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This chapter is a work in progress and not
yet finished
Capacitor
A capacitor is a component that can store electric energy in
the form of charges of opposite polarity on two metal plates
insulated from each other with a dielectric material.
 The schematic symbol
for a capacitor reflects this, two parallel lines represents the
plates, the void between them is the insulating dielectric
material. The two other lines, perpendicular to the parallel lines,
represents
the two terminals of the capacitor that connects each plate to
the electric circuit. Capacitance is written with the letter C
and and is measured in farad (F). One farad is a very high
capacitance so capacitors normally comes in
µF (micro farad), nF (nano farad)
or pF (pico farad). If a capacitor, with number 5 for example,
in a circuit has
the value 250 nano farad it can be written as C5=250nF. In the
schematic symbol there is a '+' sign adjacent to one of the
plates. This is normally only done when the capacitor is
polarized which is a sort of capacitors where one of the plates always has to be
at a positive potential with regard to the other. More of this
later.
The inner workings of a
capacitor
So how does a capacitor work? We
don't really need to know this in order to use a capacitor. We
only need to know how it reacts and affects electric voltage and
current in a circuit. However, since it is a very common
misconception that a capacitor can hold a charge when it is said
to be charged I am going to explain the physics behind the
capacitor a bit more closely and the mechanisms that stores
energy inside the capacitor. This is not strictly necessary to
know and really outside the scope of this article so skip to the
next paragraph if you just want to know how to use a capacitor.
To understand the inner workings
of a capacitor we need to know
that molecules can have a charge. This charge can be either
positive or negative. A positively charged molecule has more
protons than electrons and a negatively charged molecule has
more electrons than protons. A charged molecule also has an
electric field around it. This field can physically affect other
charged molecules. Two molecules with opposite polarity attracts
each other and two molecules with the same polarity repels each
other. So when we apply a voltage across a capacitor, one plate
gets a surplus of electrons in its metal molecules and the other
plate gets a deficit of electrons. That is, for every electron
that is missing on one plate, there is an extra electron on the
other plate which makes the net charge of the capacitor zero. It
has no net charge, never. Neither a discharged nor a charged
capacitor. You can also think of it as when an extra electron is
attached to one plate another electron has to be taken away from the
other plate, and the electron is actually getting forced away
from the plate by the field of the first electron and the
voltage potential difference applied to the plates in the
electric circuit and it can
only do so if the second plate is connected to the same electric
circuit as the other plate.
There is a limit to how many
electrons that can be displaced on the plates in this way and
the limiting factors are the area of the plates, the distance
between the plates and the voltage across the plates. The
displaced electrons on the plates creates an electric field
which in turn creates a voltage difference between the plates.
The more displaced electrons, the greater the field and the
higher the voltage difference. The voltage created by the field between the plates is
reversed compared to the voltage applied to the capacitor in the electric
circuit. The plates can't take in more electrons on one plate
and push out more electrons on the other plate when the voltage
across the plates due to the field from the displaced electrons,
has reached the same value as the voltage that is applied
between the plates in the electric circuit. And since the
internal field is increased when more electrons are displaced it
gets harder and harder to
displace even more. When the voltage from the field in the dielectric
material between the plates has reached the same level as the
applied voltage, no more electrons can be moved which means that
the current stops. That is, the capacitor is blocking the DC
current and current is only flowing during charging or
discharging. We can also see that it takes a certain amount of work
to charge the capacitor since there is a voltage and a flowing
current during the charging. This work is stored as the field in
the dielectric material between the plates in the capacitor and
can be converted back to a current in the electric circuit, sort
of as a battery. It is actually the field between the plates that
stores the energy in a charged capacitor.
To recap things - When current is flowing in to one terminal
of a capacitor, the same amount of current must also flow out of the
other terminal. Current can only flow in to one terminal if the
other terminal is connected to the same electric circuit and
there is a voltage difference between the terminals. A capacitor
blocks DC current when it is fully charged. A capacitor can
store a finite amount of energy when a DC voltage is applied to
it. This energy can be converted back to a DC current.
