bad behaviour of triac

[lok]

Hi all,

I need to control electrical devices(like bulb,fan...etc) in home with Triac (BT136). I got a circuit from net


when control pin from microcontroller is low the device should be off right? but in my case when I switch on the main switch for the bulb without turn on it by controller, bulb is blinking for the first time. What is the problem Iam not able to understand the trouble.

 

[Ogu Reginald]

The microcontroller should be used to switch ON and OFF the input of the optical isolator which triggers the gate of the triac.

 

[lok]

The microcontroller should be used to switch ON and OFF the input of the optical isolator which triggers the gate of the triac.

Iam connected the bulb through a mechanical switch. bulb blinks when mechanical switching occurs even if the opto isolator is in OFF state

 

[red_alert]

Your schematic is wrong (you missed some key components). You may encounter parasitic triac turn-on when switching on the mains.

This is the complete schematic from the MOC3021 datasheet:

 

[lok]

thanks for the reply..I need to know is it working as a low pass filter (I mean the 360 ohm resistor and 0.05 capacitor)??

 

[red_alert]

In your schematic, triac's A2 (MT2) terminal is floating? (the image is incomplete).

Yes, it looks like a low pass filter but a simple resistor between pin 6 and MT2 should also work.

Being a random phase driver, it might turn on the triac at anytime so EMI could occur.

 

[FvM]

The standard MOC30xx circuit shown by red_alert is comprised of a RC circuit and a series resistor. The RC cicruit can be understood as a low pass or a snubber, in any case it limits dV/dt for the main and the opto triac. As said it's necessary to prevent triggering by voltage transients, at least with some loads and at sites with unpleasant mains waveforms.

The series current limiting resistor is also required to keep the triac gate current and opto coupler current within rated values.