Spot welder triac control. Feasibility?

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#1
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Hi Guys:

I'm making a homemade spot welder as a hobby project. I salvaged an old AC stabilizer and recovered the autotransformer. With that core I calculated that I can make a decent 2KW spot welder (well inside the light-medium duty commercial spot welders)

Now, my problem is that, for tiny metal sheets and/or wires I'd like to control the output current of the welder (to set to a max amount, not continous regulation).

I though first in a transformer with a tapped primary in order to reduce the "open" voltage and then control the max output current to some extent with a selector or relays.
Then I realized that make taps into a 2mm dia. wire is somewhat difficult (but not impossible) and let's the winding deformed (I need to attach a bar as secondary or a very big cable) so, begun to think what about a triac control.

I looked for spot welder schematics and there are none with triac control. May be a triac SWITCH (on off) but not control.

Is this impossible? or expensive ? Unreliable to fit in a commercial welder? the primary full current would be no more than 15A at overload (+-10A@220V at 2KW)
Does anyone here have built a spot welder? and tried to do such control?

thanks in advance
Nachus

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You should be able to do what you want using phase control. I would suggest using an "Alternistor" instead of a common triac. These are made by Teccor/Littlefuse and hook up the same way as triacs, but they are a little more rugged and perform more like back-to-back scrs. I used to work on large resistance welding equipment and one style used Ignitrons to phase control a 400kw transformer so in theory, it should work for you.

Tom Pappano
Tulsa, Oklahoma

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How do I get my AVR to fire a ignitron? Where do I buy them? Digikey doesn't have them listed.
(tongue planted firmly in cheek).

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ST has to good triac with "snuber less" it is the same than "Alternistor"
or use a std triac 25A with a snubber.
Thierry

Thierry Pottier

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Thanks Guys: I can get here in Argentina some "alternistors", more of the type Thierry says (The ST snubberless triacs) I'll try that, with microcontroller phase angle, and timing.

Thank you
Nachus