triac triggering circuit problem

[nikhilt007]

hi ,
i want to control bulb using pwm there is one switch connected to i/p pin mcu(pic16f877a)when i press on that butten my pwm should increase slowly and reaches maxim
these iam getting when i connected to led ,from the same pin iam connected to i/p of triac and triggerring bulb but bulb intensity is not increasing smoothly insteade of it its onle getting on when iam trigerring i also tested it with using interrupt pin to detect zero crossing its only working like these so please help me
thanks in advance

 

[Tahmid]

Take a look here: https://www.edaboard.com/threads/181497/

This should clear all your confusions.

Hope this helps.
Tahmid.

 

[FvM]

I understand that you are driving he triac directly from µC, so we don't need to discuss about different opto triac types involved in the design?

Triac control is phase angle control rather than PWM. According to the fact, that a triac can be only switched on and is staying on until the next zero crossing, the trigger pulses must have a specific timing related to zero crossing. Do you know how?

P.S.: Referring to the quoted link https://www.edaboard.com/threads/181497/ , it should be noted that the suggested "Circuit C" does not work. The trigger voltage has to be applied between G and MT1. In other words, an opto-triac needs to short MT2 to G, using the required series resistor and possibly RC filters.

AC trigger circuits (opto-triac, diac) are always operating in Quadrant I + III, gate and MT2 have the same voltage polarity related to MT1.

 

[nikhilt007]

hi iam tested all these iam getting just on and off when controlling through pwm(created from pic) and also with out pic iam triggered triac by connecting 0-5v as input to moc adjusting pot i am only getting just on and off is there is any chance to problem in ac voltage

 

[Tahmid]

Have you read the thread I referred to?

I assume the frequency of PWM is much greater than the line frequency. So, what happens is that the triac is triggered very soon after the zero-crossing and hence the light stays on for that entire cycle, and so on.

Remember that the triac latches once you've fired it on after the zero-crossing. So current flows through the triac (and hence through load) until the next zero-crossing.

 

[tejashs]

I dont think PWM is suitable for this purpose, you should try PPM along with zero crossing input.
if you must use PWM, your PWM time period must be atleast 5 to 10 times your supply period though it suspect it may cause flickering.