General   (General discussion, talk about anything.)

Tonker,

You could use FETs, or other similar devices so that a signal derived from the mains waveform triggers the conduction from your other source. As it stands, there will be some lag, but a filter can delay the leading signal enough to pull them together. (Each half cycle +/-, being handled separately)

That said, I don't know if that is the technique which they actually use, I would have to look it up, I suspect the commercial devices use a phase-locked loop, as it is available as a cheap single IC. Again I would need to look up the circuit they use.

Sadly, I am not in a position to be able to offer to make one for you. I would if I could.

If I get the time, I will peruse some of the circuitry online. There is usually a reference design with the examples in use being a variant of the reference design. I'll let you know if I find anything, but don't hold your breath, I'm up to my eyes with stuff to do.

A grid connected inverter is such a device.

Reference design for a GCI was easy to find, though may well not be the most up to date.

Reference design grid connected inverter