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NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Experiment No-3
Object: - To observe the wave form of single phase full converter with Freewheeling Diode.
Apparatus required:1.
2.
3.
4.
5.
6.
7.
Single Phase Full Converter kit.
Freewheeling Diode.
Connecting cords
CRO
CRO probes
Multimeter
Resistive and inductive load
Theory:The circuit arrangement of a single – phase full converter is shown in fig. with a highly inductive load so
that load current is continuous and ripple free. During the positive half – cycle, thyristors T1 & T4 are
forward biased; and when these two thyristoers are fired simultaneously at ω t = α , the load is connected
to the input supply through T1 & T4 . Due to inductive load, thyristors T1 & T4 will continue to conduct
beyond ω t = 𝜋 even though the input voltage is already negative. During the negative half cycle of the
input voltage , thyristors T3 & T2 are forward biased ; and firing of thyristors T3 & T2 will apply the
supply voltage across thyristors T1 & T4 as reverse blocking voltage. T1 & T4 will be turned off due to line
or natural commutation and the load current will be transferred from T 1 & T4 to T3 & T2 .figure shows the
regions of converter operation & figure shows the waveforms for input voltage, output voltage & output
currents.
During the period from α to π , the input voltage Vs & input current Is is positive , and the power flow
from the supply to the load. The converter is said to be operated in rectification mode. During the period
from π to π + α, the input voltage Vs is negative & the input current Is is positive, and there will be
reverse power flow from the load to the supply. The converter is said to be operated in inversion mode.
This converter is extensively used in industrial applications up to 15 KW.
Depending on the value of α, the average output voltage could be either positive or negative and it
provides two – quadrants operation.
𝟐 𝑽𝒎
The average output voltage Vdc =
cos α
𝛑
Vdc can be varied from
𝟐 𝑽𝒎
𝛑
to -
𝟐 𝑽𝒎
𝛑
Maximum average output voltage Vdcmax =
by varying α from 0 to π.
𝟐 𝑽𝒎
𝛑
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
The characteristics of a single – phase bridge circuit given above are obtained by assuming that the load
current is constant. This assumption is valid if the value of inductance on the load side (of DC reactor) is
very high.
This is possible only for converters used as regulated DC power supplies, where high value inductance
(series filter choke) & capacitor (parallel) are used to reduce the ripple in DC output voltage.
However applications of controlled rectifiers in which the output voltage is not filtered and only the
rectified voltage is used (eg. Battery chargers, speed control of motors) the load current is not constant. In
this SCRs will turn off by natural commutation at the end of every half cycle & load current will be
discontinuous.
Trigger circuit:
Phase control IC TCA 785 is used to control thyristors, triacs and transistors.
The trigger pulses can be shifted with in a phase angle between 0o and 180o.
Fig1(a) Triggering Circuit
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Fig1 (b) Firing pulses
Circuit description:
 9 volt step down transformer is used to provide synchronization signal.
 IC has in built three functional blocks
1) Zero crossing detectors
2) Sync ramp generator and
3) Control comparator
 Sync ramp signal is connected internally to control comparator.
 Depending on the control voltage set by pot meter (SET SPEED), comparator gives two pulses
Q1 and Q2 exactly 180o apart.
 Pulse transformer PTX1 and PTX2 is used to isolate the firing pulses from power circuit.
Full controlled bridge:
All four trigger signals G1K1, G2K2, G3K3 and G4K4 from control card are connected to respectively
SCR’s T1, T2, T3, and T4.
 Full Control Bridge is to be configured using four SCR’s T1, T2, T3, and T4.
 100 volt/ 1 amp isolated AC supply is connected internally to the circuit.
 Freewheeling diode is provided on panel.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Fig2 (a) Full Controlled Bridge Rectifier Circuit with R-L Load
Fig2 (b) Full Controlled Bridge Rectifier Waveform with R-L Load
Working procedure:








Connect 230 volt AC mains to the unit.
Connect lamp load provided with system.
Trigger signals from control card are connected internally to the respective SCR’s
Make power on to the unit.
Vary the firing angle by adjusting SET pot meter.
Every time note the average DC output voltage. Also note the corresponding firing angle as
observed on CRO.
Tabulate the results. Trace the observed waveform on graph paper.
Instead of lamp load connect R-L load provided with system.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING




Make power on to the unit.
Vary the firing angle by adjusting SET pot meter.
Trace the observed waveform on graph paper.
Connect freewheeling diode across output and observe the effect of freewheeling diode on output.
OBSERVATION :S. No.
Control Voltage
(Volt VC)
Firing Angle
(α)
Observed wave form-
**Trace out the various wave forms of CRO on the graph paper.
CONCLUSION -
Output Voltage
(Volt V0)
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
EXPERIMENT No.-4 & 5
OBJECT:-Plot the various (Input, Output, Transfer) Characteristics) of PNP & NPN
transistor in common emitter and common base configuration.
APPARATUS REQUIRED-:
S. No.
Name of instrument
Range
1.
D.C. Voltmeter
0-1 Volt
2.
D.C. Voltmeter
0-10 Volt
3.
D.C. Ammeter
0-50 mAmpear
4.
D.C. Ammeter
0-25 micro Ampear
PROCEDURE-Connect the electrical circuits as shown in fig.1 (a). Keep the meter selector switch on
microampere side. In the connections collector bias as well as base bias both are negative with respect to
emitter.
