i am asked to make a module for digital firing angle for thyristors.i intend to do so using zero-crossing detector and 555 timer.plz do give me ur useful suggestions.thnx

 

Once detected the zero crossing you activate the 555 delay and when the delay elapsed the circuit fires the triac. Remember that delay should be from 0 up to 10 millisecs max with a 50 Hz main or 0 up to 8 millisecs for 60 Hz main.

At delay = 0 your duty cycle will be 100%, while at 10 millisecs your duty cycle will be zero. So if you want to devide by 100 you will need increments of 100 microsecs in your delay system (555).

Alberto

 

Since the trigger pulse has to be referenced to the input of the TRIAC, you will need an optoisolator to trigger the device safely. Here is a link to one optotriac - http://www.datasheetcatalog.com/datasheets_pdf/M/O/C/3/MOC3020-M.shtml

 

thank you all for your precious time & suggestions.for the aforementioned project,i need to interface:
1)zero-crossing detector
2)555 timer
3)optocoupler
4)thyristor

how should i proceed with designing and simulation of this on proteus?any circuit diagram/helping material?

 

I'm not sure what you mean by "digital", a 555 is an analog device.

What is the digital requirement?

 

@thatoneguy:i have to design an alternative to microcontroller based digital firing angle.say,if the input=0v(logic0),it detects the zero-crossing.then after a certain delay,when the input jumps to 5v(logic1)....thyristor is fired!!!

 

@thatoneguy:i have to design an alternative to microcontroller based digital firing angle.say,if the input=0v(logic0),it detects the zero-crossing.then after a certain delay,when the input jumps to 5v(logic1)....thyristor is fired!!!

So while the input is low nothing happens, when the input is high, the chopper(triac cutting up sine wave) is active at a preset xx mS delay, or is the delay adjusted digitally as well?

 

delay adjusted digitally as well.

 

Hello,

What do you want, digital or analog?
A microcontroler is digital, a 555 is analog.

Bertus

 

Is this a homework assignment?

What is the exact need, so useful information can be given.

A Digital solution would use a microcontroller (or a lot of logic ICs), a transformer, regulator, and opto-isolated Triac, and various passive components.

 

i have to design an alternative to microcontroller based digital firing angle.say,if the input=0v(logic0),it detects the zero-crossing.then after a certain delay,when the input jumps to 5v(logic1)....thyristor is fired!!!

makleo, you are making some confusion. Detecting the zero crossing is not a digital function but an analog function, since you are exploring the sin wave hunting for the zero value. You will generally use a comparator, a bridge rectifier and a transformer for this purpose. Now if your circuit works you will see on your scope a pulse rising every time the sin wave cross the zero. Now, to dim your lamp (mind you got to be a resistive load) you need to delay the firing of the triac and for this purpose you have suggested to use a 555, (here again an analog component) setting up the proper RC you will handle the required delay to obtain your dimmer.

So you see that there is no digital staff but only analog. If you want a digital dimmer then you will need to use a microcontroller or digital ICs but this is another story

Alberto

 

One point, Alberto's comment that a 10 ms delay corresponds to 0% applies to 50 Hz systems only. For 60 Hz systems, a delay of 8.33 ms corresponds to 0%.

 

hnx 2 all 4 their time.let me clear one thing.we(group of 4) were askd to make a 'digital firing angle'.......actually we do not have the code for microcontroller 8051.so,we decided to make analog firing angle using:
1)zero-detector
2)555 timer
3)thyristors
nw plz tell me how 2 proceed.i've simulated zero-detector on Proteus.project has to be submitted on PCB.the issue is to generate adjustable delay via 555 & then fire the gate of thyristor.i am sure the ambiguity is over now.

 

What you are making is not a digital solution, but if the prof approves of your method, your solution is workable.

ZCD starts one shot timer which triggers triac at xx.xx mS. This would allow chopping at any point in the phase.

What frequency mains is this for?
Is the timer/phase adjustment via potentiometer OK as it is an analog solution to start with?

 

let me clear the ambiguity.i need to write code in C for 8051.using time delay and keypad interfacing,design a digital firing angle 4 thyristors...........i've simulated the zero-detection on proteus.plz explain how to proceed further.thnx a lot

 

Is there a reason you can't use the microcontroller for the delay and control the thyristors directly/through an optocoupler?

Using a 555 for this I guess has us confused.

 

Makelo u can sonsult to Dr.Habshi Najashi for programming purposes........

 

i hope the confusion is over now........8051 with keypad and LCD interfacing is used for delay.........optocoupler 4 firing thyristors.i think it'll be put together as follows:
"AC input(60Hz)--->zero-detector--->8051microcontroller--->optocoupler--->thyristor"....
isn't it?time 2 simulate it on proteus................!

 

i am waiting for your urgent reply...........time running out!!!

 

i hope the confusion is over now........8051 with keypad and LCD interfacing is used for delay.........optocoupler 4 firing thyristors.i think it'll be put together as follows:
"AC input(60Hz)--->zero-detector--->8051microcontroller--->optocoupler--->thyristor"....
isn't it?time 2 simulate it on proteus................!

Yes, that is a way to do it, you only need to write the code to provide the input of delay, then timers for triggering the triac, it's mostly software/programming work from here.