# TRIAC Circuit Explanation

Discussion in 'The Projects Forum' started by Randa Mohamed, Aug 27, 2015.

1. ### Randa Mohamed Thread Starter New Member

Jun 19, 2015
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0
Hello everyone

I have a project where im using TRIACs to switch several AC voltages from a transformer.
I found the following circuit and I need help in explaining how it works assuming that LIVE and NEUTRAL are from the transformer.

2. ### Lestraveled Well-Known Member

May 19, 2014
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1,217
Your circuit uses a PIC micro controller. It has to be programmed. Do you have the program and a programmer?

3. ### Randa Mohamed Thread Starter New Member

Jun 19, 2015
25
0
Actually i don't want to use this PIC. My plan is to control the TRIAC via an opto-coupler. I'm concerned about the TRIAC and transformer connections. I'm interested in TRIAC functionality. Please explain to me how it should work in such case. Do I need to use a zero crossing detection circuit ?

4. ### ian field Distinguished Member

Oct 27, 2012
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791
You need the right kind of opto-coupler with AC output (opto diac) connected from gate to the MT furthest from the gate via a current limiting resistor - the value could be 100 - 180R depending on the mains voltage.

Jul 18, 2013
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Look up Fairchild App note AN-3006.
Shows the use of Zero crossing and random phase drivers and opto's.
Max.

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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How are you going to sense the Zero crossing, can you not use a simple triac dimmer circuit....?

7. ### Randa Mohamed Thread Starter New Member

Jun 19, 2015
25
0

@MaxHeadRoom Looking at the Fairchild App was very helpful and it clarified some concepts. @ian field I think the below circuit is what you were talking about.
I have some questions regarding this circuit from the app note a3006

Is the snubber RC is for the triac only or do i need another snubber to protect the optocoupler. Why they have connected the snubber circuit that way? Why not use an RCD snubber ? And how they got the values for the RC circuit ? do these values depend on the load only and how is that ?

[/QUOTE]

@Dodgydave I'm currently trying to design a voltage zero crossing circuit to turn the triac on and a current zero crossing circuit to turn it off. I'm planning to use comparators.

Jul 18, 2013
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The snubber is used to prevent spurious switching noise on the AC line, however with zero cross turn on and off type switching it may not be relevant.
Max.

9. ### ian field Distinguished Member

Oct 27, 2012
4,447
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Wherever they come from - transients can unintentionally trigger a triac, they have a Dv/Dt specification stated in the datasheet.

The marketplace has had for some time - "snubberless triacs". Probably worth a look.

10. ### Lestraveled Well-Known Member

May 19, 2014
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Here is a simple zero crossing circuit I like. It uses a bi-polar opto-isolator. You can use two normal opto-isolators in parallel and get the same result.

Are you looking for just on/off, or are you looking for a dimming function?

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11. ### ian field Distinguished Member

Oct 27, 2012
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791
Those AC input optocouplers were fairly common in modems.

Look for any PC internal modem cards that don't have a bridge rectifier in the input section.

Jul 18, 2013
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The OP mentions full phase on/off zero-switching, no phase angle control.
You need a sync circuit as per the PDF when using a Pic.
The MOC316x are zero crossing opto's.
Max.

Last edited: Sep 4, 2015
13. ### Randa Mohamed Thread Starter New Member

Jun 19, 2015
25
0
From my research the on/off switching noise depends on the load, if the load is resistive then the voltage and current are in phase so having a voltage or current zero crossing circuit is enough. My aim is to switch on/off an inductive load where the voltage and current aren't in phase. Basically the triac should switch on at zero volts and switch off at zero amps.

@ian field First time to hear about "snubberless triacs" I'll research about

Well this circuit should do, my aim was to convert the voltage and current signals into squarewaves using a comparator then detect the falling edge part using an xor gate, i think its gonna be more precise.