Light dimming using triac and optocoupler is very common and basic circuit, I have successfully tested the dimming for 1 channel (1 triac, 1 opto-coupler) now I want to add one more light dimmer with same MCU, whenever I start 2 channels (2 triac, 2 opto-couplers) circuit I have got fluctuation in one of the 2 channels.

I am giving trigger input to the NPN transistor from XMC1100 series micro-controller, I also have running ZCD circuit ref for my micro-controller, so there is no problem in input side.

I have also tried to implement an RC snubber circuit (R= 470E 1/4W and C=10nF 400v), but there is still fluctuations in one dimming channel.

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(I want to make it clear that I have made 2 identical PCBs, For both the PCBs different channels have fluctuations, and other channel have perfect dimming output, also Ihave checked hardware multiple time. All the components are same value, I have also swapped triacs but same output.)

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Here is my circuit component description:

  • Triac = BT138-800
  • Opto-coupler = MOC3021s-TA1 (SMD Version of MOC3021)
  • NPN transisto = MMBT3904
  • 3.3V VCC,230V AC Supply
  • \$\begingroup\$ And This Same Circuit Works Perfect Without Any Fluctuations and With Both Channel With This Triac "T850-6G" , i am not able to Differentiate why the BT139 works For Any random Channel And Fluctuating in Other Channel with Same Driving Components. \$\endgroup\$ – JkumaR Feb 16 at 6:55
  • \$\begingroup\$ Decscribe fluctations in terms of hystereis, frequency, intensity, Triac surge current (10x?) and pulse width then jitter. You are getting crosstalk either from line dV/dt or radiated on logic or triac \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 at 7:05
  • \$\begingroup\$ disconnect the correctly-working circuit and see if the other one recovers. \$\endgroup\$ – Jasen Feb 16 at 7:13
  • \$\begingroup\$ Fluctuations in Light (Load), i dont have CRO/DSO to Check The waveforms. @SunnyskyguyEE75 \$\endgroup\$ – JkumaR Feb 16 at 7:38
  • \$\begingroup\$ then use your eyes \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 at 7:39

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