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BRIEF DESCRIPTION OF COMPONENTS:
1) Transformer:-
A transformer is a static device which consists of two or more stationary circuits interlinked by a common magnetic circuit for the purpose of transferring electrical energy between them.
• E.M.F equation of a transformer:
e=4.44Baft
B=maximum flux density in the magnetic circuit.
a=area of cross-section of the core.
f and t are frequency and no. of turns respectively.
• Step up and step down transformer:
A step up transformer is one whose secondary voltage is greater than its primary voltage. Step down transformer is opposite to the above. A transformer is made from two or more coils of insulated wires wound around a core made of iron. When the voltage is applied to one coil, it magnetizes the iron core. This induces a voltage to other coil. Turns ratio of two sets of windings determines the amount of voltage transformation. (e1/e2=n1/n2).with a step up or step down transformer the voltage ratio between primary and secondary will mirror the turns ratio. Insulation is provided between the turns of wire to prevent the shorting.
• Comparison of three phase and single phase:
Three phase transformer takes less space than single phase.3 phase is lighter, smaller cheaper and it is more efficient. Three separate single phase transformer are suitably connected to form a three phase transformer bank. The advantage is that when one single phase transformer is damaged, the remaining two units may be used in open delta at reduced capacity.
• Construction:
Three single phase transformer are positioned at 120 degree to each other. If balanced sinusoidal voltage is applied to the three phase winding, the flux will be sinusoidal and balanced. If the three legs are carrying these flux are merged, total flux in the merged leg is zero. Each legs carries both low and high voltage winding.
 Three phase transformer group:
 Group 1: no phase displacement (yy0, dd0, dz0)
 Group 2: 180 degree phase displacement (yy6, dz6, dd6)
 Group 3: 30 degree lag phase displacement (dy1, yd 1, yz1)
 Group 4: 30 degree lead phase displacement (dy11, yd 11,
yz 11)
The zigzag connection is one of the examples of sectionalized winding and its effect is to reduce the third harmonic from line to neural voltage as well as line to line voltage
• Centre tapped transformer:
A Centre tapped is a connection made to a point half way along the winding of a transformer or inductor. Taps are sometime used in the inductor to coupling the signal.it may not necessarily at the half way point, but rather closer to one end.in a rectifier Centre tapped transformer and two diodes can form a full wave rectifier that allows the both half cycle of the ac waveform to contribute to direct current, making at smoother than a half wave rectifier.in a audio power amplifier a centre tapped transformer is used to drive the push pull output stage.
Here in our project we used 230v, 12-0-12,2A step down transformer.
Fig 1:
A typical picture of a Centre tapped transformer:
2) Diodes:-
A diode is a two terminal electronic component that conduct electric current in only one direction. A crystalline piece of semiconductor material is connected to two electrical terminals.
The most common function of a diode is to allow the electric current to pass in only one direction. (This is referred to be as forward bias function. When “terminal is positive with respect to “n” terminal).and it blocks the current in opposite direction. (When “p” is negative with respect to “n” and it is referred to be as reversed bias).
This unidirectional behavior is called rectification. It is used to convert alternating current to direct current and exact modulation from radio signal in radio receiver.
Diode can have more complicated behavior than a simple on-off switch. This is due to their complex non-linear electrical characteristics which can be tailored by varying the construction of p-n junction.
Here in our project we use diode 5408 and diode 4001.
Fig 2: AN IN 5408 DIODE
3) Zener diode:-
A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property.
A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor
Fig 3:-Zener diode showed with typical packages. Reverse current − iZ is shown
material and the doping concentrations.
A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V even if reverse bias voltage applied across it is more than its Zener voltage. The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications.
The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of doping on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient. In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6V diode is the component of choice in temperature-critical applications. Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.
All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode". A TYPICAL PICTURE OF A ZENER DIODE
Fig 4:- Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the change of voltage scale between the forward biased (positive) direction and the reverse biased (negative) direction.
• APPLICATION OF THE ZENER DIODE:-
Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.
Fig 5:- This circuit shows a typical voltage reference or regulator.
A typical voltage input UIN, is regulated down to a stable output voltage UOUT. The intrinsic voltage drop of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, Resistor R is used to limit current through the circuit.
In the case of this simple reference, the current flowing in the diode is determined using Ohms law and the known voltage drop across the resistor R.
IDiode = (UIN - UOUT) / R
• Shunt regulators are simple, but the requirements that the ballast resistor be small enough to avoid excessive voltage drop during worst-case operation (low input voltage concurrent with high load current) tends to leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller loads.
Zener diodes in this configuration are often used as stable references for more advanced voltage regulator circuits.
• These devices are also encountered, typically in series with a base-emitter junction, in transistor stages where selective choice of a device centered on the avalanche/Zener point can be used to introduce compensating temperature co-efficient balancing of the transistor PN junction. An example of this kind of use would be a DC error amplifier used in a regulated power supply circuit feedback loop system.
• Zener diodes are also used in surge protectors to limit transient voltage spikes.
• Another notable application of the zener diode is the use of noise caused by its avalanche breakdown in a random that never repeats.