# When to use a non-zero crossing opto-TRIAC and when a zero-crossing opto-TRIAC?

I have two simple questions.

From what I know, if we want to use an opto-TRIAC (with another TRIAC of course) to control an AC motor, if we want to use it as an ON/OFF switch, then I have to go for a "zero crossing opto-TRIAC" right? From what I know, this kind of opto-TRIAC doesn't not trigger the TRIAC if the AC voltage is high and only triggers it when it is a light signal in the IR LED and there is a zero crossing, so for ON/OFF aplications of AC loads this kind of optocoupler is the best, right?

A "non zero crossing optotriac" triggers the triac independant of the level of AC voltage, right? If there is an IR LED ON this will trigger the opto-TRIAC (almost) at the same time and it does not care if there is an AC peak, right? If this is right, a non zero crossing opto-TRIAC would be perfect for a phase control application like a dimmer, right?

If what I said is correct, can you help me to understand this image?

I found it here.

The image for the zero crossing opto-TRIAC makes sense to me according what I said, but the image for the non-zero crossing opto-TRIAC doesn't make sense to me if we are talking about a resistive load. Is there an error in the graph of the voltage on the load?

• I agree to your understanding of the two versions. The drawn phase shift between voltage and current in the non-zero-cross version seems to be a bug, this is (more or less) a diagram of an inductive load.
– Jens
Apr 17 at 21:07
• Thanks man! appreciate it Apr 17 at 23:00

You are correct and the image is wrong. It should look like this:

Figure 1. The (roughly) corrected diagram.

• The load current trace for a resistive load will be in-phase with the voltage while the triac is on. It will turn on with the LED and turn of at the next zero-cross after the LED is turned off. This is as you suspected.
• With the triac off full mains voltage will appear across it via the load resistance. When the triac is on the voltage across it will just be the forward voltage drop of the triac.
• Thanks dude! now its clear and thanks for fixing the image ! Apr 17 at 23:00
• For highly inductive loads, a peak-firing circuit is preferable to zero-crossing. But I have not found any peak-firing opto-triacs, so you need a non-zero-cross type with extra circuitry to implement peak firing. Apr 17 at 23:25
• I didnt know about that @PStechPaul thanks man, ill give it a read, thanks man! Apr 18 at 4:14
• Switching an AC voltage into an inductive load at the zero crossing causes a phenomenon known as DC offset. This can cause a transient current twice as large as the steady state current. You may experience this when a microwave oven turns on and it makes a noise for a few seconds as the DC offset dissipates (although it may also be from an initial current surge). For a deeper explanation: relaytraining.com/what-is-dc-offset-ask-chris Apr 18 at 4:45
• @CblueX, you might be interested in what I've written about the internals of a zero-cross SSR: lednique.com/tag/zero-cross. Apr 18 at 5:43