For some crude work (i.e. massive connectors, buttons) I bought a 125W transformer soldering gun. It is a large (and heavy) transformer and a NO push button powering the transformer.
It is said this is dangerous for semiconductors because there is a large current in the soldering loop which induces too large currents in the board. Also I have read the situation is worst at the moment you press or release the button (in other words at power on and power off) - is it true?
I would like to replace the button with a triac controlled by a MCU and fire the triac only at mains zero crossing. I hope to get many advantages.
- It would be safer for components that are already on the soldered board.
- The soldering gun has a LED light powered from the secondary windings. When the soldering gun is off the LED is also off. When I add a capacitor to the LED and keep the soldering on at some low duty when it is "off" the loop would keep some temperature (but not overheat) and the LED will stay on.
- If I use two whick wires instead of the loop and power the gun for only one mains half cycle (or even only part of that cycle) I could use it for spot welding - with 50Hz mains frequency one half cycle should deliver about 1J to the joint - is it reasonable value?
But I don't have enough knowledge about transformers. AFAIK transformer powering the loop should be mostly resistive load so using the zero crossing triac switching should be OK. But when the loop is damaged or not present the secondary winding is open and the transformer primary will act as an inducor - can something go wrong? Would the energy in one half cycle be enough to get a small spot weld (e.g. to connect a soldering strip to a battery)?