common collector configuration of a transistor.....

COMMON COLLECTOR CONFIGURATION OF A TRANSISTOR

COMMON COLLECTOR CONNECTION

In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

                                                       Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by 

COMMON COLLECTOR CIRCUIT 

A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input and the output circuits to measure VCE and VCB

INPUT CHARACTERISTICS

                                                Common Collector Input Characteristic Curve

• It  is  a  curve  which  shows the  relationship  between the  base  current,  IB and the collector base voltage VCB at constant VCE This method of determining the characteristic is as follows.

• First, a suitable voltage is applied between the emitter and the collector. Nextthe  input  voltage  VCB  is  increased  in  a  number  of  steps  and  corresponding values of IE are noted.

• The base current is taken on the y-axis, and the input voltage is taken on the x-axis. Fig. shows the family of the input characteristic at different collector- emitter voltages.

• The following points may be noted from the family of characteristic curves.  1.Its  characteristic  is  quite  different  from  those  of  common  base  andcommon emitter circuits.

2.When VCB increases, IB is decreased.

Output Characteristics

• It is a curve which shows the relationship between the emitter current l and collector-emitter voltage, the method of determining the output characteristic is as follows.

• First,  by  adjusting  the  input  a  suitable  current  IB  is  maintained.  Next  VCB increased in a number of steps from zero and corresponding values of IE are  noted.

• The above whole procedure is repeated for different values of IB. The emitter current  is  taken  on  the  Y-axis  and  the  collector-emitter  voltage is  taken  on the X-axis.

• Fig shows the family of output characteristics at different base current values. The following points are noted from the family of characteristic curves.

                                         Common Collector Output Characteristic Curves

1.This  characteristic  is  practically  identical  to  that   of  the  common  emitter circuit.

2.Its current gain characteristic for different values of VCE is also similar to that of a common emitter circuit.
moreE detail .. you can pictourical view of transistor onthis link biploar transistor!!!
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phasor diagram & equivalent ckt of transformer...!!!

phasor diagram for transformer
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Phasor Diagrams & Equivalent Circuit Of a Transformer On-No Load And On Load Condition

The resulting equivalent circuit as shown in Fig. 16 is known as the exact equivalent circuit. This circuit can be used for the analysis of the behaviour of the transformers. As the no-load current is less than 1% of the load current a simplified circuit known as `approximate' equivalent circuit (see Fig. 16(b)) is usually used, which may be further simplified to the one shown in Fig. 16(c).

On similar lines to the ideal transformer the phasor diagram of operation can be drawn for a practical transformer also. The positions of the current and induced emf phasor are not known uniquely if we start from the phasor V1. Hence it is assumed that the phasor  is known. The E1 and E2 phasor are then uniquely known. Now, the magnetizing and loss components of the urrents can be easily represented. Once I0 is known, the drop that takes

place in the primary resistance and series reactance can be obtained which when added to E1 gives uniquely the position of V1 which satisfies all other parameters. This is represented
in Fig. 17(a) as phasor diagram on no-load.


Next we proceed to draw the phasor diagram corresponding to a loaded transformer. The position of the E2 vector is known from the flux phasor. Magnitude of I2 and the load power to be known. But the angle θ2 is defined with respect to the terminal voltage V2 and not E2. By trial and error the position of I2 and V2 are determined. V2 should also satisfy the Kirchoff's equation for the secondary. Rest of the construction of the phasor diagram then becomes routine. The equivalent primary current I02 is added vectorially to I0 to yield I1. I1(r1+jxl1)is added to E1 to yield V1. This is shown in fig. 17(b) as phasor diagram for a loaded transformer.

PMMC

PMMC....!! BY electricalforyouonly....!!

Permanent Magnet Moving Coil Instrument (PMMC)

The permanent magnet moving coil instrument is the most accurate type for D.C. Measurements. The working principle of these instruments is the same as that of the d’Arsonval type of galvanometers, the difference being that a direct reading instrument is provided with a pointer and a scale

(Fig) Permanent magnet moving coil instrument

Construction of PMMC Instruments

•     The constructional features of this instrument are shown in Fig.
•     The moving coil is wound with many turns of enameled or silk covered copper wire.
•     The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings.
•     The coils move freely in the field of a permanent magnet.
•     Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping.
•     Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt.
•     The coil itself, therefore, provides electro magnetic damping.

