Capacitor is another important passive component commonly observed in electrical and electronic circuits. Some times it is also called a condenser. Basically it is used to store electrical energy (by charging) and to supply this stored energy when required (by discharging). This component has property of staring charge across its two plates; hence it is also named as capacitor. They are available in fixed and variable types; their capacity is measured in unit "Farad". Typically they are available in micro Farads.
The capacitor utilization is very high in any analog circuit.
Characteristics of a Capacitor:
lt can store an electrical charge.
lt will not permit any voltage to exist on its terminals until same of the voltage charge is rnoved from one of its plates to anather plate.
lt consist of two parallel plates, insulating material called dielectric separates the plates. Each plate has an equal quantity of positive and negative changes when it is neutral. When the voltage source is connected to the plates of the capacitor, the charge is redistributed. The negative charges of the upper plate are attracted to the positive electrodes of the power source. The positive charges on the lowest plate are attracted towards the negative electrodes of the power supply. The result of this action is a device containing a positively charged plate and a negatively charged plate.
The charges on the plates of a capacitor develop an energy field in the dielectric material and are known as electric field. It consists of lines of force between the charged plates of capacitor. The lines of force are concentrated in the dielectric material; they therefore store energy in the dielectric material.
RELATION BETWEEN C, Q AND V
C = Q/V
C = Capacitance of capacitor in Farad
O = Charge on plate of capacitor in Coulombs
V = Voltage across the capacitor in Volts.
UNIT OF CAPACITANCE
The basic unit of capacitance is Farad. The application of the capacitor in electrical and eleetronic devices usually requires a unit with value much less than 1 Farad. Typical units of capacitance are the microfarad (10-6 F) and picofarad (1 0-12F)
FAILURE IN CAPACITORS
1. Open circuit problems
2. Short cireuit problems
• Due to dielectric breakdown
• Environmental shock
• Mechanical shock
• Thermal shock
• Misuse-poor assembly
• Prolonged starage
• Manufacturing imperfection
• Impurities in eleetrolytic
TESTING OF CAPACITORS
Electrolytic and high value eapaeitors can be eheeked for open eireuit, short eireuit and leaky behaviar by means of an ohmmeter operated on the highest seale.
If ok the capacitor will slowly charge to show a high value of resistance. In cas e of open circuit the charging action would be absent.
Troubles in Capacitors
Capacitor can become open or short-circuited. In either case, this capacitor is useless because it cannot store charge leaky capacitor is equivalent to a partial short circuit where the dielectric gradually loses its insulating properties under the stress of applied voltage, lowering its resistanee. A good capacitor has very high resistance of the order of mega ohms. A short-circuited capacitor has zero ohm resistance, or continuity. The resistance of a leaky capacitor is lower than normal.
In addition to the possibility of an open connection in any type of capaeitor, eleetrolytic develop high resistance in the electrolyte with age, particularly at high temperatures. After service of a few years, if the electrolyte dries up, the capacitor will be partially open. Much of the capacitor action is gone, and the capacitor should be replaced.
A leaky capacitor reads R less than normal with an ohmmeter. However, D.C voltage tests are more definite. In a circuit the D.C. voltage at one terminal of the capacitor should not affect the dc voltage at the other terminal.
Except for electrolytic, capacitors do not deteriorate with age while stored, since there is no applied voltage. Electrolytic capacitors, like the dry cells should be used fresh from manufacture. The reason is the wet electrolyte.
A capacitor offers low impedance to AC, but very high impedance to DC. Hence capacitors are used when we want to couple alternating voltage from one circuit to another, while at the same time blocking the DC voltage from reaching the next part of the circuit. The capacitor is also used as a bypass capacitor, where it bypasses the AC through it without letting the AC signal to go across the circuit. Capacitors are also used in tuned circuits in series or parallel with an inductor.
Electrolytic capacitors are used in circuits that have a combination of DC voltage and AC voltage. The DC voltage maintains the required polarity across the electrolytic capacitor. A common application is Electrolytic fılter capacitors eliminate 60 Hz AC ripples in a DC power supply. Another use is for audio coupling capacitors in transistor amplifiers.