PNP COMMON BASE TRANSFER CHARACTERISTICS
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Input characteristics:1. Adjust collector to emitter voltage VCE (using VR2) at some suitable values (say at – 2V) and
keep in constant.
2. By adjusting input supply (using VR1) set the base to emitter voltage, so that base current shows
value say 20mA. Note down base to emitter voltage VBE. Increase VBE in small steps and note the
corresponding base current IB.
3. Repeat step no. 1&2for other values of VCE 9 say -6V, -8V etc.).
4. Plot a graph by taking base voltage VBE along X-axis and base current IB along Y-axis as shown
in fig.1 (b).
5. Draw a tangent to VBE –IB curve & determine its slope. The reciprocal of the slope gives the value
of input resistance of transistor.
Output characteristics:1. Adjust the base current IB to 50mA using VR1.
2. Set collector voltage VCE to 0.5V and note down the corresponding collector current IC. Gradually
increase the collector voltage in small steps (i.e. make it -2V, -2.5V, -3.0V, -----10V etc) and note
the corresponding values of collector current IC keeping the base current IB constant.
3. Repeat steps 1 & 2 for other values of base current IB (say 75mA, 100mA etc).
4. Plot a graph by taking collector voltage VCE along X-axis fig.1( C ) & collector current IC along
Y-axis.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
5. Draw a tangent on a VCE- IC curve and determine its slope, reciprocal of the slope gives the values
of output resistance of transistor.
Transfer characteristics:1. Adjust collector voltage at suitable values (say VC = -4V) and maintain it constant.
2. Adjust base current IB to a suitable small but measurable value and note down the corresponding
collector current IC. Increase IB in small steps and note down the collector current IC each time.
3. Plot a graph by taking base current IB along X-axis and collector IC along Y-axis as shown in fig.1
(d). The slope of the graph gives the values of current gain.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
PNP COMMON EMITTER TRANSFER CHARACTERISTICS
PROCEDURE:Connect the electrical circuit as shown in fig.2 (a). Keep the meter selector switch on milli ampere
side. In the connections collector bias is negative with respect to base & emitter bias is positive with
respect to base.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Input characteristics:1. Adjust collector to base voltage VCB (using VR2) at some suitable value (say at -2V) and keep it
constant.
2. By adjusting input supply set the emitter current to a small but measurable value say 5mA, note
down the corresponding emitter to base voltage VEB. Increase VEB in small steps and note down
the corresponding emitter current IE.
3. Repeat the step no. 2 & 3 for other values collector voltages (say -6V, -8V etc).
4. Plot the graph by taking emitter base voltage VEB along X-axis and emitter current IE along Yaxis as shown in fig. 2(b).
5. Draw a tangent to VEB – IE curve & determine its slope. The reciprocal of the slope gives the
value of input resistances of transistor.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Output Characteristics:1. Adjust the emitter current IE to a suitable value (say 10mA).
2. Set collector voltage VCB to 0.5V and note the corresponding collector current IC. Gradually
increase the collector voltage in small steps (i.e. make it -2V, -2.5V, -3.0V, -----10V etc) and note
down the corresponding values of collector current IC keeping the emitter current IE constant.
3. Repeat steps 1 & 2 for other value of emitter current IE (say 15mA, 20mA etc).
4. Plot graph by taking collector voltage VCB along X-axis fig. 2 ( C ) & collector current IC along
Y-axis.
5. Draw a tangent on a VCB – IC curve and determine its slope, reciprocal of the slope gives the value
of output resistance of transistor.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Transfer characteristics:1. Adjust collector voltage at suitable value (say VCB = 4V) and maintain it constant.
2. Adjust emitter current IE to a suitable small but measurable value and note the corresponding
collector current IC. Increase IE in small steps and note the collector current IC each time.
3. Plot a graph by taking emitter current IE along X-axis and collector IC along Y-axis as shown fig.
2 (d). The slope of the graph gives the value of current gain B.
PROCEDURE: - Same as PNP Procedure for NPN.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Input characteristics: - Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Output characteristics: - Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
Transfer Characteristics: - Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Input characteristics: - Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Output characteristics: - Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
Transfer Characteristics- Proceed in the same manner as in case of PNP Common Base
Characteristics and plot the graph.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
OBSERVATION TABLE FOR PNP -
Common Emitter Characteristics
A) For Input Characteristics (VBE – IB for constant VCE)
S.
No.
1.
2.
3.
4.
5.
6.
7.
8.
Base Voltage
Base Current IB in mA
VBE (Volts)
VCE = -2V
VCE = -6V
VCE = -8V
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
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B) For Output Characteristics ( VCE – IC for constant IB)
S. No.
Collector Voltage
Collector Current IC in mA
VCE (Volts)
IB = 50mA
IB = 75mA
IB = 100mA
1.
2.
3.
4.
5.
6.
7.
8.
C) For Transfer Characteristics (IB - IC for constant VCE)
Constant value of Collector Voltage VCE = ----------------Volts
S. No.
1.
2.
3.
4.
5.
6.
7.
Base Current (IB)
Collector Current (IC)
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
8.
Common Base Characteristics
A) For Input Characteristics ( VEB – IE for constant VCB)
S. No.
Emitter Voltage
VEB (Volts)
Emitter Current IE in mA
VCB = -2V
VCB = -6V
VCB = -8V
1.
2.
3.
4.
5.
6.
7.
8.
B) For Output Characteristics ( VCB – IC for constant IE)
S. No.
1.
2.
3.
4.
5.
