Fully Controlled Rectifier

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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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


capacitor reduces the ripple in output voltage waveform.

ntrol circuit consists of pic18F4550 microcontroller, LM 358 op-amp IC


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


amp IC an accurate


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


coupler – provide isolation for PIC18F.


5. PROGRAMME FLOW CHART

6

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

10

Fig.10 input voltage and triggering pulse from PIC18F4550

11

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

;******************************************************************************

;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

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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

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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:[email protected]

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Fully Controlled Rectifier