Magnet Systems

•     Old style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.
•     Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.
•     The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.

Control

•     When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown.
•     This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

Damping

•     Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and Scale

•     The pointer is carried by the spindle and moves over a graduated scale.
•     The pointer is of lightweight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade.

•      This helps to reduce parallax errors in the reading of the scale. When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil.

Torque Equation. 

The torque equation of a moving coil instrument is given by

As the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.

 Errors in PMMC Instruments

The main sources of errors in moving coil instruments are due to

•     Weakening of permanent magnets due to ageing at temperature effects.
•     Weakening of springs due to ageing and temperature effects.
•     Change of resistance of the moving coil with temperature.

krichoff voltage law

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(kvl law)

The sum of all the voltages around the loop is equal to zero. v1 + v2 + v3 + v4 = 0

This law is also called Kirchhoff's second law, Kirchhoff's loop (or mesh) rule, and Kirchhoff's second rule.
The directed sum of the electrical potential differences around any closed circuit must be zero.

KVL may also be stated as " the algebraic sum of various potential drops across an electrical circuit is equal to the electromotive force acting on the circuit"

dc machine
it can work as dc motor as well as generator also...

the main part of dc machine are following written below..

1. stator
2. rotor

which consist  following things ....
a). magnetic-field system
b). armature
c). commutator

further elaborating them we get...following things


1. Yoke or magnetic frame

2. Magnetic poles

3. Field coils

4. Inter poles (or) Commutator poles

5. Armature and Armature coils

6. Commutator and Brushes

7. Bearings and End covers


Yoke or magnetic frame

• •    It is made of cast iron for small machines. For large machines, it is made of cast steel.

Function of yoke

i)    It provides mechanical support for the machine and acts as a cover 
for the machine

ii)    It forms the portion of a magnetic circuit.

Magnetic poles

• •    The field magnet consists of pole cores and pole shoes.

• •    The pole cores and pole shoes are built with thin laminations of annealed steel and are held together using rivets or under hydraulic pressure.

• •    These magnetic poles are fitted to the yoke using screws.

• •    An advantage of pole cores built up of laminations is that eddy current losses in the pole faces are minimized.

Function of pole shoes

(i)    They spread out the flux in the air gap and also reduce the reluctance of the magnetic path at its larger cross section.

(ii)    They support the exciting coils or field coils.

      Field coils

• •    Field coils are wound with enameled copper wire. Sometimes cotton insulation is used.

• •    The coils are dried in vaccum and then impregnated with an insulating compound.

• •    Sufficient space is left between the layers for ventilating purposes.

• •    The exciting coils on the inter poles are connected in series with the armature.

• •    So they carry the full armature current and are made up of a few turns of heavy conductor.

Interpoles or Commutating poles

Inter poles are present in the high capacity dc machines.

Function of Interpoles or Commutating poles

 (i) To improve commutation and

(ii) To reduce armature reaction

Armature and Armature coils

•    When the armature is revolving, it is subjected to alternating magnetization. 
•    This causes hysteresis loss. 
•    To minimize this loss, low hysteresis steel containing a few percentage of silicon is used in the armature. 
•    To reduce the eddy current loss, laminations are provided .

•    The armature coils are wound with single cotton covered or double cotton covered or enameled insulated wires.
•    These coils are put into the insulated slots of the armature to avoid short circuit between the armature conductors and core. 
•    The slots are closed by fiber or wooden wedges to prevent the conductors from plying out due to the centrifugal force caused by the rotation of the armature. 
•    The armature is impregnated with varnish and dried. In medium and small machines, circular conductors are used. 
•    In large machines, rectangular strips of conductors are used for winding the armature coils.

Commutator

•    It is made of wedge shaped segments of hard drawn (or) drop forged copper insulated from each other by thin layers of built-up mica. 
•    The segments are held together by clamping them using Vshaped end rings and insulated from the segments by V shaped micanite rings. 
•    Each segment is provided with a riser to which the leads of the armature coils are soldered.

Brushes

•    The purpose of brush is to carry the current from the commutator to the external circuit. 
•    It is made of carbon or copper. For low voltage machines, it is made of copper-carbon compound. 
•    The brushes are placed in the brush holder, which is kept pressed against the commutator by a spring. 
•    The brush holder is fitted to a spindle, which is insulated from the machine. 
•    Connections from the brushes are taken by means of flexible pigtails, made of copper ribbons.