Collector Voltage
Collector Current IC in mA
VCB (Volts)
IE = 10mA
IE = 15mA
IE = 20mA
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
6.
7.
8.
C) For Transfer Characteristics (IE - IC for constant VCB)
Constant value of Collector Voltage VCB = ----------------Volts
S. No.
1.
2.
3.
4.
5.
6.
7.
8.
Emitter Current (mA)
Collector Current (mA)
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Experiment 05
OBJECTIVE: study and performance of jone’s chopper.
APPARATUS REQUIRED .: Jone’s chopper kit, connection lead, multi-meter, CRO
THEORY: In many industrial applications, it is required to convert a fixed-voltage dc source in
to a variable voltage dc source. A dc chopper converts directly from dc to dc and also known as a
dc to dc converter. A chopper can be considered as dc equivalent to an ac transformer with a
continuously variable turn’s ratio. Like a transformer, it can be used to step down or step up a dc
voltage source.
Choppers are widely used for traction motor control in electric automobiles, trolley cars, marine
hoists, forklift trucks, and mine haulers. They provide smooth acceleration control, high
efficiency, and fast dynamic response. Choppers can be used in regenerative breaking of dc
motors to return energy back into the supply, and this feature results in energy saving for
transportation systems with frequent stops. Choppers are used in dc voltage regulators, and also
used, in conjunction with an inductor, to generate a dc current, especially for the current source
inverter.
PRINCIPLE OF STEP-DOWN OPERATION:
The principle of operation can be explained by figure. When switch SW is closed for a time t1,
the input voltage Vs appears across the load. If the switch SW remains off for a time t2, the
voltage across the load is zero. The waveforms for the output voltage and load current are also
shown in fig. the chopper switch can be implemented by using a (1) power BJT (2) power
MOSFET (3) GTO (4) force-commutated thyristor.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
The average output voltage is given by
And the average load current, Ia = Va / R, where T is the chopping period, K=t1/T is the duty
cycle of chopper, and f is the chopping frequency. The rms value of output voltage is found from
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Assuming a lossless chopper, the input to the chopper is the same as the output and is given by
The effective input resistance seen by the source is
The duty cycle K can be varied from 0 to 1 by varying t1, T of f. therefore, the output voltage V o
can be varied from 0 to Vs by controlling K, and the power flow can be controlled.
1. Constant-frequency operation: operation: the chopping frequency f (or chopping period
T) is kept constant and the t1 is varied, the width of the pulse is varied and the type of
control is known as pulse-width modulation (PWM) control.
2. Variable frequency operation: the chopping frequency f is varied. Either on time t1 or
off time t2 is kept constant. This is called frequency modulation. The frequency has to be
varied over a wide range to obtain the full output voltage range. This type of control
would generates harmonics at unpredictable frequencies and the design would be
difficult.
CHOPPER USING SCR :
In this SCR is used as switch. Thus SCR is to be made ON & OFF depending on chopping
frequency.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
The process of making switch off is called commutation. There are two types of
commutation:
1) Current commutation 2) voltage commutation.
CURRENT COMMUTATION:
In this type of commutation the conducting SCR is turned off by injecting a pulse of reverse
current through it.
The current pulse is usually generate by an L-C circuit and is initiated by gating an auxiliary
SCR is self commutated. The reverse current pulse must be of sufficient magnitude to reduce
the anode current of main SCR to zero. A distinctive feature of the current commutation is
that, the reverse voltage across the device is applied through a diode connected in antiparallel to SCR. Since this is limited to about one volt the turn of time of the device increases
in comparison to voltage commutation.
VOLTAGE COMMUTATION:
In this type of commutation the conducting thyristor is turned off by impressing a large
reverse voltage across it.
This is done by switching a charged capacitor.
Figure shows the circuit diagram of the type A chopper.
SCR1 is the main power switch & SCR2 is auxiliary. The circuit operates in the following
modes.
MODE 1 : It is assumed that prior to this mode SCR1 has been conducting and that the
capacitor C is charged to Vin volts in the direction shown in figure. The gating of SCR (Aux)
by signal ‘G1’ starts this mode. SCR1 is almost instantaneously turned off. The load current
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
assumed to be constant during this mode, now flows through C and SCR2. The capacitor C
discharges from –Vin to +Vin , almost linearly. The output voltage Vo jumps to 2 Vin at the
start of this mode and falls linearly to zero. The duration of this mode is given by
T1 = 2Vin C / Idc………………………………..A
The mode ends when diode D3 is forward biased and is conducting. The capacitor is charged
to +Vin at the end of the mode. The turn of time for SCR1 is t1/2.
MODE 2 : The load current freewheels through diode D3 in this mode. The duration of the
mode is ( T = Ton – T1 ).
MODE 3 : This mode occurs when the main thyristor SCR1 is gated. An oscillatory circuit is
formed with L1, C1, SCR1 and D1. The capacitor voltage changes sinusoidally from +Vin to
–Vin.
The duration of this mode is given by.
T2 = π √𝐿1 𝐶1 ……………………………………..B
The turn off time for SCR is t3/2.
DESIGN EQUATIONS : the components L1 & C1 can be calculated for given values of
load current Idc and SCR turn off times tq1 and tq1’.
