I'm designing a PCB where an MCU has to command a TRIAC to drive a coil, when googling an schematic for an SSR, I came across 2 snubber designs, and while researching them this question: Triac snubber: Where to put current limiting phototriac resistor?

For what I gathered one design protects the optocoupler and the other protects the whole circuit but special care should be paid to the values of the components.

I was wondering if I could combine both approaches like so, or if some interaction that I'm unaware of between them will cause the circuit to not work properly? enter image description here

  • \$\begingroup\$ Follow datasheet for optotriac. \$\endgroup\$
    – user263983
    Apr 5 at 15:44

The two circuits are for different purposes.

R17 and C18 are a snubber. They reduce spikes and quick changes in voltage (dv/dt) that can damage or misfire the triac.

R15 and C16 may be required for inductive loads. With an inductive load, for the first instant after the gate pulse, enough current may not flow through the triac to keep it ON. C16 stretches the gate pulse until current flows through the triac.

Edit: further explanation: R15 and C16 are not an effective snubber because the resistor value is too high. Also, R14 is connected to the middle. It might be possible to find values of R15 and C16 that could serve both purposes, but the cap value would be large and therefore expensive. So, you could omit 2 cheap parts but one part would be more expensive.

  • \$\begingroup\$ As why the snubber is double? One is not enough? \$\endgroup\$
    – user263983
    Apr 5 at 19:13
  • \$\begingroup\$ @Mattman944 Can you explain this ? "C16 stretches the gate pulse until current flows through the triac." \$\endgroup\$
    – mastermind
    Apr 5 at 19:57
  • \$\begingroup\$ electronics.stackexchange.com/questions/558590/… I have same problem here.. Can you please see it. \$\endgroup\$
    – mastermind
    Apr 5 at 22:08
  • 1
    \$\begingroup\$ @mastermind - Without C16, since the gate signal is derived from the MT2 terminal, when the triac turns on, the gate signal will disappear. For the triac to remain on, current must flow. If the load is inductive, current will not flow immediately. \$\endgroup\$
    – Mattman944
    Apr 6 at 0:07
  • 2
    \$\begingroup\$ @mastermind The current takes time to increase through an inductive load, but that rate might be too slow to rise to a level high enough to latch the triac on in the short time the opto is pulsed. C16 doesn't go through the TRIAC and just sits there charged and waiting. When the opto pulses, the capacitor can dump its stored charge through the triac to raise the current high enough to latch on. I think the reason the snubber can't also be used for this is the R17 is too small so the cap discharges too C18 discharges too quickly. In some cases, the snubber might be able to do double duty \$\endgroup\$
    – DKNguyen
    Apr 6 at 0:07

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