opamp circuit for power electronics

I have made a phase angle control thyristor circuit which ouputs full brightness for 0v input and lamp off for 5v input . I have made an opamp circuit to give 5v for 0v input and 0v for 5v input. So that i can include this circuit in between and have fullbrightness for 5v input and lamp off for 0v . every thing is working good but when i switch off the control circuit the lamp suddenly goes to full brightness before switching off. please share your view regarding this.

Thankyou
ramesh

Hi rameshdanas,

I guess (based on your description) that you are looking at a zero crossing event some how and are creating a delayed turn on pulse for the thyristor (delay = brightness control in sin wave). If this is the way you are doing the control then you need to be careful that as the deleay gets longer and you get closer to another zero cross event that you don't go over. In other words if the turn on pulse is wide enough you may be straddling the zero cross event and inadvertantly triggering the thyristor on the wrong side of the zero cross event - this would turn your thyristor on for the full next cycle (which is full bright). I have done a circuit like this myself and did have some trouble with the triggering of the thyristor on the wrong edge of the trigger pulse (falling as well as rising). Hope that helps and I haven't completely messed you up.


Regards


dfullmer