From equation A & B
tq1 = Vin C / Idc…………………………………….C
t’q1 =
𝜋
2
√𝐿1 𝐶1 …………………….. D
from equations C & D ,
C = ( Idc tq1/Vin )
And
L1 = ( 2/𝜋 t’q1)2 / C
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LAB SUBJECT CODE: EX-504
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The peek current of SCR1 is
I Tp = I1 + Vin √𝐶⁄𝐿1
The peek voltage SCR1 & SCR2 is
(VT1)p = ± Vin
The peek current through SCR2 is
I T2p = Vin √𝐶⁄𝐿1
The peek voltage across the freewheeling diode D3 is,
Vom = 2 Vin
IMPORTANT FEATURES OF VOLTAGE COMMUTATED CHOPPER :
1) The average load voltage is given by
Vo = ( Vin Ton / T ) + ( Vin t1 / T )
=
𝑉𝑖𝑛
𝑇
Ton +
2 𝑉𝑖𝑛 𝐶
𝐼𝑑𝑐
The effective on period of the chopper is therefore larger than the period Ton and varies with the
load current .
2) Since the charge on the capacitor must be reversed when SCR1 is turned on, the
minimum on period of the chopper is ( t3 + t1 ). Thus T must be increased if low values
of Vo are to be obtained.
3) At start SCR2 must be gated before SCR1 so that a charge is established on C. further if
SCR1 fails to commutate C is completely discharged. The commutation therefore cannot
be resumed in the next cycle and SCR1 must then be turned off by some other means, e.g.
switching off Vin.
4) The circuit cannot be operated at no load.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
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CONTROL CIRCUIT :
Class D type of forced commutation is used in this chopper circuit & pulse width modulation
(PWM) control is used.
PWM CONTROL CIRCUIT :
Basic fixed frequency clock of desired chopping rate is obtained from 555 astable timer. This
clock is given as trigger to monostable 74721 IC. The pulse width of 74721 is adjustable from 15
to 80% of T. on every next rising edge of clock pulse the 74721 is triggered.
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
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When Q output of 74721 is high, this high output drives UJT relaxation oscillator. These pulses
through pulse transformer drives the main SCR ( Points T1 & T2 ). Depending on R C values of
monostable ( C is fixed internally & hence only pulse width is to be adjusted by external
potmeter ) the Q will become Lo &
will becomes high. At this instant, there will be no
trigger pulses at T1 T2. But a single pulse trigger during Lo to high transition of
will appear
at A1 and A2. This will trigger auxiliary SCR. Once auxiliary SCR is triggered reverse voltage
appear across main SCR1 and turn it off & as C charges auxiliary SCR will automatically turn
off. Now main SCR is off till rising edge of next clock pulse.
On rising edge of next clock above cycle repeats.
For proper commutation process (1) it is assumed that capacitor ‘C’ is charged initially before
applying 1st trigger to main SCR.
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LAB SUBJECT CODE: EX-504
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(2) Auxiliary SCR must be turned off & C should be charged before applying next to main SCR.
JONE’S CHOPPER-SCR T1 is the main thyristor, whereas SCR T2 , capacitor C, D2 and
autotransformer (T) forms the commutating circuit for the main thyristor T1. Therefore the
special feature of this circuit is the tapped autotransformer T through a portion of which the load
current flows. Here L1 and L2 are closely coupled so that the capacitor always gets sufficient
energy to turn off the main SCR T1.
If the main thyristor T1 is on for a long period, than the motor will reach the maximum stedy
state speed determined by the battery voltage, the motor end the mechnicl load characteristics. If
thyristor T1 is off, the motor will not rotate. Now, if thyristor T1 is alternately on an Off in a
cyclic manner, the motor will rotate at some speed between maximum and zero.
Let us assume that initially capacitor C is charged to a voltage Edc with the polarity shown in
figure. As shown ion fig. SCR T1 is triggered at time t = t1, current flows through the path C A –
T1 – L2 – D1 - CB and capacitor C charges to opposite polarity, i.e. plate B positive and plat A
NAME OF LABORATORY- POWER ELECTRONICS
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negative. However, diode D1 prevents further oscillation of the resonating L2 C circuit. Hence,
capacitor C retains its charge until SCR T2 is triggered. In figure the capacitor voltage wave
forms are drawn at bottom plate B of capacitor .
Now, at time t = t3 SCR T2 is triggered. Current flow through the path CB – T2 – T1 – CA.
therefore , discharge of capacitor C reverse – biases SCR T1 and turns it off. The capacitor again
charges up with plate A positive and SCR T2 turns off because the current through it falls below
the holding current value when capacitor C is recharged.
The cycle repeats when SCR T1 is again triggered. The use of autotransformer insures that
whenever current is deleverd from DC source to the load, a voltage is indused in L2 in the
correct polarity for changing the commutating capacitor to a voltage higher than Edc. Thus, the
autotransformer measurably inhances the reliability of the circuit.
At t5, the bottom plate B of capacitor C reaches the peek value. Since at t5, the capacitor is
charged to a voltage greter than Edc, diode D1 is again forward biased. Capacitor C now
discharges to a value lower than Edc. The time duration t3 to t4 is the circuit turn off time
presented to SCR T1.
PROCEDUREOBSERVATION TABLES.NO
1
2
3
4
5
RESULT-
Vout Measured
Ton
Toff
Vout calculated
NAME OF LABORATORY- POWER ELECTRONICS
LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Experiment No.6
OBJECTIVE: Study and performance of different types of commutation of SCR.
APPARATUS REQUIRED: Thyristor commutation kit, connection leads, Multi-meter, and
CRO.
