What is Capacitor?
A capacitor is an electronic component characterized by its capacity to
store an electric charge. A capacitor is a passive electrical component
that can store energy in the electric field between a pair of conductors
(called “plates”).
In simple words, we can say that a capacitor is a device used to store
and release electricity, usually as the result of a chemical action. Also
referred to as a storage cell, a secondary cell, a condenser or an
accumulator. A Leyden Jar was an early example of a capacitor.
Capacitors are another element used to control the flow of charge in a
circuit. The name derives from their capacity to store charge, rather like
a small battery.
Capacitor Symbol and Unit
There are two capacitor symbols generally used in electronics. One
symbol is for polarized capacitors, and the other is for non-polarized
capacitors.
Capacitor Symbol of Polarized and Non-Polarized Capacitors
In the above diagram, the symbol with one curved plate represents a
Polarized Capacitor. The curved plate represents the cathode (negative)
of the capacitor, and the other plate is anode (positive). Sometimes a
plus sign is also added to the positive side.
Types of Capacitors
There are several types of capacitors for different applications and
functions. Following are the Main and Most Common Types:
- Ceramic Capacitors
Thru-Hole and SMD Type Ceramic Capacitor
These are non-polarized capacitors made out of two or more alternating
layers of ceramic and metal. The ceramic acts as the dielectric and the
metal acts as the electrodes.
Ceramic Capacitors are also called “Disc Capacitors.”
A code of 3 Digit is generally printed on the body of this type of
capacitors to tell their capacitance in pico-farads. The first two digits
represent the value of the capacitor and the third digit represents the
number of zeros to be added.
- Electrolytic Capacitor
Thru-Hole and SMD Type Electrolytic Capacitor
These type of capacitors are generally used where large capacitance is
needed. Anode of electrolytic capacitors is made of metal and is
covered with an oxidized layer used as dielectric. The other electrode
can be either wet non-solid or solid electrolyte.
Electrolytic capacitors are polarized. This means that correct polarity
must be used when supplying DC voltage to it. In simple words positive
lead of the capacitor must be connected with positive terminal and
negative plead to the negative terminal. Not doing so will damage the
capacitor.
These capacitors are grouped into following 3 Types depending on their
dielectric:
Aluminum electrolytic capacitors.
Tantalum electrolytic capacitors.
Niobium electrolytic capacitors.
- Film Capacitor
Thru-Hole and SMD Type Film Capacitor
These are most common type of capacitor used in electronics.
Film capacitors or plastic film capacitors are non-polarized. Here an
insulating plastic film acts as the dielectric. Electrodes of these types of
capacitors can be aluminum metal or zinc reactive metal. They are
applied on one or both sides of the plastic film thus forming a metallized
film capacitor. Sometimes a separate metallic foil is used over the film
thus forming a film or foil capacitor.
Film capacitors are available in different shapes and sizes and offer
several advantage over paper type capacitors. They are highly reliable,
have long life and have less tolerances. They also function well in high
temperature environment.
- Variable Capacitor
Thru-Hole and SMD Type Variable Capacitor
These are non-polarized variable capacitance type of capacitors. They
have moving and fixed plates to determine the capacitance. They are
generally used in Transmitters and Receivers, Transistor Radios etc.
These capacitors are grouped as:
Tuning capacitors; and
Trimmer capacitors
How Capacitor Works?
You can imagine a capacitor as two large metal plates separated by air,
although in reality they usually consist of thin metal foils or films
separated by plastic film or another solid insulator, and rolled up in a
compact package. Consider connecting a capacitor across a battery.
A simple capacitor connected to a battery through a resistor
As soon as the connection is made, charge flows from the battery
terminals, along the wire and onto the plates, positive charge on one
plate, negative charge on the other.
Why? The like-sign charges on each terminal want to get away from
each other. In addition to that repulsion, there is an attraction to the
opposite-sign charge on the other nearby plate. Initially the current is
large, because in a sense the charges can not tell immediately that the
wire does not really go anywhere, that there is no complete circuit of
wire.
The initial current is limited by the resistance of the wires, or perhaps by
a real resistor. But as charge builds up on the plates, charge repulsion
resists the flow of more charge and the current is reduced. Eventually,
the repulsive force from charge on the plate is strong enough to balance
the force from charge on the battery terminal, and all current stops.
The existence of the separated charges on the plates means there must
be a voltage between the plates, and this voltage be equal to the battery
voltage when all current stops. After all, since the points are connected
by conductors, they should have the same voltage; even if there is a
resistor in the circuit, there is no voltage across the resistor if the current
is zero, according to Ohm’s law.
The amount of charge that collects on the plates to produce the voltage
is a measure of the value of the capacitor, its capacitance, measured in
farads (f). The relationship is C = Q/V , where Q is the charge in
Coulombs.
Large capacitors have plates with a large area to hold lots of charge,
separated by a small distance, which implies a small voltage. A one
farad capacitor is extremely large, and generally we deal with
microfarads ( μf ), one millionth of a farad, or picofarads (pf), one
trillionth (10-12) of a farad.
Consider the above circuit again. Suppose we cut the wires after all
current has stopped flowing. The charge on the plates is now trapped,
so there is still a voltage between the terminal wires. The charged
capacitor looks somewhat like a battery now.
If we connected a resistor across it, current would flow as the positive
and negative charges raced to neutralize each other. Unlike a battery,
there is no mechanism to replace the charge on the plates removed by
the current, so the voltage drops, the current drops, and finally there is
no net charge left and no voltage differences anywhere in the circuit.
The behavior in time of the current, the charge on the plates, and the
voltage looks just like the graph above. This curve is an exponential
function: exp(-t/RC) . The voltage, current, and charge fall to about 37%
of their starting values in a time of R ×C seconds, which is called the
characteristic time or the time constant of the circuit.
The RC time constant is a measure of how fast the circuit can respond
to changes in conditions, such as attaching the battery across the
uncharged capacitors or attaching a resistor across the charged
capacitor. The voltage across a capacitor cannot change immediately; it
takes time for the charge to flow, especially if a large resistor is
opposing that flow. Thus, capacitors are used in a circuit to damp out
rapid changes of voltage.