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I've seen in several places this snubber circuit composed of 2.4K and C1 in this particular case.

My question is - what's the purpose of this network exactly and how are the values for the resistor and capacitor calculated?

Does it provide a phase shift so that power triac could be turned on on zero crossing and if so, how does it work?

It's said, that it is designed for power triacs with sensitive gate Igt < 50mA. What do I do if my power triac's gate is not sensitive (> 50mA)?

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The network is there to prevent high speed/voltage transients from causing problems, such as spurious triggering of the triac.
Triacs have a dv/dt rating (how quickly the voltage across them changes) above which they may trigger without a gate pulse.

To show how this can cause issues, here is a rough approximation of the circuit without the capacitor:

Triac no snubber

Here is the simulation, notice when the current approaches zero and the main triac tries to turn off, there is a transient - this spike passes through the control triac, causing a spurious trigger and keeping the main triac turned on.

Triac no snubber sim

So we add the capacitor back in:

Triac snubber

Now we simulate, and see the circuit functions as expected. The snubber shunts the transient pulse to ground and prevents the main triac from staying on.

Triac snubber sim

Further reading:

2N6073B SPICE Model (used in above LTSpice simulations)

Explanation of maximum ratings for thyristors

Thyristor theory and design considerations

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Explanation below is very simplified, but should be ok in general.

With inductive load, phase of current is shifted. When triac stops conduct (at zero current), some voltage already exists. So, voltage on triac pins quickly rises from zero (~1.4v) to some higher value. But triac has dV/dt limit. If exceded, it can swich self on without conrol signal (not desired). So, extra parts (snubber) may be needed to keep dV/dt in valid range.

You have 2 triacs - main, and in opto-coupler. If main triac is 3Q snubberless (high dV/dt, BTA16-600CWRG), then you can "protect" optocoupler only. In this case componens will be smaller, than for "classic" snubber. Your schematic is for this case.

Now, how to calculate. The most easy to read is Panasonic Application Note 030, Driving Triacs with Phototriacs.

There are special snubberless optocoplers like FOD420, but those are more expensive.

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    \$\begingroup\$ I think your use of "triac closes" and "self-open" are opposite to most people's. Use "triac conducts" or "triac switches on" or "off" to avoid confusion. \$\endgroup\$
    – Transistor
    Sep 22, 2020 at 11:45
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    \$\begingroup\$ @Transistor thank you very much! Corrected terms as you suggesed. \$\endgroup\$
    – Vitaly
    Sep 22, 2020 at 15:29
  • \$\begingroup\$ It reads well now. Thanks. \$\endgroup\$
    – Transistor
    Sep 22, 2020 at 15:30
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In the original circuit, the 2.4 kΩ resistor and C1 are a snubber circuit for the Optotriac. This gets rid or transients and voltage spikes that can cause auto triggering or destroying the Opto altogether. There actually should be a snubber on the Triac itself, even though they say snubberless. That is incorrect and can lead into frying your Triac.

Each manufacturer has different parameters concerning their parts, so read all you can on parts used then go from there.

Here is a good circuit to start with but just remember, if you are doing PWM on the optotiac then use a non-zero opto. Zero cross is for continuous duty, not PWM. Please remember if the load is on the high side that means it is energized all the time-not good, use the low side always.

Optotriac typical application circuit from Fairchild (now On Semi)

(Image source: Mouser - Fairchild MOC3020M datasheet)

Please look up Onsemi data sheet, also Vishay, Little fuse, Fairchild, Toshiba, and STMicro specifically application note AN5114.

If all you want a better picture of how to design a better Triac circuit, you have to read the application notes. Each manufacturer is different, but a general circuit can be built for all.

If a non sensitive trigger Triac is used then for your circuit you posted, use a 1.2 kΩ in place of the 2.4 kΩ and use a 200 nF C1. Another good note is ALWAYS use a snubber regardless if they say not to. This goes for the Triac as well as the Opto triac. Also do not forget the inductor, which may not be needed according to your circuit demands. If it is a motor control then use one.

Do the research first then build a good circuit.

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