Electrical – BT136 TRIAC not closing on LOW signal to the MOC3021


I am trying to make an AC dimmer. I am following this schematics for my circuit,

Circuit Link

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as you can see in the below picture that I am able to turn the signal on after a short while of zero voltage

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The blue signal is from my micro- controller (I'm using a PIC18f45 instead of an arduino) to the moc3021 and yellow signal is from the 12 V AC (I am trying to control 220V AC but this oscilloscope can't seem to read higher voltage so this is just to show that I am able to send a signal at zero voltage)

I am not able to dim the light buld. Even with that setup the bulb is still very bright. I tried shortening and lengthening the HIGH signal to the MOC but it doesn't seem to affect it

From what I have understood the moc3021 is a random phase triac driver meaning that I should be able to open and close the gate of at any time of the wall AC phase. The triac should have turned off when the signal turned low but instead the bulb is still very bright.

Am I missing a concept here ? what should I do for it to turn off when the signal goes low?

Best Answer

You are missing the concept here, yes. Triacs (and SCRs) have a strong positive feedback effect internally: they can be turned on with gate current, but not off.

To regulate power to a light bulb, you need to change when the gate is turned on -- the later in the half-cycle that you turn on the gate, the less power the bulb gets, and the dimmer it gets.

Note that this all may not work with LED and CFL bulbs -- I've kind of lost track of the magic that they put in between the base and the active elements, but I know they don't necessarily like dimmers.