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Bearings and End covers

•    End covers are made of cast iron or fabricated steel are fitted to both ends of the yoke. 
•    To these end covers either ball bearings or roller bearings are fitted and armature shaft is mounted over these bearings. 
•    These bearings are lubricated with grease or hard oi

Armature winding fundamentals

•    In modern dc machines, drum type of armature windings is provided. In this type, the coils are wound and placed into the insulated slots of the armature. 
•    A coil with more than one turn is known as multi turn coil. 
•    The coils are always placed in two slots which are approximately one pole pitch apart. 
•    The portion of a coil placed inside a slot is called coil side and each coil will have number of conductors. 
•    The distance between the two coil sides of a coil is termed as coil pitch, which is expressed in terms of
number of slots.
•    The ends of a coil are connected to two different commutator segments on the commutator. 
•    The distance in terms of number of commutator segments, between the commutator segments to which the ends of a coil are connected is called commutator pitch.
•    The double layer windings are present in the dc machine. In double layer winding there will be two coil sides belonging to two different coils in each slot. 
•    If a coil side of one coil is laid in the top layer of a slot, the other coil side of the same coil occupies the bottom layer of the slot usually one pole pitch.

Drum type of winding has two types.

1. Lap winding

2. Wave winding

Lap winding

•    In lap winding each coil is connected in series with next coil under the same pole pair. 
•    If the start of one coil is connected to one particular commutator segment, the end of that coil and start of the next coil are connected to the next commutator segment either to the right or left of the first commutator segment. 
•    This process is continued till all the coils are connected. Here the commutator pitch is always + 1. 
•    In lap winding there are as many parallel paths as there are number of poles. 
•    The emf induced in any generator is equal to the emf induced in all the coils in a parallel path. 
•    Therefore lap winding is used in low voltage large current dc machines.

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three point starter

the three point starter in order to limt high current for protection purpose


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   The above figure shows, the motor is connected to supply terminals through three terminals L, A, and F available in the starter, it is called as three point starter. 

•    In this starter the resister elements are mounted behind an insulating board. 

•    The end of the resistor elements meet at the brass studs fixed on the front side of the board. 

•    The handle of the starter is fixed to a point so as to be moved over the studs against a spring tension.

•    When the starter handle is moved and makes contact with first stud full voltage is applied to series combination of resistor elements and armature. 

•    Therefore a reduced voltage is applied to the armature due to drop in the resistor elements and starting current is limited to safe value. 

•    At the same time full voltage is applied across the shunt field and this establishes normal flux. 

•    As the starter handle is moved towards right from one stud to next stud the resistor elements are cut out one by one and the voltage applied to armature increases step by step. 

•    Finally when all the resistance elements are cut out the handle is in on position and full voltage is applied across the armature terminals. 

•    Now the current flowing through the no volt release coil develops an attractive force over the soft iron piece fixed to the starter handle. 

•    At normal voltage this attractive force holds the handle in on position against the spring tension at the pivoted end of the handle. 

•    When the applied voltage falls below certain value or at the interruption of supply voltage, the attractive force developed at the no volt release coil over the handle may not be sufficient to deep the handle in on position against the spring tension. 

•    Therefore the handle flies back to off position immediately and avoids restarting of motor on resumption of supply. 

•    If such arrangement is not there in the starter and the handle is on position, on resumption of supply full voltage would be applied to the armature terminals. 

•    Also, the voltage release coil is connected in series with the shunt field, the starter handle is released to off position whenever the field circuit be opened. 

•    This avoids racing or motor to high speed.

•    The over load relay is connected in series with the armature circuit and carries load current.

•    When motor is over loaded the current through over load relay increases. If this current exceeds a predetermined value, the mmf produced by the over load coil lifts the pivoted iron piece underneath the coil. 

•    This iron piece short circuits the no voltage release coil and destroy its attractive force over the handle. 

•    Therefore the handle is released to off position. 

•    When the handle moved to right and resistance elements are cut out, they are in turn, included in the field circuit. 

•    Since the resistance of these elements is very small, inclusion of them in the field circuit does not affect the field current appreciably. 

•    Another important point is that the armature and shunt field circuits are closed against each other through the starting resistor element even when the handle is in off position. 

•    Therefore the energy stored in magnetic circuit gets dissipated slowly through starting and armature resistances. 

•    Thus inductance is avoided.

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