I have a circuit where I am using triacs to switch home appliances like fan, tube light etc. This is the circuit I am using:

No snubber

(TRIAC1, TRIAC2, TRIAC3 come from arduino GPIO pins)

First load is a fan with a capacitor based regulator. (Sorry for showing fan and tubelight as a resistor). Speed is controlled by rotating a knob which changes the series capacitance. Lower capacitance means lower fan speed.

As seen from the circuit, I didn't use a snubber because I thought using a snubberless triac will do the job, however the following incident made me rethink about the situation:

1) When Bulb and tubelight are off and fan is ON, and I try to regulate the fan speed by rotating the knob, sometimes there is a flicker in the tubelight and bulb. Flicker is very short lived but still visible. Flicker doesn't happen at higher speed. It usually happens at lower speed levels. For ex - There are these levels: OFF, 1, 2, 3, 4, 5 (5 means direct connection to Live). When I switch speed from 2 to 1 (or vice versa), I see a short lived flicker in the tubelight.

2) If the fan is running at low speeds - 1 and 2, and I try to turn the fan OFF by pulling the GPIO (TRIAC1) LOW, the fan won't turn OFF. Instead it produces humming sound and continues to rotate at a lower speed which indicates partial turn ON of triac.

Now in order to fix this, I am planning to use a snubber. I read a few articles and this is the circuit I could come up with:

Circuit with snubber

Rs and Cs are usually mentioned as 39 ohms and 0.01 uF respectively. However I have a few questions:

1) Is my assumption of spurious triggering correct? If yes, will the proposed circuit improve the behavior?

2) Usually snubber design for inductive load shows a pure inductor as the load and a parallel RC circuit as the snubber. Here, the load is LC due to the presence of capacitor based regulator. Should there be a modification in snubber circuit due to this?

Here is the fan regulator I am talking about:

Fan regulator


Fixed the snubber circuit.

Added the image of fan regulator.


Tried the snubber. It didn't work. I am assuming some sort of resonance when the regulator is kept at low speeds. This resonance might be preventing the triac from turning off completely.


2 Answers 2


1st Read the MOC3021 datasheet, you have a correct circuit for inductive loads. Not only the snubber, also the trigger circuit is different
2nd: the snubber goes connected across the triac, not between live and neutral.
3rd: Triac isn't suposed to drive capacitive loads, so why don't you use a phase angle control instead using capacitor in series, as this isn't the good manufacturing practice.

EDIT: Motor and capacitor can cause serious problems, not only for triac but for entire mains network, because it is an LC circuit which can lead to ferroresonance. At this point you can induce large overvoltage spikes that will damage apliances, yours and neighbours arround you.

  • \$\begingroup\$ Thanks Marko. Sorry, I made a wrong snubber circuit in a hurry. I was aware of the snubber connections and I have made necessary corrections. I saw the different circuit for inductive loads but triac datasheet claimed that since the triac is snubberless, I don't need additional circuitry. I am not using phase control because it produces a humming sound. LC circuit should have those issues even if I am not using a triac, right? Many reputed companies (like Le Grand, Anchor etc) are making fan regulators based on capacitors and people are using them without any noticeable issues. \$\endgroup\$ Feb 27, 2016 at 12:56
  • \$\begingroup\$ @Whiskeyjack Are you sure on capacitor based regulators? Can you link a schematics of such regulator. \$\endgroup\$ Feb 27, 2016 at 13:00
  • \$\begingroup\$ Yes Marko, I am very sure. I personally have opened one up as well as made a similar circuit on bread board and tested the speed control. I have added the image in the question. Also I'd like to add that in earlier days, triac based fan regulators were common. They used RC based timing circuit to fire the triac and do a phase control. Now a days, companies have moved to series capacitor based ones to get rid of the humming sound. \$\endgroup\$ Feb 27, 2016 at 13:06
  • \$\begingroup\$ Regarding the schematics, this contains a rotary switch which changes the effective series capacitance depending upon knob position. Small 0.25 watt (200 kohm) resistors are in parallel to capacitor and they serve as discharge resistors. 1 watt resistors are in series and prevent a sudden spike when knob is rotated. (Value is 4-5 ohms). \$\endgroup\$ Feb 27, 2016 at 13:14
  • \$\begingroup\$ @Whiskeyjack I do believe now. Probably the capacitance is calcullated such that the whole circuit is far from resonance at 50/60Hz, also the resistors help to "eat" the spike. Then you should look for triac suited for capacitve load. \$\endgroup\$ Feb 27, 2016 at 13:34

Try connecting the gate of the TRIAC to T1 using a resistor (260 Ohms) or so. This is seen in many SSR's and it seems to solve most of the false triggering. Also look for an opto driver which has higher dv/dt so that most of the transients are eliminated.

  • \$\begingroup\$ I am using FOD4208 which has better specs and now I am not facing this problem that frequently. About the resistor, I don't think it's required because I am using snubberless triacs. Resistor is a must in case of sensitive gate triacs but snubberless ones are pretty resistant to false triggering in that way, I have read. \$\endgroup\$ Jun 29, 2018 at 9:17
  • \$\begingroup\$ I tested the same setup using FOD4208 and found that the triac still false triggers when the regulator is turned fast, though it is much better compared to MOC. \$\endgroup\$
    – Zac
    Jun 29, 2018 at 18:34
  • \$\begingroup\$ So, how did you solve it? I am using BTA316X triac + FOD4208. \$\endgroup\$ Jun 29, 2018 at 19:09
  • \$\begingroup\$ I still haven't found a solution which is 100% accurate. Using triac and opto with higher dv/dt and subberless versions seems to improve the immunity against transients. As you pointed out, the gate connection to T1 doesn't seems to add any value while using snubberless triacs. \$\endgroup\$
    – Zac
    Jul 1, 2018 at 6:07
  • \$\begingroup\$ The Overvoltage protected ACST series triac's from STMicroelectronics seems to have good characteristics out of those I have tested. \$\endgroup\$
    – Zac
    Jul 1, 2018 at 6:10

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