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Digital AC Light Dimmer Design

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Junior Member level 3
Jun 18, 2015
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Dear All,

I' trying to implement a digitally controlled light dimmer.

I started from here

Digital Light Dimmer.jpg

I construct the same circuit with TIC206D Triac.

I faced a problem for the 1K resistor feeding the Triac gate current it was burned once I started the circuit. I increased the (1/4) Watt to (1/2) Watt and even 1 Watt but all got burned too!

I started a trial testing for resistor values to replace it until I could run the circuit without having the resistor burned at a resistor value of 15 Ohms. In this case I found the gate current is 155mA so the resistor dissipated power is 0.432 Watt so I think using 1 Watt resistor will be OK. I already run the circuit like this for about an hour without a problem although I noticed the Triac and 15 ohm resistor are getting heated too much. But no burns of fires!

My first question is this setup is OK and this circuit can be used for long run and stable?

My second problem is that I can trigger the Triac and switch it on based on the signal from Arduino on the positive AC half cycle correctly but once turned on it continues on the remaining half cycle of the positive and the whole negative half cycle too and switch off only at the beginning of the new cycle. What I expect is the Triac should be turned off at every zero crossing of the AC sine wave i.e. twice per cycle once when changing from positive to negative and once when changing from negative to positive but it does on zero crossing on one direction only. At the end I can control the firing angle correctly for one half of the AC cycle only but the other half is totally passed to the load. I have checked the signal from my Arduino and it is perfect and being sent in both half cycles of the AC wave.

Appreciate if anybody could help me to solve this problem and get a stable working circuit.

Ehab Zaky

From the MOC3021datasheet, that resistor should be around 180 ohms. You could check the datasheet for further circuit design tips.

What's the pulse duration from your Arduino? I hope it doesn't pass the next zero crossing point.

Thank you very much for your help.

I have checked 180Ω Resistor value 1 Watt and it got burned too!

I could measure the current passing through it in little seconds before it gone. It was around 125mA so I think the resistor power should be 2.8125 Watt. I don't think this is normal power rating. Correct me if I'm mistaken please.

For the pulse width it is very small compared to the cycle. I set it to 100 Micro Seconds.

I have attached a screen shot from my oscilloscope to show the case in clearer. It shows the pulse from Arduino and the resulted waveform.


Ehab Zaky

You may try the suggestions presented in the following threads:

Triac/Optoisolator combo - why won't it turn off?

Problem with MOC3011 & BT136

Your triac is a gate sensitive one so it could easily encounter false triggering due to capacitive coupling, dV/dt and such. One possible solution is to put a resistor between gate and M1 terminal, to ensure that the gate signal is OFF when the triac is not conducting. You could also use a simple RC snubber across the triac (even you're not dealing with an inductive load).

- - - Updated - - -

That's from the datasheet:


Thank you very much for your support.

Unfortunately I could not have it to run at all. I tried a resistor between M1 and gate started from 100K till short circuit no change. I tried also the circuit from datasheet not working too. I tried also a capacitor across M1 and M2 but this also not working.

After all I decided to change the triac itself. I used BTA12-600b which worked perfectly! I could use the 1K resistor as per the original circuit with no heat on both triac and gate resistor like before. I think the TIC206D triac is either not suitable at all for this application or the items I have having some problem.

red_alert thanks again for your suggestions.

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