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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.
2
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
12
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.
3
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
4
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
5
(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
6
FLOW CHART
Fig. 5 flow chart
7
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
8
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
9
Fig.8 output voltage waveform measured across the load firing angle >90o
Fig.9 output voltage waveform measured across the load with C filter
10
Fig.10 input voltage and triggering pulse from PIC18F4550
11
Fig.10 triggering pulse For T1,T2 nd T3,T4 - PIC18F4550
12
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
;******************************************************************************
;Configuration bits
CONFIG WDT=OFF; disable watchdog timer
CONFIG MCLRE = ON; MCLEAR Pin on
CONFIG DEBUG = ON; Enable Debug Mode
CONFIG LVP = OFF;
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CONFIG FOSC = INTOSCIO_EC
;******************************************************************************
;Variable definitions
; These variables are only needed if low priority interrupts are used.
; More variables may be needed to store other special function registers used
; in the interrupt routines.
UDATA
WREG_TEMP RES 1 ;variable in RAM for context saving
STATUS_TEMP RES 1 ;variable in RAM for context saving
BSR_TEMP RES 1 ;variable in RAM for context saving
vipin res 1
UDATA_ACS
EXAMPLE RES 1 ;example of a variable in access RAM
;******************************************************************************
;EEPROM data
; Data to be programmed into the Data EEPROM is defined here
DATA_EEPROM CODE 0xf00000
DE "Test Data",0,1,2,3,4,5
;******************************************************************************
;Reset vector
; This code will start executing when a reset occurs.
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RESET_VECTOR CODE 0x0000
goto Main ;go to start of main code
;******************************************************************************
;High priority interrupt vector
; This code will start executing when a high priority interrupt occurs or
; when any interrupt occurs if interrupt priorities are not enabled.
HI_INT_VECTOR CODE 0x0008
bra HighInt ;go to high priority interrupt routine
;******************************************************************************
;Low priority interrupt vector and routine
; This code will start executing when a low priority interrupt occurs.
; This code can be removed if low priority interrupts are not used.
LOW_INT_VECTOR CODE 0x0018
bra LowInt ;go to low priority interrupt routine
;******************************************************************************
;High priority interrupt routine
; The high priority interrupt code is placed here to avoid conflicting with
; the low priority interrupt vector.
CODE
HighInt:
retfie FAST
15
;******************************************************************************
;Low priority interrupt routine
; The low priority interrupt code is placed here.
; This code can be removed if low priority interrupts are not used.
LowInt:
movff STATUS,STATUS_TEMP ;save STATUS register
movff WREG,WREG_TEMP ;save working register
movff BSR,BSR_TEMP ;save BSR register
movff BSR_TEMP,BSR ;restore BSR register
movff WREG_TEMP,WREG ;restore working register
movff STATUS_TEMP,STATUS ;restore STATUS register
retfie
;******************************************************************************
;Start of main program
; The main program code is placed here.
Main:
MOVLW B'00011000'
IORWF OSCCON,1,0
CLRF PORTB
CLRF LATB
MOVLW 0X0E
MOVWF ADCON1,0
BCF 300005H,1,0
16
MOVLW 0X00
MOVWF ADCON0,0
MOVLW B'00101100'
MOVWF ADCON2,0
MOVLW 0X01
MOVWF ADCON0,0
MOVLW 0XFF
MOVWF TRISB,0
MOVWF PORTA,0
CLRF PORTB
movlw 0x00
MOVWF TRISD,0
MOVLW B'00001111'
MOVWF PORTD,0
BCF PORTD,5,0
BCF PORTD,6,0
BTFSS PORTB,0
GOTO neg
GOTO pos
pos BCF PORTD,5,0
CALL delay1,1
BSF PORTD,5,0
CALL delay2,1
BCF PORTD,5,0
L7 BTFSC PORTB,0
GOTO L7
17
goto neg
neg
BCF PORTD,6,0
call delay1,1
BSF PORTD,6,0
call delay2,1
BCF PORTD,6,0
L6 BTFSS PORTB,0
GOTO L6
goto pos
delay1
BSF ADCON0,1,0
L12 BTFSC ADCON0,1,0
GOTO L12
MOVF ADRESH,0,0
L5 NOP
DECFSZ WREG,0
GOTO L5
18
RETURN
delay2
movlw 0x10
L11 DECFSZ WREG,0
GOTO L11
RETURN
;******************************************************************************
;End of program
END
10. REFERENCES
1. PIC18F4550 datasheet
2. LM 358 datasheet
3. P.S Bimbhra Power Electronics
By Vipin A M, Natiional Institute of Technology Calicut
Email:vipin.am@gmail.com
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