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?

  • \$\begingroup\$ Just a note, you say it is an 800W unit, I believe you meant Joules. 3F at some 23V is 800J, which discharged in .1ms (0ms seems a little too fast) leads to a whopping 8MW. \$\endgroup\$ – Vladimir Cravero Feb 28 '16 at 10:26
  • \$\begingroup\$ @VladimirCravero he says "Ws" for watt-seconds, i.e. Joules. But I agree, correct units to be preferred. \$\endgroup\$ – Brian Drummond Feb 28 '16 at 10:59
  • \$\begingroup\$ I thought it was some fancy plural form of W... English can be odd sometimes \$\endgroup\$ – Vladimir Cravero Feb 28 '16 at 11:16
  • \$\begingroup\$ Just a thought; why do you need the HF add-on at all? You're already using a short-pulse power source at 0.035 max duration, so why not just add a flyback xfmr & drive it by the output from the MOSFETs? \$\endgroup\$ – Robherc KV5ROB Feb 28 '16 at 13:27
  • \$\begingroup\$ This is hardly an answer, so I'm putting it in comments. There is some work already done by another individual so I'll link that here: pulsearcwelder.blogspot.com/2012/08/… . I'd also like to point out that you will need Argon gas as Oxygen and Nitrogen will not work so well from what I've read. Good luck. \$\endgroup\$ – Dave Feb 28 '16 at 16:05

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.

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