I want to use an optocoupler and triac to control a ceiling fan. How do I calculate the snubber resistor and capacitor value?

Will the inductive load of the fan change my snubber value? enter image description here

  • \$\begingroup\$ What is the role of C1 in this circuit? \$\endgroup\$
    – Tom Hanks
    Mar 27, 2020 at 18:02
  • 1
    \$\begingroup\$ @Tom: Ask your own question rather than try to hijack someone else's. \$\endgroup\$
    – Transistor
    Mar 27, 2020 at 21:23

2 Answers 2


I may have a few pointers for you:

The snubber is necessary in two cases : * You are using a 4 quadrant TRIAC (always use a snubber then) * You are trying to control an inductive load

Snubber will help the TRIAC properly turn off when needed. The turn off parameter is the crossing zero dI/dt. In case of resistive load, this value is zero or close to that; however, when the load is inductive, the phase between I and V increases this value.

My rule of thumb to calculate the turn off commutation of a given inductive application is the following:

$$ \biggl( \frac{dI}{dt}\biggr)_{application} = k \cdot 0.5 \cdot I_{T (RMS)} $$ (with IT(RMS) the evaluation of the current of my load in permanent mode). k = 3 in case of universal motor and 1 for every other case.

The key parameter to look at is the turn off commutation of your TRIAC, aka (dI/dt)c. The TRIAC only turns off if the slope of the current when crossing zero is BELOW the value in the datasheet of the TRIAC:

$$ \biggl( \frac{dI}{dt}\biggr)_{application} < \biggl( \frac{dI}{dt}\biggr)_{datasheet} $$

If you see that the TRIAC you use has a low (dI/dt)c, then only do you need to setup a snubber.

Example: Inudctive load of 3A RMS and 4A rated TRIAC. (dI/dt)app = 3 TRIAC Z0405MF, (dI/dt)c = 1.8 ==> TOO LOW ==> SNUBBER necessary

When it comes to calculating the values for the snubber, the minimum value of the capacitance should be 10 nF to 47 nF and the minimum value of the resistance should be 47 Ohm to 680 Ohm.

For more details I refer to the AN from STMicro

I hope all of this helps.

  • \$\begingroup\$ I'm confused. Isn't dv/dt the more critical triac parameter for inductive loads? \$\endgroup\$
    – RodB
    Sep 13, 2018 at 21:56

Snubber is essentially the capacitor of it, but the resistor is needed to prevent harmful (=destructive) too fast rising anode current (Ia ) peak from snubber's capacitor through the triac when the triac is triggered to ON state. In addition the resistor dampens oscillations that can occur with inductive load and snubber's capacitance.

The capacitor over the triac is needed to prevent too fast rising of the anode-cathode voltage Uak when the triac tries to turn off. No inductive load is needed for too fast Uak rise, a resistive load can well be enough if there's no capacitance inserted.

Mains voltage also has unwanted peaks, which sould be somehow suppressed. The snubber helps. The charging of snubber's capacitor slows down the rise of Ua because the load causes voltage drop. Snubber's resistor must not be too high because that would make the voltage rising speed limitation non-existent, Stray capacitances hopefully help over the start step at the turn-off

Inductive load will shift the turn-off further over the zero crossing of the voltage but the turn-off happens when the current drops under the hold level. A big inductor can even in that situation have remarkable energy which at the turn-off kicks Uak ultrafast up. The snubber must eat that pulse, That happens if the resistor isn't too big and the capacitance is high enough.

So, to design the snubber you must know the AC voltage, its unwanted peak estimations (can be found from design norms), how fast the anode current is allowed to rise (=dIa/dt -limit)at turn-on, how fast Ua is allowed to rise (=dUa/dt -limit) at turn-off , the hold current, what are the variation limits of the load (resistance, inductance) and what's the triggering angle variation range.

With these numbers you search with a circuit simulator which snubber values in all load and triggering situations prevent the excessive dIa/dt and dUa/dt. As well the oscillations should be kept in control because they+input voltage can be too much.

Here's one whitepaper which contains the principles of the design math, if that's needed. Also all explanations there are useful https://www.st.com/resource/en/application_note/cd00004096-rc-snubber-circuit-design-for-triacs-stmicroelectronics.pdf

I have seen a case that no snubber alone was enough, there was needed also a saturating inductor to prevent too high dIa/dt at turn-on. The problem was the load which took a current peak due the capacitance.


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