# Confused between 2 terminals of Triac

1. ## Confused between 2 terminals of Triac

Hai , iam building a AC dimmer circuit using Triac, When going through all the available circuits online, iam confused between 2 forms..

In some of the Circuit, the input AC Line is fed to Main terminal 2 of the Triac and in some other circuits Load is connected to main terminal 2 and Ac line is connected to terminal 1.

Which is the best possible result??

2. ## Re: Confused between 2 terminals of Triac

Both circuits have the opto triac between A2 and G, in so far, they are connecting the triac in the same way. Circuit 2 has an additional filter suppressing fast pulses that might trigger the opto triac.

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3. ## Re: Confused between 2 terminals of Triac

Originally Posted by FvM
Both circuits have the opto triac between A2 and G, in so far, they are connecting the triac in the same way. Circuit 2 has an additional filter suppressing fast pulses that might trigger the opto triac.
Oh,, i guess iam missing some knowledge here, Iam new to Triacs and analog circuit. But what confused me is in the diagrams above in the first one Load is connected to top part and input line is connected to bottom,, but in the next circuit it is vice versa.

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4. ## Re: Confused between 2 terminals of Triac

I see, I didn't pay attention to this point. It doesn't actually matter with an isolated trigger circuit. Triac is a bidirectional switch, either A1 or A2 can be connected to the input side.

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5. ## Re: Confused between 2 terminals of Triac

Originally Posted by FvM
I see, I didn't pay attention to this point. It doesn't actually matter with an isolated trigger circuit. Triac is a bidirectional switch, either A1 or A2 can be connected to the input side.
Yeah i read the same thing.. But i was learning more about triacs and somewhere i read that the order we choose plays a big role, And that the firing has to come from source connected to terminal 2.
is there something like that ?

6. ## Re: Confused between 2 terminals of Triac

Hi,

With the given circuits you are in quadrant I and III.

There are triacs that need different drive current in different quadrants.
And there are triac not able to trigger in all quadrants.

--> You have to check datasheets individually.

Klaus

7. ## Re: Confused between 2 terminals of Triac

Originally Posted by KlausST
Hi,

With the given circuits you are in quadrant I and III.

There are triacs that need different drive current in different quadrants.
And there are triac not able to trigger in all quadrants.

--> You have to check datasheets individually.

Klaus
i am using a Triac Called BT138-600E.. let me check its datasheet

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It says it triggers in all 4 qudarants.. So iam good to go with any of the above circuit ?

8. ## Re: Confused between 2 terminals of Triac

Any triac can trigger in quadrant I and III, G-A1 voltage has same polarity as A2-A1. AC trigger circuits with opto triacs will always work.

The problem is only for quadrant IV, positive gate voltage and negative A2 voltage, which isn't supported by all triac types.

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9. ## Re: Confused between 2 terminals of Triac

You can use any on of terminal as incoming and another one as outgoing. Main specialty of TRIAC is bidirectional current flow

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