I'm using one BT139-800 TRIAC to switch a 35 W single-phase motor and one TRIAC for switching three ballasts for six fluorescent lamps with a total power of 108 W.

I'm triggering the gate of the BT139 with an MOC3063 opto-TRIAC. I'm using the circuit below for both loads with the noted resistor and capacitor values.

enter image description here

The problem I've encountered is that when I connect the circuit to the mains both TRIACs are already triggered and conducting.

I've started debugging by removing the opto-TRIAC which did not solve the problem. After that I've tried removing the 360 Ω resistor between gate and terminal. Obviously the TRIAC did not conduct and I was able to trigger it by carefully shorting the pins with a screwdriver. Thus I figured the problem is in the resistor, but I'm not sure what its value should be. Is there an easy way of determining the correct value? Are there any other problems with the circuit?

Also, how do I know that the snubber circuit has the correct values? The motor has a 2.5 μF starter capacitor. I guess I could eliminate the snubber circuit for the fluorescent lamps since the ballast has a power factor of 0.98 and the opto-TRIAC is zero-crossing. Is that correct?

  • \$\begingroup\$ The other source of trigger is a sudden dV/dt applied across anode cathode, which the snubber ought to attenuate but might not if the PFC has a large input capacitance . \$\endgroup\$ Commented Jun 22, 2020 at 17:30
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I find it unlikely that both triacs (for the lamps and the motor) would encounter this same problem. \$\endgroup\$
    – Cezerb
    Commented Jun 22, 2020 at 17:44
  • \$\begingroup\$ I would not choose such a sensitive gate TRIAC when driven by a low impedance Optoisolated Triac. without a small gate cap. I wonder what layout effects have or Gate Voltage exists? \$\endgroup\$ Commented Jun 22, 2020 at 18:06
  • \$\begingroup\$ I don't understand how this circuit is supposed to work properly. When you apply power, the opto-triac is presumably off and through the load and the \$360\Omega\$ resistor, voltage is applied to the gate, which in my opinion makes it switch on immediately and by default. Maybe you could try to put the opto-triac in series with the two \$360\Omega\$ resistors? \$\endgroup\$
    – HarryH
    Commented Mar 10 at 13:01

1 Answer 1


From the malfunctions you mention, I think you have reversed the A1 and A2 terminals of the Triac. Try swapping them.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.