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I've been looking at this schematic for triac control by varying the gate pulse timing. The description states that the triac switches off automatically at zero crossing and when there is no gate pulse. Does this mean this a zero-crossing detecting triac driver? I looked up the datasheet for the triac listed there (MOC3052-M) and it states that the opto circuit is couple to a non-zero crossing triac driver. I'm a little confused.

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The description states that the triac switches off automatically at zero crossing and when there is no gate pulse. Does this mean this a zero-crossing detecting triac driver?

No. Triacs have the characteristic that, once triggered, they stay on until the current through them falls to zero (or very close). This makes them very easy to use in AC circuits because the voltage (and current)falls to zero every half-cycle. It also makes them almost useless in DC circuits - unless you want something to latch on until power is switched off.

The term "zero-cross triac driver" refers to when the triac is turned on.

See my answer to Confusion with TRIAC firing and zero crossing point for more details and some graphics on this subject and when to use zero-cross and not.

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  • \$\begingroup\$ Ah now I get it. Its the triac itself and not the driver. So how is an optocoupler such as the MOC3041 that has a zero crossing detection built into it differ from one that does not have this detection (especially since the detection is anyway done through the H11AA1)? \$\endgroup\$ – electrophile May 28 '16 at 15:14
  • \$\begingroup\$ See my answer to Using AC current to trigger triac. \$\endgroup\$ – Transistor May 28 '16 at 15:21
  • \$\begingroup\$ Yep I just read this and this leads me to my next Q: in the event the gate pulse is not fired close to zero but a little ahead in the half-cycle when the corresponding AC voltage across the triac is far above (or below) zero, would the triac still turn on and conduct? or it wont and that would be the whole point of a zero-crossing detection? \$\endgroup\$ – electrophile May 28 '16 at 15:27
  • \$\begingroup\$ See my answer to Activating SSR for an AC motor via PWM input. The OP was very confused about how the triac worked. "Q: in the event the gate pulse is not fired close to zero but a little ahead in the half-cycle ..." If the pulse is still on at zero-cross then the triac will fire. If the pulse has turned off the opto won't remember it so no, the triac won't turn on. \$\endgroup\$ – Transistor May 28 '16 at 15:32
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This is two different things. The zero crossing detection is happening with the the H11AA1 circuit, send pulse to "Dig Out". For each detection, the Arduino send another pulse using to the MOC3052 "Dig In", wich is just a optocoupler to protect the Arduino side, from the 220V power side. To adjust the power (Dimmer), you need to control the time (delay) after get a zero cross to send a pulse to MOC3052.

The schematic that you look, is similar to this one, wich can be easier to understand. http://cdn.instructables.com/FQZ/NYV7/H8CVG9TK/FQZNYV7H8CVG9TK.MEDIUM.jpg

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  • \$\begingroup\$ I'm aware of how the zero crossing is detected by the Arduino. And while there is a pulse to turn on the triac optocoupler there is no mechanism to ensure that the triac is off after a specified duration. This, as the articles states, is because the triac can detect a zero-crossing and automatically switches off when there is no gate pulse present. This makes it a zero-crossing detection driver similar to the MOC3041, however its datasheet states that its a non-zero detection triac driver. That is what I'm confused about. Also the schematic in the link cannot control inductive loads. \$\endgroup\$ – electrophile May 28 '16 at 14:32
  • \$\begingroup\$ A TRIAC, by its very nature, will turn off when the current through it gets small enough to kill the TRIAC's self-latching behavior, and isn't what's being referred to when "zero cross switching" is being talked about. What is being referred to is sending a pulse into the TRIAC's gate strong enough to turn on the TRIAC when the AC across MT1 and MT2 is very close to zero volts and traveling away from zero volts in either direction. \$\endgroup\$ – EM Fields May 28 '16 at 15:02

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