triacs that work with zero crossing optocoupler drivers

Thread Starter


Joined Oct 13, 2018
Hi All, I have a moc3043 zero crossing triac optocoupler which I used to drive a rs1000 triac. This worked OK using the triac to switch the neutral line for safety.
I know nothing about the rs1000 so I assumed the moc3043 was going to work for any triac. This was not the case using a bta16-600b triac. As this triac like many others require a 50ma current to switch on I assume that at zero crossing there is not enough current to switch on this triac.
Can anyone suggest which specific triacs will work with a zero crossing triac optocoupler driver? rs1000 is not available any more.

schmitt trigger

Joined Jul 12, 2010
I believe that I understand what you are saying, but I could be wrong.............. let me rephrase:

Your concern is that very near the zero crossing point, the instantaneous voltage will be too low to develop the necessary 50 mA current?

The series resistor in a random-firing optocoupler is sized such that at peak voltage, the maximum peak repetitive current thru the optocoupler is not exceeded.

In a 240 volt world, this would mean 338 volts peak. The MOC3043 datasheet indicates that the Peak Repetitive Surge Current (PW = 100 μs, 120 pps) is 1 amp. Therefore, the required resistor is, as Dodgydave shows in the attached schematic, 390 ohm.

To satisfy a worst case main Triac trigger current of 50 mA, that would mean that 19,5 volts (better say 22 volts with forward voltage drops) would be required to trigger the Triac.

A loss of 22 instantaneous volts in a 240 volt line means that you would be losing the first 3 electrical degrees. Is that significant in your application?

If so, because the application would always trigger very close to the zero crossing, I would say it is safe to bet that you could significantly lower your resistor value. That would provide you with an extra degree or so of conduction. But in case the phototriac accidentally triggers because of high dV/dt at a high instantaneous voltage, you could damage the device.


Joined Aug 12, 2014
But in case the phototriac accidentally triggers because of high dV/dt at a high instantaneous voltage, you could damage the device.
In the circuit shared by @Dodgydave above, C1 and R4 together act as a "snubber" to help protect against that.

There's also another method which places a snubber across the TRIAC gate instead of, or in addition to, the snubber across the load. Here's an example circuit with both:

I confess I don't remember how best to calculate values for either snubber configuration, but I know they can be pretty effective when executed properly.


Joined Aug 12, 2014
This is a typical circuit to drive a Triac in the Neutral Load , you can lower R2 to 180 ohms.

View attachment 201752
I meant to ask this earlier - does it make any difference if you have the load on the line side or the neutral side of this circuit? Would you design anything differently if you were moving the load?

All of the SSRs that I've checked in the past said line and load terminals were interchangeable, and my very weak understanding of TRIACs would tell me that it makes no difference, but the thread starter's initial question, and your wording in this response made me wonder if I'm missing something.


Joined Jun 22, 2012
Triacs will work switching the Live or Neutral, either will work, most Triac dimmers switch the live side, and put the load in the Neutral, like my post #3..