SCR False Triggering [SOLVED]

[anishkgt]

Hi All,

I've been trying to get my First SMD boards working. Seems every time i fix something another one seems to be popping up. First, it was the USB bridge not being detected and now the SCR seems to be triggered and conducts continuously after powering on the PCB.

The through hole version works very well but the SMD does not seem to be working. Are there are in standard procedures that needs to be followed when selecting SMD that would run on the mains. Should they be THD. I've got only 4 MOVs(R16, R40), one Cap (C10) and another Cap (C16) that are through hole.The load for the moment is the a Incandescent bulb but was designed to switch a microwave oven transformer for spot welding.

The schematic is quite simple, the peak of a sine wave is detected and the SCR are triggered. i am certain its the MCU that is triggering, i've removed the optocoupler (OPT2) and still load conducts.

[anishkgt]

After removing each component i was able to isolate the problem. Removing the scrs stopped the bulb from switching on when the pcb was. i checked the resistivty of Cathode to Anode on both the scrs both read OL.

So it wasn't the SCR either, then what could it be ?

[anishkgt]

Thank you Blueskull. Appreciate it. just checked my through hole design and i was surprised.