I tried to simplify schematic, and replace AC-DC convertor to inductor-less, based on KP3310SGA:

• https://oshwlab.com/speed/ac-speed-control-v3

Also, tweaked schematic a bit to route all components on single side, and added board for one more grinder model.

Unfortunately, when received PCB and started to test KP3310SGA, found one serious disadvantage. Turn on time takes 5-10 seconds after power on :). This may be acceptable for some appliances, but not usable with grinder.

So, i had to abandon v3 hardware in favor of v4:

• https://oshwlab.com/speed/ac-speed-control-v4

AC-DC uses LNK3204, but other advantages of v3 still exists:

• More PCB variants, for actual grinder models
• Single side SMD components mounting

Those are not tested yet. Just ordered.