THEORY: A thyristor is normally switched on by applying a pulse to gate. Once the thyristor is
turned ON, gate looses the control and some means are required to turn it OFF. The
turn off means that the forward conduction of the thyristor has ceased and
reapplication of positive voltage to the anode will not cause the current flow without
applying gate signal.
The process of turning off thyristor is called COMMUTATION.
Forced commutation:
In some thyristor circuit, the input voltage is DC & the forward current of thyristor is
forced to zero by an additional circuitry called commutation circuit it turn OFF
thyristor. This technique is normally applied in choppers.
Method of forced commutation:
1. Class A: Self commutation by resonating the load.
When SCR is triggered anode current flows & charges C. After time t, depending upon
the value of L, C, R direction of current flow through SCR reverse and the SCR will turn
OFF.
2. Class B: Resonant pulse commutation
In class A method of commutation, the commutating components L, C, carry the load
current being they are in series with load.
In class B, commutating component L & C are connected parallel to SCR.
Initially the capacitor C is charged to voltage Ec. When the SCR is fired the capacitor will
discharge it & at the end of the discharge will have a reverse voltage. Since the SCR is
conducting, the negative voltage in will produce a negative current I. when this current
will become equal to load current, the SCR will be turned OFF.
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3. CLASS C: Complementary commutation is used for parallel invertors or bridge
invertors
where load is to be complemented between pair of SCR’s. As shown in circuit
diagram- when SCR1 is conducting LOAD1 will get energized & capacitor C will get
charged to supply voltage. When SCR2 is fired, the capacitor will apply a reverse
potential across SCR1 and turn it off. The LOAD2 get energized and capacitor C will be
charged in the opposite direction.
If SCR1 is turned ON, SCR2 will get turned off. Thus firing of one SCR commutates the
other.
4. CLASS D : Auxiliary commutation
In this configuration, the load current is carried by only one of the SCR. To turn off the
main SCR, auxiliary SCR is turned on which in turn commutates the main SCR. As
shown in the circuit diagram, initially capacitor C is charged to voltage Ec. when main
SCR1 is turned ON, the capacitor will discharge through it and also through inductor. At
the end of the discharge, capacitor voltage will be reversed. The reverse discharge
through SCR1 is prevented by diode D. when auxiliary SCR1 is prevented by diode D.
when auxiliary SCR1 is fired, the capacitor will discharge through SCR1 and turn it off.
Since reverse voltage is applied to SCR1 immediately after turning on auxiliary SCR, this
is also known as voltage commutation.
5. Class E :External Pulse Source For Commutation
SCR is triggered by applying gate voltage. Once the SCR is fired, load current will flow
through load.
To turn SCR OFF, Base drive is applied to the transistor Q1.This will connect auxiliary
supply E2, across SCR1, turning it off. Pulse width of the base drive must be of sufficient
duration to keep Q1ON, for the duration of the turn –OFF time of SCR.
6. CLASS F: Line commutation
If the supply in an alternating voltage, load will during the positive half cycle. During the
negative half cycle SCR will turn OFF due to the negative polarity across SCR.
PROCEDURE:
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1) For Class A type commutation:
Fig (1. ) Class A Commutation Circuit
1. Connect trigger input for class A type commutation, at gate and cathode of
SCR.
2. Make power ON to unit.
3. Connect CRO across SCR1 or load. When SCR is triggered output appears across RL.
4. As this charge condense C, at time t, depending on L, C & R, the SCR turns off. It is
turned on by next trigger pulse.
5. Trace the waveform on graph paper.
2) Repeat the same procedure for class B type of commutation.
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Fig (2. ) Class B Commutation Circuit
3) For class C type commutation:
1. Connect G1 K1 & G2 K2 trigger to SCR1 & SCR2 of class C type of circuit.
2. Make power on. As G1 K1 & G2 K2 trigger are complementary, SCR1 & SCR2 are
SCR2 are triggered at t1 & t2 respectively.
3. When SCR1 is trigger at t1, SCR2 remains off.
4 At time t2, SCR1 to turn off.
5. This makes SCR1 to turn off.
6. Observe the waveform across SCR1 or SCR2.
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Fig (2. ) Class C Commutation Circuit
Trace the waveform on graph paper.
4) For Class D type commutation:
Fig (4 ) Class D Commutation Circuit
1. Connect A1 A2 trigger to auxiliary SCR of D type of commutation circuit.
2. Connect only T2 trigger point to T2 of main SCR1 of the circuit.
3. Make power ON to the unit.
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(Only Auxiliary SCR2 will get trigger first & charge C)
4. Now connect the output voltage across RL on CRO.
5) For Class E type of commutation:
1. Make power on to the unit.
2. Provide manual trigger by press to ON switch to trigger SCR.
3. Measure the voltage across SCR & load resistance RL, on multi-meter.
4. To make the SCR OFF, provide pulse by PRESS to ON switch. This makes transistor
Q on and injects current in SCR in reverse direction to make it off.
5. Q on and injects current in SCR in reverse direction to make it off. Check when SCR is
turned off, by measuring voltage across SCR.
Fig (5) Class E Commutation Circuit
6) For Class F type Commutation
1. Make power on to the unit.
2. Connect CRO across RL and observe the waveforms.
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3. Trace the waveform on graph paper.
Fig (6) Class F Commutation Circuit
RESULT- Trace the waveform of CRO on graph paper
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LAB SUBJECT CODE: EX-504
NAME OF DEPARTMENT-ELECTRICAL ENGINEERING
Experiment No. -7
OBJECTIVE: Study of series inverter and observe the waveform on CRO.
