9 | capacitor
What is capacitor?
A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals. A capacitor is a passive two-terminal electrical component used to store energy elector-statically in an electric field. The effect of a capacitor is known as capacitance.
A capacitor is a device that is used to store charges in an electrical circuit. Capacitor originally known as a condenser.
A capacitor works on the principle that the capacitance of a conductor increases appreciably when an earthed conductor is brought near it.
Figure 1
capacitor symbols
Capacitor Principles
A capacitor has two plates separated by a distance having equal and opposite charges. Construction of capacitor consisting of two conductors in close proximity and insulated from each other.
The two plates are known to as +plate and -plate.
Figure 2 construction
Characteristic
If positive charges with total charge +Q are deposited on one of the conductors and an equal amount of negative charge −Q is deposited on the second conductor, the capacitor is said to have a charge Q.
The main function of capacitors is to store electrostatic energy in an electric field, and give this energy to the circuit, when necessary. They allow the AC to pass but block the flow of DC to avoid a hazardous breakdown of the circuit.
Figure 3 principle of capacitor charging
Formula Capacitance
Charge (Q) stored in a capacitor is the product of its capacitance (C) and the voltage (V) applied to it. The capacitance of a capacitor should always be a constant, known value.
The capacitance is the ratio of electric charge (Q) to the voltage (V) and the mathematical expansion is following.
C = Q ÷ V
Where,
Q is the electric charge in coulombs
C is the capacitance in farad
V is the voltage between the plates in volts
The capacitance is determine by three factors.
effective area – capacitance is directly proportional to the effective area if the plates. The effective area is improved by increasing the number of plates or the size of plates.
distance between plates – as the spacing increases, the effect that one plate has upon the other is reduced and the closer the plates the greater the capacitance.
dielectric material – the dielectric constant signifies the degree to which the capacity of a capacitor can be increased by replacing the air between plates.
Figure 4 capacitor basic
Types of Capacitors
The different types of capacitors are following.
Electrolytic Capacitor
Mica Capacitor
Paper Capacitor
Film Capacitor
Non-Polarized Capacitor
Ceramic Capacitor
The primary difference between film capacitors and other forms of capacitors is their dielectric properties. These include polycarbonate, polypropylene, polyester (Mylar), polystyrene, Teflon, and metalized paper. Regarding capacitance range, film type capacitors are available in ranges starting from 5pF to 100uF.
A non-polarized ("non polar") capacitor is a type of capacitor that has no implicit polarity. Electrostatic capacitors are non-polarized type, they can be connected in any direction of polarity, and there is no difference.
A polarized ("polar") capacitor is a type of capacitor that have implicit polarity (it can only be connected one way in a circuit). Polarised capacitor is one that must be run with the voltage across it in a certain polarity. They can only be connected with fixed terminal polarities. Positive and negative terminals are marked.
Capacitor applications
Capacitors have many important applications. They are used, for example, in digital circuits so that information stored in large computer memories is not lost during a momentary electric power failure; the electric energy stored in such capacitors maintains the information during the temporary loss of power.
Capacitors play an even more important role as filters to divert spurious electric signals and thereby prevent damage to sensitive components and circuits caused by electric surges.
Capacitor in Series .
Let us connect n number of capacitors in series. V volt is applied across this series combination of capacitors.
Formula: Total capacitance,
Ct =((1÷c1)+(1÷c2)+(1÷c3))
Now, if Q coulomb be the charge transferred from the source through these capacitors.
Total voltage , Vt = Q ÷ C
Individual capacitor voltage ,
Vc1 = Q ÷ C1
Vc2 = Q ÷ C2
Vc3 = Q ÷ C3
Total voltage , Vt = Vc1 + Vc2 + Vc3
Capacitor in Parallel
A capacitor is designed to store the energy in the form of its electric field, i.e. electrostatic energy. Whenever there is a necessity to increase more electrostatic energy storing capacity, a suitable capacitor of increased capacitance is required. A capacitor is made up of two metal plates connected in parallel and separated by a dielectric medium like glass, mica, ceramics etc.
The dielectric provides a non-conducting medium between the plates and has a unique ability to hold the charge, and the ability of the capacitor to store charge is defined as the capacitance of the capacitor.
Formula: Total capacitance,
Ct = C1 + C2 + C3
The total amount of charge (q) accumulated is directly proportional to the voltage source (V). When a voltage source is connected across the plates of the capacitor a positive charge on one plate, and negative charge on the other plate get deposited.
Vt = Vc1 = Vc2 = Vc3
Capacitor in DC
When the capacitor is connected to the DC voltage source, initially the positive terminal of the DC supply pulls the electrons from one terminal and pushes the electrons to the second terminal. When the capacitor is connected to the DC voltage source, initially the positive terminal of the DC supply pulls the electrons from one terminal and pushes the electrons to the second terminal.
Capacitor in AC
An electrical capacitor is an electrical device that stores up electricity or electrical energy and improves an AC circuit's power factor.
A charging current will flow into the capacitor opposing any changes to the voltage, at a rate equal to the rate of change of electrical charge on the plates. The larger the capacitance, the more charge has to flow to build up a particular voltage on the plates, and the higher the current will be. The higher the frequency of the voltage, the shorter the time available to change the voltage, so the larger the current has to be. The current, then, increases as the capacitance and the frequency increase.
Sine wave when capacitor in AC voltage is leading over current by 90° shift when looking at the sine wave through a oscilloscope.
Figure capacitor connected to AC supply.
Figure voltage leading current in phase shifted by 90 °.
Capacitive Reactance (XL)
The formula proves that if either the frequency or capacitance is to be increased, the overall capacitive reactance would decrease. Similar to a perfect conductor, the capacitor reactance would reduce to zero as the frequency approaches infinity.
The unit is in ohm.
Formula XL
Example calculation
1) Given three capacitors are connected in series with given value C1=2, C2=8, C3=10, calculate the total capacitance.
Formula: Ct = (( 1÷C1) + ( 1÷C2) + ( 1÷C3))
Solution: Ct = ((1÷2)+(1÷8)+1÷10) ) = 0.725 farad
2) When three capacitors are connected in parallel with C1=2, C2=4, C3=6, total capacitance is.
Formula: Ct = C1 + C2 + C3
Solution: Ct = 2+4+6 = 12 farad
3) A capacitance value 350 farad, calculate the energy Q, if voltage is 120 volts AC.
Formula: Q = C x V
Solution: Q = 350 x 120 v = 42,000 coulomb
4) Given supply voltage is 230 volts / 50 hertz, a capacitance value is 250 farad.
i. calculate the capacitive reactance (XL)
Formula: Xc = 1 ÷ ( 2 x 3.14 x 50hz x C )
Solution: Xc = 1 ÷ ( 2 x 3.14 x 50 x 250 ) = 1.27 Ω
ii. find the current stored , Ic
Formula: Q = C x V
Solution: Q = 250f x 230v =57,500 coulomb
Ic = Q ÷ V = 57500 ÷ 230v = 250 ampere