I have the TinyG controller and one of the chinese solid state relays to trun on/off the spindle...with this setup, sometimes (most times actually!) the router "hunts" around for speed, like the power is mometarily being switched off/on...thought it might have something to do with the voltage and its effect coming thru the relay having on the variable speed router...today I did some testing and at 12v from an old ATX power supply, and 24v from my main supply, the relay powers the router fine...
TinyG struggling with spindle relay
TinyG struggling with spindle relay