APPARATUS REQURIED- series inverter kit, connecting leads, CRO, multi-meter.
THEORY: inverter is used to convert DC voltage to AC. SCR is used as the basic control
element. In AC circuits, once the SCR is fired by gate pulse, it automatically turns off
when the current through SCR goes to natural zero. This is called as natural
commutation.
However in DC circuit, to make the SCR OFF, external circuit is required. This is
called as forced commutation of SCR.
Inverters are classified according to the method of commutation.
Class A – Self commutation by resistive load.
Class B – Self commutated by an LC load.
Class C – C or LC switched by a load carrying SCR.
Class D – C or LC switched by an auxiliary SCR.
Class E – External pulse source for commutation.
Class F – AC line commutated.
SERIES INVERTER
The series inverter uses class ‘A’ type of commutation. Commutating components L & C are
applied in series with the load to form an under damped circuit. Since the SCR’s turn off by
themselves, when the current becomes zero, this inverter is classified as self commutated
inverter.
THE OPERATION OF THE INVERTER:
When SCR1 is triggered, current flows from the supply, EDC charge up capacitor C to a voltage
approaching to EDC. The current then reverses and flow back to supply via diode D1 & C
discharges. During the reverse current flow, turn off time is presented to SCR1.
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SCR2 triggered next and a similar cycle occurs in the lower half the circuit with a negative
going pulse of voltage appearing across C.
TRIGGER CIRCUIT
Inverter frequency is depending on the frequency of trigger circuit. 555 oscillator provides two
complementary output Q & Q’. Frequency can be adjusted by potentiometer of 555 oscillators.
Complementary outputs Q & Q’ drives transistor trigger circuit which alternately triggers
SCR1 & SCR2.
Fig1 (a) Triggering Circuit
PROCEDURE:
1. Make power on to the unit.
2. Measure DC supply voltage of the battery (EDC).
3. Connect the CRO across load & observe the output waveform. Trace the waveform on
graph paper.
4. Change the frequency by freq. adjusts pot & notes the change in frequency of output
voltage.
5. Observe and trace the waveform on CRO for
1. Trigger
a) 555 output
b) complimentary output of flip-flop
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c) Output of pulse transformer.
2. Voltage across SCR1 & SCR2.
NOTE:
1. If dual trace CRO is used for comparing the waveform, the CRO must have FLOATING
INPUT.
2. Do not use the dual trace CRO if the inputs have common ground.
Fig1(b) SCR series inverter
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Fig1(c) Control circuit series inverter
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Experiment No.-8
OBJECTIVE: study of step up chopper using SCR and observe waveform on CRO.
APPRATUS REQUIRED: step up chopper kit, connection leads, CRO, multi-meter.
THEORY : when SCR is conducting (on state of chopper) the applied voltage Vdc generates
current I through the least resistance path i.e. through SCR and simultaneously energy is stored
in inductor L. now the moment SCR is switched OFF. (Off state of chopper). Point P becomes
positive with respect to point Q due to inductor L, diode D forward biased positive with respect
to point Q due to inductor L, diode D forward biased and the stored energy in inductor L is
transferred to the capacitor through diode D and the capacitor C starts charging C gets fully
charged to the potential at point Q. the potential at point Q will be more than that of point P (due
to inductor L connected in line). Now as point Q is at higher potential than point P. diode D
becomes reverse biased thus blocks the charge at capacitor C. in other words, we can say that,
capacitor C retains its charge higher than the applied voltage Vdc due to the presence of diode D.
The polarities of capacitor C will be as shown in fig. i.e. its upper plate will acquire positive
charge C will be as shown in fig. its upper plate will acquire positive charge and the lower plate
acquire negative charge.
Since the capacitor is charged at a higher voltage than the input supply d.c. the output voltage (
available across the load) Vo will be more than the input voltage Vdc. We can thus say that the
circuit works like a step up transformer increasing the level of output voltage available across the
load. If the current flowing in the circuit is assumed to be constant through (during ON and OFF
period of the chopper), the energy stored in the inductor and the energy transferred to the
capacitor from the inductor can be calculated.
Energy stored in the inductor during ON state of the chopper
= Vdc × I × Ton
……………… (1)
Where, I is the current flowing in the circuit and Ton is the ON period of the chopper.
Energy transferred to the capacitor C from inductor L during the OFF state of the chopper.
= (Von – Vdc) × I × Toff
…………….... (2)
Since energy stored in the inductor is the same as the energy transferred to the capacitor. From
equation (1) and (2)
(𝑉𝑑𝑐 × 𝐼 × 𝑇𝑜𝑛 ) = (𝑉𝑜 − 𝑉𝑑𝑐 ) × 𝐼 × 𝑇𝑜𝑓𝑓
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𝑉𝑜 = 𝑉𝑑𝑐
Where
𝑇𝑜𝑛 +𝑇𝑜𝑓𝑓
𝑇𝑜𝑓𝑓
𝑇𝑜𝑛 +𝑇𝑜𝑓𝑓
𝑇𝑜𝑓𝑓
……………………. (3)
is the reciprocal of duty cycle of the chopper.
