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PHASE-ANGLE CONTROL OF SCR USING AT89C51 Silicon-controlled rectifiers (SCR) are solidstate semiconductor devices that are usually used in power switching circuits. SCR controls the output signal by switching it ‘on’ or ‘off’ thereby controlling the power to the load in conte!t. "he two primary modes of SCR control are phase-angle fired-where a partial waveform is passed passed to regulate the power. #n the phase-angle controller the firing pulse is delayed to turn on the SCR in middle of every half cycle. "his means that every time a part of an $C cycle is cut the power to the load also gets cut. "o deliver more or less power to the load the power to the load also gets cut. "o deliver more or less power to the load. "he phase angle is increased or decreased thereby controlling the throughput power. "here are several ways to control the firing angle of SCR. "his article ar ticle %esc %escri ribe bess a micr microc ocon ontr trol olle lerr $" &'C &'C-ba base sedd-an angl glee cont contro roll ller er.. $ microcontroller can be programmed to fire SCR over the full range of half cycles from * to &*+ - to get a good linear relationship between the phase angle and the delivered output power. ,asi asicall cally y the ero ero-c -cro rosssing sing dete etecto ctor circu rcuit int interru errup pts the the mic microc rocontr ontro olle ller afte afterr ever every y * ms. "his "his int interru errupt pt com command mandss the the microcontro microcontroller ller to generate generate some delay delay (in the range of ms to 'ms). "he user can increase or decrease the delay in intervals of ms using switches. "he SCR is then fired through through the opto-coup opto-coupler. ler. "his repeats after every * ms. C#RC#" %/SCR#0"#12 "he complete circuit is divided into two sections3 . "he ero ero-cr -cross oss dete detecto ctorr section section 4. "he "he cont contro roll sect sectio ion n "he ero-cross detector section. 5ig. shows the circuit diagram of the erocrossing detector and the power supply. supply. "he main section section of the circuit are a rectifier rectifier regulated power supply supply and ero-cross ero-crossing ing detector. detector. "he 46*7 $C mains is stepped down by transformer 8 to deliver the secondary output of
'7 **m$. "he transformer output of 'v 6**m$. "he transformer output is rectified by a full-wave bridge rectifier comprising diodes % through %9 $nd then regulated by #C :&* (ic6). Capacitors C4 and C6 are used forpassing the ripples present in the regulated 7 power supply. $ capacitor above *;5 is connected across the output of the regulator #C while diode %< protects the regulator #C in case their input is short to ground. =/% acts as the 0ower-on indicator and resistor R limits the current through =/%. "his regulated 7 is also used as biasing voltage for both transistors ("and "4) and the control section. $ pulsating %C voltage is applied to the base of transistor " through diode % and resistors R and R4. >hen the pulsating voltage goes to ero the collector of transistor " goes high. "his is used for detecting the pulse when the voltage is ero. 5inally the detected pulse from ‘C’ is fed to the microcontroller of the control section. "he control section. 5ig 4 shows the circuit diagram of the control section for the phase-angle control of SCR. #t comprises a microcontroller $"&'C opto-coupler ?C"4/ =/% module and a few discrete components. 0ort * (0*.* though 0*.:) of $"&'C is used for interfacing data input pins %* through %: of the =C% module. 0ort pins 04.< 04. and 04.: of the microcontroller control the registers select (RS) read@write (R@>) and enable (/) input pin of the =/% module respectively. 0reset 7R is used for controlling the contrast of the =C% module. 0ush Ato-on switches S S4 and S6 are connected with the pins 0.* 0. and 0.4 through diodes %' %* and % respectively. /!ternal interrupt pin #2"*(06.4) of the microcontroller is connected to SS4 and S6 "BR1B d4 %6 and %9 respectively. "he role of different switches is shown in "able . "he output of the ero-crossing detector from ‘C’ is fed to the e!ternal interrupt pin #2"(06.6) of the microcontroller. 0ort pin 04.* is connected with pin 4 of the opt-coupler(?C"4/)."he output pin of ?C"4/ is used for triggering the gate of SCR "D2<*9. "he $node of SCR #S C122/C"/% "1 "B/ =1$% (,=,) >#"B "B/ 46*v $C supply.
$ 4?B crystal along with capacitors C and C9 are connected to the microcontroller pins & and ' to provide the basic clocE to the microcontroller. 0ower on reset is derived by using capacitor C< and resistor R<. Switch S9 is used for a manual reset. "he operation "he complete operation can be well understood with the help of waveform in 5ig.6. . "he waveform at point ‘$’ is a fully rectified wave that is fed to the base of ". 4. >hen the base voltage falls below *.:7 transistor " is switched off pulling the output higher. "his results in a very short positive pulse which is available at the collector (at point ‘,’) as shown in the second waveform. 6. $s this positive pulse is inverted by transistor "4 it produces one negative pulse of the same width at ‘C’. "his is shown as the thied >aveform. 9. "his negative pulse is fed to the interrupt pin (#2") of the microcontroller which acts as an interrupt for the microcontroller. "he microcontroller then generates a positive pulse on 04.* (at point ‘%’) after some delay. "his turns ‘off’ the internal =/% of the optocoupler (?C"4/) $2% $ 01S#"#7/ 0=S/ #S 0R1%C/% $" 1"0" ‘/’. "his is used for triggering (fire) SCR. . %epending on the time delay in between the interrupt and the pulse on port pin 04.* of the microcontroller the SCR is fired in the middle of the half-wave cycle. <. "wo different waveforms one for 9 ms delay and the other for & ms delay A are shown in fig.6. #n the case of 9ms delay the output positive cycle of the $C wave is <* per cent of the input. "herefore nearly <* per cent of the power is delivered to the load (the dotted line shows part of waveform that has been cut). #n the second case of & ms delay the output cycle is 4* per cent of the input cycle so only 4* per cent of the power is delivered to the load.
