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Hi I am working on a project using a triac to do phase control dimming of 120V ac powered devices. I recently translated my project from perfboard to SMT using an SMT fabrication service. The project appears to be working correctly, except for the snubber filter section. Despite having a snubber filter, I can see on my scope that the ac output still has a hard cut-in in the middle of the waveform and I can hear an audible buzzing from the board at dimmer settings where the ac waveform turns on mid cycle. I am testing with a 43W incandescent bulb.

I have checked that the components are the correct value and were installed correctly. Is there anything in component selection that might cause this? Here is the relevant schematic section showing the triac and the snubber filter:

Triac + Snubber Schematic

Here are the components I picked for Q1, R13, and C6 respectively:

Q1 Triac R13 Snubber Resistor C6 Snubber Cap

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  • \$\begingroup\$ Describe how your 2f phase control is done with PWM \$\endgroup\$
    – Tony Stewart EE75
    Jan 26 at 1:36

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R13 and C6 are not really suitable for mains operation. C6 should be X2 rated. The capacitor for C6 you're currently using is piezo electric, so it is most likely causing the buzz you hear. R13 isn't surge rated.

If you want to slow the edge, you need an inductor in series with the load like all good phase control light dimmers have. R6/C13 is only for snubbing the triac.

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Snubbers act like a pressure relief valve around the switch and slow the resulting dI/dT for inductive loads which reduces the peak voltage. The problem is you an incandescent light is not inductive load so your snubber won't really slow anything at all.

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  • \$\begingroup\$ I am measuring the voltage at J5 pin 2. This is a digitally controlled dimmer, so it provides phase control by turning on the triac at an early or later point in the waveform. Perhaps 'cuts in' would have been more accurate. \$\endgroup\$
    – Emmett Palaima
    Jan 26 at 13:39
  • \$\begingroup\$ @EmmettPalaima Understood. So you're seeing ringing or something when it cuts in? \$\endgroup\$
    – DKNguyen
    Jan 26 at 14:23
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    \$\begingroup\$ I now understand that the waveform looked normal for a non-inductive device, but I was concerned with the fact the I was hearing a buzzing noise from the PCB. After replacing the SMT ceramic cap with a bigger film capacitor I no longer have that issue. \$\endgroup\$
    – Emmett Palaima
    Jan 27 at 21:09
  • \$\begingroup\$ @EmmettPalaima So what was the buzzing? Was it just the piezo behaviour of the ceramic cap acting as a speaker and not actually a problem with the cap functioning? Or something else? EDIT: Oh, I see the other answer did say it might have just been piezo behaviour. \$\endgroup\$
    – DKNguyen
    Jan 27 at 21:26

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