In case Ton =Toff = T from equation (3) we get
𝑉𝑜 = 𝑉𝑑𝑐
𝑇+𝑇
𝑇
𝑉𝑜 = 𝑉𝑑𝑐
𝑉𝑜 = 2Vdc
2𝑇
𝑇
……………………….. (4)
Thus the output voltage can be obtained as twice the input voltage. The output voltage can even
be more twice the input voltage if Ton is increased and Toff is decreased with Ton=Toff. In other
words, we can conclude that the above circuit works as a step up chopper and the output voltage
more than the source voltage can be obtained by controlling Ton and Toff for the chopper. This
circuit is very useful for obtaining higher voltages. The large inductor L also helps in minimizing
the ripples as the output.
PROCEDURE
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LAB SUBJECT CODE: EX-504
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(A) PWM CONTORL CIRCUIT:
Fig(1) PWM TRIGGER CIRCUIT
(1)
(2)
(3)
(4)
(5)
Do not connect trigger to SCR (keep G1,K1&G2,K2 open)
Make power on to the unit
Make S1 switch ON.
Observe the Q and Q’ output on CRO (dual trace).
Also observe pulses at G1, K1 & G2, and K2 at the output of trigger circuit on CRO
(dual trace).
(6) Changes the pulse width by varying PW adjust potentiometer.
(7) Note minimum Ton & maximum Ton by varying potentiometer.
(8) Trace the waveform on graph paper.
(B) STEP UP CHOPPER :
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
1.
2.
3.
4.
5.
6.
7.
C load is connected at the output of chopper internally & Connect lamp load
externally.
Keep switch S1 OFF, connect G1,K1 & G2,K2 points of trigger circuit to make
power on to the unit. Measure supply voltage points.
Make power on to the unit. Measure supply voltage Edc.
Chopper supply voltage = Edc = ………… volt.
Make S1 switch on.
Measure the DC voltage across load Vo on multi-meter.
Connect CRO across main SCR T1 and observe the voltage waveforms.
Vary the potentiometer of trigger circuit slowly & note output voltage, Ton & Toff
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OBSERVATION TABLE
S. No.
Ton
Toff
1
2
3
4
5
RESULT-Observe the wave from CRO& Trace it.
CONCLUSION-
LAB QUIZES1. What is the role of input and output filters in choppers?
2. What is the method to reverse the current flow of a chopper’s load?
3. What is a difference between a step-down and a step-up chopper.
4. What is the duty cycle equal to?
5. Which type commutation techniques used inStep chopper.
6. What is the purpose of the thyristor commutation circuits?
Output Voltage Vo
(Volt)
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Experiment-12
OBJECT: -To observe the wave form of single phase full converter with R and R-L load.
APPARATUS REQUIRED:8. Single Phase Full Converter kit.
9. Connecting cords
10. CRO
11. CRO probes
12. Multimeter
13. Resistive and inductive load
THEORY:The circuit arrangement of a single – phase full converter is shown in fig. with a highly inductive
load so that load current is continuous and ripple free. During the positive half – cycle, thyristors
T1 & T4 are forward biased; and when these two thyristers are fired simultaneously at ω t = α ,
the load is connected to the input supply through T1 & T4 . Due to inductive load, thyristors T1 &
T4 will continue to conduct beyond ω t = 𝜋 even though the input voltage is already negative.
During the negative half cycle of the input voltage , thyristors T3 & T2 are forward biased ; and
firing of thyristors T3 & T2 will apply the supply voltage across thyristors T1 & T4 as reverse
blocking voltage. T1 & T4 will be turned off due to line or natural commutation and the load
current will be transferred from T1 & T4 to T3 & T2 .figure shows the regions of converter
operation & figure shows the waveforms for input voltage, output voltage & output currents.
During the period from α to π , the input voltage Vs & input current Is is positive , and the power
flow from the supply to the load. The converter is said to be operated in rectification mode.
During the period from π to π + α, the input voltage Vs is negative & the input current Is is
positive, and there will be reverse power flow from the load to the supply. The converter is said
to be operated in inversion mode.
This converter is extensively used in industrial applications up to 15 KW.
Depending on the value of α, the average output voltage could be either positive or negative and
it provides two – quadrants operation.
The average output voltage Vdc =
Vdc can be varied from
𝟐 𝑽𝒎
𝛑
to -
𝟐 𝑽𝒎
𝛑
𝟐 𝑽𝒎
𝛑
cos α
by varying α from 0 to π.
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Maximum average output voltage Vdcmax =
𝟐 𝑽𝒎
𝛑
The characteristics of a single – phase bridge circuit given above are obtained by assuming that
the load current is constant. This assumption is valid if the value of inductance on the load side
(of DC reactor) is very high.
This is possible only for converters used as regulated DC power supplies, where high value
inductance (series filter choke) & capacitor (parallel) are used to reduce the ripple in DC output
voltage. However applications of controlled rectifiers in which the output voltage is not filtered
and only the rectified voltage is used (e.g. Battery chargers, speed control of motors) the load
current is not constant. In this SCRs will turn off by natural commutation at the end of every half
cycle & load current will be discontinuous.
PROCEDURE:Trigger circuit:
Fig1(a) Triggering Circuit


Phase control IC TCA 785 is used to control thyristors, triacs and transistors.
The trigger pulses can be shifted with in a phase angle between 0o and 180o.
Circuit description: 9 volt step down transformer is used to provide synchronization signal
IC has in built three functional blocks1) Zero crossing detector, 2) Sync ramp generator and
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3) Control comparator.



Sync ramp signal is connected internally to control comparator.
Depending on the control voltage set by pot meter (SET SPEED), comparator gives two
pulses Q1 and Q2 exactly 180o apart.
