Transformer spot welder logic.

[firewalker]

What is the logic behind a transformer spot welder (welding battery tabs type) controlled my an mcu?

Zero cross detector -> Wait ~20 msec (50 Hz) -> Enable TRIAC (or any suitable device) -> Wait X*20 msec amount of time -> Turn off the TRIAC.

To play with the power just allow more cycles (increase X)?

Alexander.

[tautech]

I think I would apply the KISS principle.
Adjustable timer switching the Triac gate feed from a Triac controller.

[jebcom]

Yes, you're right to use zero-crossing detect and allow for a selectable number of cycles. If you have multiple taps on your transformer, you can also allow selection of taps to control heat. If you're doing a quantity of joints, the electrode tip will degrade, and you will need to either dress the tip or make adjustments in your number of cycles or voltage. Yes you do want to keep it simple, but you also need enough room for adjustment to get ideal welds in your range of working conditions.

[eneuro]

Adjustable timer switching the Triac gate feed from a Triac controller.

I saw many spot welders with triacks and made myself one using this huge transformer

However what about AC mosfets switch and current sensor to controll more tricky transformer primary?

Just testing small optoisolated mosfets gate power supply to be able test those AC switches.
I guess some kind of snubbers will be needed to this AC switch circuit.
Did you tried drive transformers using such AC mosfet switches?

[nctnico]

Usually only part of a 50Hz cycle is used. So wait until the zero crossing and then wait for the right firing angle to trigger the TRIAC.