1 1.INTRODUCTION Phase controlled rectifiers employing thyristors are extensively used for changing constant ac input voltage to controlled dc output. In phase controlled rectifiers a thyristor is turned off by ‘natural commutation’ or line commutation in which the anode to cathode voltage become negative causes commutation of thyristor. In industrial applications rectifier circuits makes use of more than one thyristors fully controlled rectifier with line commutated thyristor which employs no commutation circuitry very simple less expensive and are therefore widely used in industries where controlled DC power is required.
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1.INTRODUCTION
Phase controlled rectifiers employing thyristors are extensively used for changing constant ac
input voltage to controlled dc output. In phase controlled rectifiers a thyristor is turned off by
‘natural commutation’ or line commutation in which the anode to cathode voltage become
negative causes commutation of thyristor. In industrial applications rectifier circuits makes use
of more than one thyristors fully controlled rectifier with line commutated thyristor which
employs no commutation circuitry very simple less expensive and are therefore widely used in
industries where controlled DC power is required.
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2.BASIC CIRCUIT OPERATION
Basic operation can be explained with the help of circuit diagram shown in fig 1. In which four
thyristors are used in which thysistor T1 and T2 forward biased during the positive half cycle of
the input AC supply. The SCRs T1 & T2 starts conducting only after a firing delay angle
specified by the controller. During negative half cycle T3 & T4 conducts for the same firing
angle. Thus the average output voltage varies according to the firing angle.
Relation between average output voltage and firing angle is given by
Fig.1 fully controlled rectifier
Vo = (2Vm/п) cosα
Where, Vo= Average output voltage in volts
Vm = peak voltage of the input AC supply in volts
α = firing angle in degrees
T4
12
LOAD
T1
12
single phase AC supply
T3
12
T2
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3.BLOCK DIAGRAM
With the development of digital electronics controlling of power circuits become very simple,
cheap and efficient. A PIC18F4550 microcontroller is used for the controlling the output voltage
it has following functions
• Read the firing angle as an input voltage using internal ADC module
• Read the zero crossing output pulses
• Generate firing delay after crossing of each zero of the input AC voltage for both
negative and positive half cycle.
• Generate the triggering pulse for all four SCRs.
A pulse transformer is used to supply the firing pulses to SCR which provide protection for control circuit
from power circuit. A filter capacitor reduces
Circuit Description:
The control circuit consists of pic18F
triggering pulse can be generated by using the proper synchronization with input supply this is
done with the help of zero crossing circuit.
which also reads the input from zero crossing detection circuit and the potentiometer which
isused to give the reference (firing angle)
transformer input.
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electronics controlling of power circuits become very simple,
A PIC18F4550 microcontroller is used for the controlling the output voltage
Read the firing angle as an input voltage using internal ADC module
ad the zero crossing output pulses
Generate firing delay after crossing of each zero of the input AC voltage for both
negative and positive half cycle.
Generate the triggering pulse for all four SCRs.
Fig.2 block diagram
A pulse transformer is used to supply the firing pulses to SCR which provide protection for control circuit
capacitor reduces the ripple in output voltage waveform.
ntrol circuit consists of pic18F4550 microcontroller, LM 358 op-amp IC
triggering pulse can be generated by using the proper synchronization with input supply this is
crossing circuit. Firing pulse is generated by pic microcontroller
reads the input from zero crossing detection circuit and the potentiometer which
(firing angle).the output pulse from PIC is given to the pulse
electronics controlling of power circuits become very simple,
A PIC18F4550 microcontroller is used for the controlling the output voltage
Generate firing delay after crossing of each zero of the input AC voltage for both
A pulse transformer is used to supply the firing pulses to SCR which provide protection for control circuit
amp IC an accurate
triggering pulse can be generated by using the proper synchronization with input supply this is
iring pulse is generated by pic microcontroller
reads the input from zero crossing detection circuit and the potentiometer which
the output pulse from PIC is given to the pulse
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4. CIRCUIT DIAGRAM
Fig 3 . Circuit diagram
T22P
4M
1 2
T32P
4M
1 2
R10
3.3
k
+5 V
PIC
18F
4550
12
28
29
31
32
33
MC
LR
/VP
P
RA
0/A
N0
RD
5
RD
6
VS
S
VD
D
RB
0/IN
T0
V3V
AC
TO T2
1k
1k
1k
1k
C
3300 u
F
LO
AD
47
V2
VA
C
1k
T12P
4M
1 2
T42P
4M
1 2
1k
from P1
- +
U2A
LM
348
231
84
+5 V
from P2
P1
1k
+5 V
1k
P2
TO T3
Zero crossing detection circuit (Ref Circuit diagram)
Fig.4 Output of zero crossing detection circuit with input
When the input is greater than zero the output goes to + V sat of 5V and during the negative half cycle.
The output is followed by an opto-coupler
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(Ref Circuit diagram)
Output of zero crossing detection circuit with input
When the input is greater than zero the output goes to + V sat of 5V and during the negative half cycle.
coupler – provide isolation for PIC18F.
When the input is greater than zero the output goes to + V sat of 5V and during the negative half cycle.
5. PROGRAMME FLOW CHART
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FLOW CHART
Fig. 5 flow chart
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6.COMPONENT DESCRIPTION
Serial no: Component Name & specification Quantity
1 SCR-2P4M 4
2 PIC18F4450 1
3 PULSE TRANSFORMER 2
4 RESISTOR -1K Ω 7
5 RESISTOR-47 Ω 1
6 CAPACITOR-3300 Μf,50 V 1
7 TANSFORMER 6-0-6V, 12-0-12V 1
8 LM358 1
7. FILTER DESIGN
Expression for Filter capacitor, is given by C= 1/4fR [1+ (1/1.414RF)]
Where f= supply frequency
R=load resistance
RF=Ripple factor
Assume Ripple factor=3%, R=47 Ω, f=50 Hz
C= 2615 µF
Take C= 3300 µF
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8. RESULTS AND ANALYSIS
Fig .6 output voltage waveform measured across the load firing angle <90o
Fig.7 output voltage waveform measured across the load firing angle <90o
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Fig.8 output voltage waveform measured across the load firing angle >90o
Fig.9 output voltage waveform measured across the load with C filter
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Fig.10 input voltage and triggering pulse from PIC18F4550
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Fig.10 triggering pulse For T1,T2 nd T3,T4 - PIC18F4550
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CALCULATION OF RIPPLE VOLTAGE
From the Fig 9
Ripple voltage = (15-8)/2=3.5V
RMS value of the Ripple voltage = 3.5/1.414=2.47V
Average output voltage = 13.4 V
Ripple factor =RMS value of the Ripple voltage /Average output voltage
Ripple factor = 2.47/13.4 =18.4%
9.PROGRAMME
LIST P=18F4550, F=INHX32 ;directive to define processor
#include <P18F4550.INC> ;processor specific variable definitions