistance between its two terminals. So we should avoid it to use also.
EIA Coding System of Capacitors:
Electronic Industries Alliance (EIA) makes a standard coding system to represent a capacitance value which is described below:
From the figure we can see a number 474 is printed on capacitor. According to EIA coding system first two digits are considered as first two digits of Pico Farad value. Third digit is considered as power of 10 which will be multiplied with the numerical value of first two digits. Finally the result of multiplication will be capacitance value in Pico Farad unit.
47 x 104 pF
= 470000 pF
= 0.47 µF
Most common uses of capacitor are filter, transient limiter, coupling, power factor corrector etc. Following examples can be considered:
A two stage amplifier is shown in above figure. Two separate stages is coupled with a capacitor C3. Here Capacitor C3 is used as coupling capacitor.
Many kind of filters can be built with capacitors. A most common and simple filter is rectifier filter. It is a low-pass shunt capacitance type filter which is shown in figure below:
In the above figure the capacitor C acts as filter capacitor. Ripple of output wave from rectifier is suppressed by the filter capacitor.
Transient is a phenomena for sudden changes of either current or voltage in electronic circuitry during operation. A capacitor can act as transient limiter. A shunt capacitance or series inductance or both together may used to limit the transient.
Power Factor Corrector:
By using correct amount of switched capacitors in the power distribution line, the Power Factor can be corrected and the value becomes nearer to 1 to minimize the wasted energy to save money!!!