Hydraulic comparison
In comparison with the closed water pipe system, a capacitor
can be seen as a container with two compartments separated from each other
by a water tight, elastic wall or diaphragm. With water pressure,
energy can be stored as tension in the diaphragm. We can also
see that the diaphragm can only be flexed to a certain limit in
both ways. Water pressure on one end affects the pressure on the
other end as long as the diaphragm isn't flexed to it's maximum.
It is also easy to imagine how an AC current can pass through
with the help of the diaphragm and how a DC current is blocked
by the diaphragm.
Impedance
From above we saw that a capacitor passes AC current but
blocks DC current. This is not just an on/off function regarding
AC and DC current but the capacitor actually has a resistance
and this resistance is inversely proportional to the
frequency. When we are dealing with a resistance that is
changing with frequency, we call it impedance instead of
resistance. Impedance is still measured in ohms but is written
with the letter Z instead of R as for a resistor. The impedance
of an ideal capacitor that is subjected to a sinusoidal current
with the frequency F can be calculated with the formula:
Z=1/(2*pi*F). From this we can see that a frequency of 0Hz gives
an infinite impedance.
Impedance is a bit more complicated than this simplification.
It actually consists of one real part (resistance) and one
imaginary part (called reactance). If the imaginary part is
different from zero, the AC current and voltage will be out of
phase. The reactance for a capacitor makes the current up to 90
degrees (pi/2) out of phase with the voltage where the current
is leading. But this is out of the scope of this article.
Serial connection
Just as with resistors, capacitors can be connected in series
and in parallel. But in contrary to resistors, the total
capacitance becomes smaller when capacitors are connected in
series. The impedance still gets larger since it is calculated
with the inverse value of the capacitor.
Just as with resistors, the AC current through all capacitors
is the same and the voltage over each capacitor is the total
voltage over all capacitors, divided between the capacitors
proportionally with respect to their value. Remember that DC
current is blocked. Capacitors are normally only connected in
series in certain circuits together with other components.
Examples of such
circuits are filters and voltage multipliers. More
on this below. Serially connected capacitors can also be used as
voltage dividers for AC currents, perhaps mostly for higher
voltages. Another use to connect capacitors in series is to make
a capacitor for very high voltage. 3 capacitors with a voltage
rating of say 600V can be seen as one capacitor with a voltage
rating of 1800V when they are connected in series. The combined
capacitance only becomes 1/3 of a single capacitor, though.
Parallel connection
When capacitors are connected in parallel, the total
capacitance equals the sum of all the individual capacitors.
This is easy to imagine since we can think of it as the surface
of the plates are increased.
Parallel connecting of capacitors is a very common way to
achieve a higher capacitive value from lower values capacitors.
Perhaps mostly common in power supply circuits, where there can
be a need for very high capacitances. For high frequency
designs, such as switch mode power supplies, there can be one
more reason than just the increased capacitance. Real capacitors
are not ideal but they also have a small internal series
resistance. This is called ESR which stands for Equivalent
Series Resistance. The ESR is increased at higher frequencies
and it is also higher for larger capacitors. This resistance is
causing losses at high frequencies and large currents, such as
is present in a switch mode power supply. To lower these losses,
a design engineer might choose to use several lower value
capacitors in parallel instead of a single large capacitor. The
lower ESR for a lower value capacitor in combination with an
effective parallel coupling of these capacitors can reduce the
effects of ESR at higher frequencies a lot.
More to come...
Start,
Tools and equipment,
Voltage,
Current,
Resistor,
Power, Capacitor,
Inductor,
Diode,
LED,
Transistor,
OP-Amp,
Linear Integrated Circuits,
Digital Integrated Circuits,
Microprocessor,
Relay,
Thyristor,
Transformer
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