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When triac is fired a 12VAC dimmer has distortions!

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12VAC dimmer problem

I make a 12Vac dimmer useing hotlek mcu to drive the triac

the circuits are in the attached. . . .
i get problem when i get fire to the triac then the 12Vac was distorted.
And it casse me cannot detect the zero crossing ...

anyone can help thx!!

normal 12V ac when triac was not fired
DSC03271.jpg


distorted 12Vac when triac was fired
DSC03270.jpg
[/img]
 

Re: 12VAC dimmer problem

The basic problem is, that you can't control a triac with galvanic coupling across a bridge rectifier. Just consider the voltage difference between L and DC ground in both half periods and you'll understand, why it can't work.

It can work either with isolation (optocoupler, separated windings) or a half-wave rectifier. The strange waveforms are also caused by this problem, I think. Generally, you don't see a zero crossing, only a voltage minimum at the zero crossing terminal and it has to be loaded by a resistor connected to ground.
 

Re: 12VAC dimmer problem

that mean i cant use the bridge rectifier????
 

Re: 12VAC dimmer problem

You can use it, but only with isolation (or another special circuit means) for the gate drive. Cause the triac A1 or MT1 isn't (and can't be) connected to DC ground.
 

12VAC dimmer problem

thx for your reply

and if i use half bridge
and with MT1 connect to ground ,

then do it mean only 50% of AC can be use?

the efficiency reduce by 50%?
 

12VAC dimmer problem

Yes. And you have to double the power supply filter capacitor. But cause the purpose is to control an AC load from the same supply, I guess, transformer current capacity respectively effiency isn't a limiting factor in this application.
 

12VAC dimmer problem

transformer current capacity respectively effiency ?
i dont understand ....

can you elaborate more?

thx for your suggestion my half bridge now work
but i wonder the efficiency problem~
 

Re: 12VAC dimmer problem

but i wonder the efficiency problem
Click to expand...
Do you experience an efficiency problem in your application? I guess, you don't. That's it.
 

12VAC dimmer problem

thx for you reply again??

"Do you experience an efficiency problem in your application? I guess, you don't. That's it. "

Sorry i still dont understand about it,

i have some questions
1.1)now i use half bridge instead of full bridge to work with the dimmer, that mean half of the AC is wasted ? so the efficency is reduced by 50%?
am i correct?

1.2)And if the triac is used with half bridge?
then why dont i use SCR instead of the triac?
both can deal with AC
now triac with one directional = function of the SCR
am i right?

2) for the triac, in the datasheet it have quadrant to deal with, if i use a MCU I/O to drive it, then only
quadrant 1 and 4 can use ? right?
in which case i can output a negative pulse to the drive??

thx a lot ~
 

Re: 12VAC dimmer problem

I see, that I possibly misunderstood your circuit, that wasn't shown completely up to now.

From the L and N marking at the triac part I understood that the triac and load have been connected directly to the AC supply. The rectifier would supply the control circuit only. I thought of this circuit, cause it's the only plausible solution to my opinion, avoiding rectifier losses from the load current. As you have talking of a 12 VAC dimmer, I couldn't imagine, that you apparently designed a DC dimmer.

The only reason, to connect the triac behind a rectifier would be a DC load, e. g. a motor. I suggest to clarify your requirements first.
 

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