A Device to convert
AC it to DC.
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magnetic field in a dynamo then for half cycle of the rotation the induced e.m.f is in one
direction and for the other half it is in the opposite direction. Beside this, the e.m.f is
changing every moment .When the end of such a coil is joined to a circuit then the
current flows for half cycle in one direction and for other half it flows in the opposite
direction. The magnitude of current is also changing every moment. Hence such a
current is called alternating current.
Transformer: - It is such an instrument through which without much loss of electric
energy the e.m.f of an A.C may be increased or decreased. There is a soft-iron
laminated core in it marked A in the fig. and in one arm of which is the primary coil P and
in the opposite arms there is a secondary coil S.
The e.m.f which is to be increased or decreased is connected to the primary (p) coil and
due to this e.m.f current flows in the p coil and magnetic lines of force are produced. This
lines of force passing through the soft iron core reach the secondary. When the
magnitude of the current in the primary changes the line of force also change there.
Consequently the line of force passing through the secondary also changes and due to
this change in S coil e.m.f is induced in it. To use this e.m.f in a certain circuit the end of
the S coil are joined in the circuit. If the loss of energy in the transformer be negligible
then the power supplied to the primary will be equal to the power obtained in the
secondary.
Uses of transformer:-
(1) Step-up and step-down transformers are largely used in the electrical
transmission of A.C.
(2) Audio frequency transformer also called low frequency transformer is infact,
step-down transformer which is used in radio receiver, in radio telephone, in radio
telegraph and in television.
(3) Radio frequency transformer is of the order of mega cycle and it is used in radio
communication at this frequency.
(4) Besides of above transformer constant current transformer and constant voltage
transformer (CVT) are used to obtain constant current and constant voltage respectively.
Advantages and disadvantages of alternating current:-
The greatest advantage of the A.C is that by the help of transformer its e.m.f can be
increased or decreased and the loss of electric energy in this process is negligible. The
e.m.f of the direct current is decreased by using resistance in which much electrical
energy is wasted. The e.m.f of direct current can not be increased in a simple way.
Hence in factories where in order to work large and small machines different e.m.f are
required and for this only alternating current is used. The second advantage of the A.C
is that raising its e.m.f to a large value it may be sent at distant place. In transmitting
electric energy at high voltage and low current there is very small loss of electric energy.
Hence for transmission from one place to other only A.C is used.
(a)Electrolysis can not be done by A.C. Hence in aluminum factories as well as factories where electrolysis is
required, only D.C is used. For this reason electroplating also can not be done by A.C.
Comparison of A.C and D.C:-
(1) Direct current (D.C) flows in one direction and its value is constant where as the value of alternating current
(A.C) changes periodically with time and hence it has positive and negative values.
(2) Electric power can be transferred from one place to other by A.C with less loss of energy and with less
expense.
(3) A.C can be controlled by choke coil with least loss of energy where as D.C is controlled only by ohmic
resistance in which there is maximum loss of energy.
(4) The machines depending on A.C have longer life (as for example electric motor) in comparison to the
machine which depend on D.C.
(5) A.C is more dangerous than D.C because the people feel more shock with A.C.
Diode
A diode is a component that only allows electricity to flow one way. It can be thought as a sort of one way street for
electrons. Because of this characteristic, dioded are used to transform or rectify AC voltage into a DC voltage.
There are three types of diodes (not counting photonic diode types): silicon, germanium and zener. Silicon diodes
are the most common type. They are made of a small chip of silicon and are mostly used as rectifiers in power
supplys, absorb voltage spikes, perform logic, etc. Germanium diodes are made of a small chip of germanium,
usually in a glass case. These handle much less power than silicon diodes and are usually used to demodulate
radio signals, for temperature compensation in a circuit or to perform simple logic. The symbols for these types of
diodes are exactly the same on the schematic, except that germanium diodes will have a little "Ge" nextto the
symbol (Ge is the elemental symbol for germanium). Diodes have two connections, an anode and a cathode. The
cathode is the end on the schematic with the point of the triangle pointing towards a line. In other words, the
triangle points toward that cathode. The anode is, of course, the opposite end. Current flows from the anode to the
cathode. Diodes are rated according to the voltage and current they can handle. Diodes may or may not be drawn
with the circle surrounding them.
Light emitting diodes(LED) differ from regular diodes in that when a voltage is applied, they emit light. This light can
be red (most common), green, yellow, orange, blue (not very common), or infa red. LEDs are used as indicators,
transmitters, etc. Most likely, a LED will never burn out like a regular lamp will and requires many times less current.
Because LEDs act like regular diodes and will form a short if connected between + and -, a current limiting resistor
is used to prevent that very thing. LEDs may or may not be drawn with the circle surrounding them. In our project
LED is used for testing current flow only and not as part of rectifier.
Rectifier
There are two types of rectifier.
- Half wave rectifiers
- Full wave rectifiers
Half wave rectifiers convert the A.C to a pulsating D.C and convert only one half of the sine wave.
Half wave rectifiers convert the A.C to a pulsating D.C and convert all of the sine wave.
Rectifier Circuits
Rectifiers are devices that convert A.C voltage to D.C voltage. These rectifier circuits use the diodes and take
advantage of their property that allows current to flow only in one direction.
positive half cycle of the AC voltage
the diode can conduct allowing current
to pass through resistor. The resistor
is the “load” or the circuit that is to be
supplied with direct current. On the
negative half cycle the diode is reverse
biased and therefore, non conducting.
positive half cycle D2 and D4 conduct.
On the negative half cycle, D1 and D3
conducts. In each case the direction of
current flow through the load R1 is the
same.
We observed that in the half-wave rectifier, we lost half of our signal. To take advantage of the entire signal
we use the full-wave rectifier which is shown in fig. Full Wave Rectifier is combination of two half wave rectifiers.
For full wave rectifier we use center taped transformer.
A full-wave rectifier circuit is a circuit that rectifies the entire cycle of the A.C sine wave. A basic full wave
rectifier uses two diodes. The action of these diodes during each half cycle is shown in fig.
