Why do we prefer zero-crossing optotriacs for driving triacs?

I usually see a special type of optotriac which has internal zero-crossing detection circuit in it. This type of optotriac is generally used for driving triacs. But, I don't understand its necessity. A normal optotriac can do the same driving job, couldn't it? Why is this zero-crossing triac preferred for driving triacs?

• Are you asking why we want zero crossinga at all, or why we don't do zero crossing detection externally? – PlasmaHH Mar 23 '15 at 11:02
• I am asking why we want it. – hkBattousai Mar 23 '15 at 11:03
• Because if it switches when the voltage is 0V, it switches virtually no current in resistive loads. That's better for interferences injected in the supply, otherwise it would be the same problem as switching converters. Doesn't apply to inductive or capacitive loads though. – Mister Mystère Mar 23 '15 at 11:05
• @MisterMystère For reducing the switching power loss on the $330\Omega$ and $300\Omega$ resistors? Is this the sole reason? Why don't you post it as an answer? – hkBattousai Mar 23 '15 at 11:07
• Have a look at my Opto-triacs, zero-cross which should be of interest. – Transistor Dec 8 '19 at 9:45

1 Answer

Short answer: to reduce switching losses and harmonics rejected into the supply (EMI issues), assuming the loads are resistive.

An optotriac is an implementation of a solid state relay, which can be other things, but I'll refer to your optotriac as an SSR in the following.

Assuming resistive loads: On AC supply, a zero cross detection circuit will allow the SSR to switch when virtually no current is flowing, which reduces the switching losses from the finite turn-on time of the contacts, and also reduces interferences rejected into the supply: if a nice sine wave is cut off abruptly at random locations, you're more or less drawing a square wave of the current for a small amount of time - this represents loads of non-50Hz (or 60Hz) harmonics drawn from the supply into all the parasitic elements of the supply, which will turn into voltage interferences. The extreme case for this are switching converters such as a buck or a boost but it's essentially the same issue. Triacs already spontaneously turn off at current zero cross, so there is only need to turn on the SSR at current zero cross. For resistive loads, this is also voltage zero cross.

Assuming inductive or capacitive loads: I'm pretty sure the "zero cross" label refers to the voltage, and not the current, which means that the above does not apply to capacitive and inductive loads since voltage and current are not in phase. I would reckon in such cases that the SSR will turn on at voltage zero cross/random current, and turn off at random voltage (after the next voltage zero cross)/current zero cross because of the triac's behaviours. Turning on would waste power and reject interferences. To be confirmed though.

To my knowledge the zero cross feature exists also in SSRs which are not based on triacs, it's just less convenient when dealing with AC controls.

Addendum: Here is an illustration of the interferences I'm talking about. The first figure shows one unswitched sinewave, and one switched, which have the same power. Strictly speaking I should be comparing several cycles where they are switched at zero cross in one case, and randomly in the other but that was quicker for one cycle. The fast fourier transform in the second figure shows that much more unwanted frequencies are drawn from the supply when the sine wave is switched, including DC.

• A non-zero crossing driver would be driving the triac continuously. It keeps driving the trial all the time. Wouldn't that let the entire sine wave pass the triac? I don't understand why the sine waveform would be cut off at random locations. – hkBattousai Mar 23 '15 at 11:44
• I think I get it. You mean, zero-crossing driver prevents the starting point to coincide on the middle of the sine wave. And it guaranties that the starting point is always a zero-crossing point (0 and 180 degrees). But if the triac is to be switched only once in a while, the only benefit of a zero crossing driver will be saving switching power. – hkBattousai Mar 23 '15 at 11:49
• Triacs only turn Off at the zero crossing of current, so that shouldn't generate RF interference. I don't think the type of load matters either. When the triac is off, there's no current, so no phase shift due to non-resistive loads. What matters is that it switches On when the voltage across it is zero, so the load sees a fairly smooth sine wave ramp up, not an instant step in voltage. – tomnexus Mar 23 '15 at 12:22
• Hopefully I've made my answer clearer now. – Mister Mystère Mar 23 '15 at 12:49
• So an optical isolator with zero crossing detector cannot be used in dimming bulb applications, is my understanding correct? – Page David Jun 9 '19 at 1:17