[SOLVED] Triac false triggering

Hi

I am using a triac and also a relay to control loads from a microcontroller. In the circuit shown both A and B are connected to microcontroller. The problem is, when the triac is in OFF state, if relay is used or any other switch in my home is operated, the bulb connected to the triac flashes once. Kindly tell the reason for this if anyone are aware. Also suggest me the design parameters to be considered to avoid this.

It is called rate of voltage rise dv/dt.

Is your bulb is an incandescent one?

A small capacitor (0.1µ ceramic) across the anode to cathode of the triac may help.

Hemaditya said:

if relay is used or any other switch in my home is operated, the bulb connected to the triac flashes once

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Please, tell me that the 5V suppying the relay coil is NOT the same used to power the microcontroller.

andre_teprom said:

Please, tell me that the 5V suppying the relay coil is NOT the same used to power the microcontroller.

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Why is that?
The relay coil has diode suppression so should not interfere with the microcontroller operation.

I agree that it should not interfere, but if you probe the 5v bus with a scope meter, certainly will see a substantial voltage spike there at realy comutation during de-energize. In addition, it was not clear whether Hemaditya used a decoupling capacitor close to uC or not. Note that some cores are very sensitive, or can be configured with high brownout drop values. Anyway, as you mentioned above, the insertion of a snubber circuit should be the first action to do.

Hemaditya said:

Hi

I am using a triac and also a relay to control loads from a microcontroller. In the circuit shown both A and B are connected to microcontroller. The problem is, when the triac is in OFF state, if relay is used or any other switch in my home is operated, the bulb connected to the triac flashes once. Kindly tell the reason for this if anyone are aware. Also suggest me the design parameters to be considered to avoid this.

View attachment 125264

Click to expand...

Your controller voltage is going down the Required 5v.
use two power supply to check this.
May be relay needs a very high current to operate so the controller +5v gets down which impact the Light to be flashed.

Hi everyone

@ schmitt trigger - I tried incadescent, CFL and also florescent tube with electronic ballast. All face the same problem,except florescent tube with ordinary ballast because it take time to turn on.

@crutscow - What do you mean by anode and cathode of triac? since there are only MT1 , MT2 and gate. do you mean between MT1 and MT2?

It is definetely not because of the controller, because i have tried without the controller by driving the optocoupler directly with 5v.

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Hi everyone

I have done some reading on dv/dt. High dv/dt also turns ON a triac. I think relay switching action is generating spike on the mains which is turning on the triac. as crutscow said i have placed a capacitor in series with a resistor across MT2 and MT1. this solved the problem.

Thank you all.

Hemaditya said:

as crutscow said i have placed a capacitor in series with a resistor across MT2 and MT1. this solved the problem

Click to expand...

Despite it solved the problem, it is important to add a resistor in series with that capacitor in order to prevent long term stress due to the lack of something to limit the surge current across it. Anyway, if you are dealing with a load under rated with the TRIAC specifications, this is not absolutely an issue.