A little back story – I'm a network security engineer turned bench jeweler. I built a capacitive discharge spot welder. It's an awesome unit, and works great, but it's not exactly what I need it to be. It's a resistance welder, which means that the resistance of the workpiece generates the weld heat. Gold and silver (the metals I work in), are very very conductive to both heat and electricity. I need to convert the unit to a pulse arc welder. For reference look at the Orion welder line by Sunstone Engineering, the PUK line by Lampert, or the Pulse Arc line by ABI.
http://www.orionwelders.com/
http://www.abiusa.net/Pulse.htm
To convert the unit to pulse arc, all I need to do (in theory) is boost the voltage above the breakdown voltage of air. It has a 3 farad main bank at 25 volts making it's an 800Ws welder when charged to it's safe max. The main bank can be charged to anything from 0 to 25, with 2 output pulses anywhere from 0ms to 35ms each. The breakdown voltage of air is about 370 volts.
The unit has a main control board that interfaces with a mosfet switching board that uses 18 75v @ 209A mosfets. Basically, it just takes 5v pulses from the control board and turns the mosfets on and off accordingly.
I'd like to build a daughterboard that will sit in between the two, and switch a high frequency source on then "pass" the 5v control pulses along to the mosfet board.
This should create an ionized plasma channel through which the weld energy will then travel.
So, two questions.
1) How do I create that high frequency source? I could use the tried-but-true automotive ignition coil method. But that seems… crude. There's got to be a better way.
2) How do I protect the rest of the circuitry from being fried to bits by the high frequency addition?
Best Answer
It seems like you want a saturable transformer; one transformer winding in series with the weld current, the other winding is coupled to an RF oscillator. When the RF oscillator is active, but no weld current is flowing, the transformer core couples the two. When the weld current starts to flow, it magnetizes the transformer core (and that decouples the RF winding from the circuit). You might get by with a simple pulse into the transformer, using a fast (DIAC or SCR?) switch, instead of RF.