Pulse transformer PTX1 and PTX2 is used to isolate the firing pulses from power circuit.
Fig1 (b) Firing pulses
Full controlled bridge:
All four trigger signals G1K1, G2K2, G3K3 and G4K4 from control card are connected to
respectively SCR’s T1, T2, T3, and T4.


Full Control Bridge is to be configured using four SCR’s T1, T2, T3, and T4.
100 volt/ 1 amp isolated AC supply is connected internally to the circuit.
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Fig2 (a) Full Controlled Bridge Rectifier Circuit with R Load
Fig2 (b) Full Controlled Bridge Rectifier Waveforms with R Load
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Fig3 (a) Full Controlled Bridge Rectifier Circuit with R-L Load
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Fig3 (b) Full Controlled Bridge Rectifier Waveform with R-L Load
PROCEDURE:


Connect 230 volt AC mains to the unit.
Connect lamp load provided with system.
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





Trigger signals from control card are connected internally to the respective SCR’s
Make power on to the unit.
Vary the firing angle by adjusting SET pot meter.
Every time note the average DC output voltage. Also note the corresponding
firing angle as observed on CRO.
Tabulate the results. Trace the observed waveform on graph paper.
Instead of lamp load connect R-L load provided with system.
OBSERVATION TABLE:FOR R Load
S. No.
Control Voltage
(Volt VC)
Firing Angle
(α)
Output Voltage
(Volt V0)
Control Voltage
(Volt VC)
Firing Angle
(α)
Output Voltage
(Volt V0)
1
2
3
4
5
FOR R-L Load
S. No.
1
2
3
4
5
**Trace out the various wave forms of CRO on the graph paper.
CONCLUSION-
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LAB SUBJECT CODE: EX-504
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EXPERIMENT NO.2
OBJECT: -To observe the wave form of single phase Semi controlled converter with R and
R-L load.
APPRATUS REQUIORED:1.
2.
3.
4.
5.
6.
Single Phase Semi controlled converter kit.
Connecting cords
CRO
CRO probes
Multimeter
Resistive and inductive load
THEORY:The circuit arrangement of a single phase semi-converter is shown in figure with a highly
inductive load. The load current is assumed continuous and ripples free. During the positive half
cycle, thyristor T1 is forward biased. When thyristor T1 is fired at ωt = α, the load is connected
to the supply through T1 & D4 during the period π < ωt < π. During the period from α < ωt <
(π+α), the input voltage is negative and the freewheel diode Dm is forward biased. Dm conducts
to provide the continuity of current in the inductive load. The load current is transferred from T1
& D4 to Dm; and thyristor T1 & diode D4 are turned off. During the negative half cycle of input
voltage, thyristor T2 is forward biased, and the firing of thyristor T2 at ωt = π+α will reverse bias
Dm. The diode Dm is turned off & the load is connected to the supply through T2 & D3. This
converter operates in single quadrant (i.e. both output voltage & current have positive polarity).
The average output voltage is given by,
Vdc =
𝑽𝒎
𝛑
(1+cos α)
Vdc can be varied from 2 Vm/π to 0 by varying α from 0 to π.
The Maximum average output voltage
𝟐 𝑽𝒎
Vdc max = 𝛑
Trigger circuit:
Phase control IC TCA 785 is used to control thyristors, triacs and transistors.
 The trigger pulses can be shifted with in a phase angle between 0o and 180o.
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Fig1(a) Triggering Circuit
Fig1 (b) Firing pulses
Circuit description:
 9 volt step down transformer is used to provide synchronization signal.
 IC has in built three functional blocks 1) Zero crossing detectors
2) Sync ramp generator and
3) Control comparator.
 Sync ramp signal is connected internally to control comparator.
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

Depending on the control voltage set by pot meter (SET SPEED), comparator gives two
pulses Q1 and Q2 exactly 180o apart.
Pulse transformer PTX1 and PTX2 is used to isolate the firing pulses from power circuit.
Semi Controlled Bridge:
Fig2 (a) Semi Controlled Bridge Rectifier Circuit with R Load
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Fig2 (b) Semi Controlled Bridge Rectifier Waveforms with R Load
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Fig3 (a) Semi Controlled Bridge Rectifier Circuit with R-L Load
Fig3 (b) Semi Controlled Bridge Rectifier Waveform with R-L Load
Semi Controlled Bridge:



Both trigger signals G1K1 & G3K3 from control are connected to respective SCR’s (T1
and T2).
Half control bridge is to be configured using two SCR’s T1 & T2 and power diode D3 &
D4.
100 volt/ 1 amp isolated AC supply is connected to internally to the bridge.
PROCEDURE:

Connect 230 volt AC mains to the unit.
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 Connect lamp load provided with system.
 Trigger signals from control card are connected internally to the respective SCR’s
 Make power on to the unit.
 Vary the firing angle by adjusting SET pot meter.
 Every time note the average DC output voltage
 Note the corresponding firing angle as observed on CRO.
 Tabulate the results
 Trace the observed waveform on graph paper.
OBSERVATION TABLE:FOR R Load
S. No.
Control Voltage
(Volt VC)
Firing Angle
(α)
Output Voltage
(Volt V0)
Control Voltage
(Volt VC)
Firing Angle
(α)
Output Voltage
(Volt V0)
1
2
3
4
5
FOR R-L Load
S. No.
1
2
3
4
5
**Trace out the various wave forms of CRO on the graph paper.
CONCLUSION-