unable to trigger BT136 with MOC3021 (lamp dimmer)

Thread Starter


Joined Oct 14, 2008
I have started building a lamp dimmer circuit.
Using the zero crossing point information the micro controller is programmed to trigger the triac.
Everything is fine upto the zcd and programming part.
After connecting the triac and a lamp (230v, 15W), it didnt work.

I tried many methods to solve.....

1. I wanted to test the circuit with low voltage. I used a 230/12v transformer instead of direct 230v. The load is 1K ohm. RESULT: No output across 1K.

2. Same circuit, MT1 and MT2 are interchanged...... now its working.....I am able to control the average value of the 12v sine with firing angle (POT connected to ARDUINO)


3. Now the 12v and 1K setup is changed to 230v, 15W bulb.....RESULT: The lamp continuously glowing. In the oscilloscope the output(across the almp) is showing such a way that the firing angle is around 10 degrees.


4. I turned off the arduino and removed the MOC3021 from the circuit. Still the output remains same.


5. Irrespective of the gate control, the TRIAC keeps on conducting at every 10 degrees.


6. I changed the gate resistor positions.......swapped phase neutral connections......lamp positions (fromMT1 to MT2)...... but NO USE.

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Why the TRIAC is working good for 12v, 1K and not for 230v, 15W lamp?

Relevant schematics are attached....sorry for the missing oscilloscope results.....the results and schema are hand drawn..... i can attach them tomorrow.


Joined Jun 22, 2012
put the bulb in the mt2 side,
have you put a 10k resistor from the gate to mt1, to turn off the triac, also does the mains light come on with the moc removed.