"his change in delay is done using switches S and S4. %ifferent =/%s are used for indicating different functions as shown in "able ##. "he diodes %4 through %9 are connected in such a manner that whenever any of the three push-to-on switches are pressed it generates an e!ternal interrupt #2"*. >hen switch S is pressed for the first time it enables e!ternal interrupt #2" and displays the message ‘SCR on.’ So after every * ms e!ternal interrupt * and the message ‘SCR on.’ So after every *ms e!ternal interrupt #2" is generated which starts the entire operation. 0ressing switch S again disables e!ternal interrupt * and the message ‘SCR off’ is displayed. "he complete SCR operation gets shut off. 1n pressing S4 the delay increases by ms (firing angle will shift by &+) and firing of SCR is delayed by ms. "he power delivered to the load is also decreased by * per cent. "he ma!imum delay that can be applied is ' ms which will delay firing by an angle of <4+.>hen the limit is reached it is indicated by =/%6 and a message ‘?a!. phase angle’ is displayed on the =C%. "he glowing of the bulb goes off. Similarly when S6 is pressed the delay is decreased by ms and the load current increases by * percent. "he minimum delay is * ms which means a full positive cycle is applied. Bowever when the limit is reached it is indicated by =/%9 and a message and a message ‘?in phase angle’ is displayed. $n actual-sie single-side 0C, for phase-angle control using SCR is Shown in 5ig. 9 and its component layout in 5ig..
Software program "he software code for this proFect is written in ‘C’ programming language and compiled using the Geil ;7ision6 compiler. $fter compilation the final he! code is downloaded to the microcontroller using a suitable programmer. "he source program is well commented and easy to understand. "he main function initialies the timer ports and =C%. 5inally after enabling the e!ternal interrupt function and is automatically called when any of the three switches S through S6 is presses. . #f switch S is pressed it checEs if it is pressed for an even@odd number of times. $ccordingly it either switches ‘on’ or switches ‘off’ the SCR. ,asically it enables@disables e!ternal interrupt . "he state of the SCR is displayed by a message on the =C% and an indication comes on =/% $2% =/%4 $=S1. 4. #f switch S4 is pressed the delay is increased by & . "he light intensity of the bulb also increases. # f the limit is reached the message is displayed on the =C%. 6. 5or switch S6 the operation remains the same as with S4 but the delay is decreased by ms and the angle is decreased by &*+. #nt function is also an interrupt function and is automatically called when the ero Acrossing detector gives the pulse after every *ms. #t feeds one pulse to the gate of the SCR after the desired delay (set by switch S4 and S6). "he pulse applied is indicated on =/%. >ritedata function sends the command byte to the =C%. #t taEes one argument byte and sends it to port 0. >ritestr function writes a whole string (message) on the =C%. #t taEes the pointer as an argument that points the address of the first character of the string. "hen through the pointer it sends all the characters one by one to port 0*. ,usy function checEs the status of the busy flag of the =C%. #f the flag is set it means the =C% is not ready and the programs remain within the loop. >hen the flag is reset it means the =C% is ready and the program comes out of the loop.
Geydly function used for Eey debouncing is the fi! delay by appro!imately **ms. %elay function is a variable delay generated by timer *. "he basic delay is of ms which is rotated in the loop from to ' times to generate a minimum of ms and a ma!imum of ' ms delay. %isplay function separates each digit of the angle and converts them into an eHuivalent $SC## number before sending it to the =C% one by one for display.
TALE I ROLE OF !IFFERENT S"ITCHES Switch S S4 S6
5unction "o switch SCR on@off #ncrease delay by sec %ecrease delay by sec
TALE II CHANGE IN !ELA# USING PUSH-TO-ON S"ITCHES =/% =/% =/%4 =/%6 =/% 9
#2%#C$"#12 SCR 12 SCR 155 ,linEs when delay is increased ,linEs when delay is decreased
5ig. 3 0ower supply and ero-crossing detector circuits
5ig 43 Circuit diagram of phase angle control of SCR using $"&'C
5ig. 63 >aveforms observed at various point in 5ig. and 5ig. 4 and SCR output waveforms
5ig. 93 $ctual-sie single-side 0C, for the phase angle control of SCR using $"&'C
