Stick to TIG welder conversion

Thread Starter

gregtomko

Joined Sep 29, 2008
16
I am converting an old AC stick welder to a DC TIG, nothing fancy, no high frequency arc starting, or square wave or anything. I am just going to rectify the output from the secondary of the welder for DC, and vary the current to the primary with a large triac for remote current control. My question is, I would like to be able to use a microcontroller to drive the triac, a BTA40-800B, which needs a pretty hefty 80ma to drive the gate, so can I use a smaller triac to trigger the large one, and drive the small triac through an optoisolator from the micro? Or is there a better way I am not thinking of?

thanks for any input
 

Thread Starter

gregtomko

Joined Sep 29, 2008
16
I have seen schematics using SCRs to vary the primary current, but they have the same gate drive difficulty, and I thought a triac would simplify things. Am I overlooking something, or will the triac be ok?
 

Thread Starter

gregtomko

Joined Sep 29, 2008
16
Actually now that I looked into it a little more, I am thinking about using one of those optoisolators with a triac output to drive the main high current triac, or am I still wrong about being able to use a triac to drive the primary of the welders transformer all together?
 

Thread Starter

gregtomko

Joined Sep 29, 2008
16
Just for clarification, I am going to hook up the high current triac just like you would in a dimmer switch for a light bulb, but in place of the diac and potentiometer trigger, I will be using the optoisolator with triac output driven by the micro, for triggering.
 

Thread Starter

gregtomko

Joined Sep 29, 2008
16
Also I was looking at a APT1221 for the optoisolator, which has a 100ma rated output, which I am thinking would be sufficient to drive the BTA40-800B triac which has a max trigger current of 80ma. Oh yeah, this is going to be powering a 220v welder which runs off of a 30amp breaker.
 

JJAngleton

Joined Aug 21, 2010
14
If you ever got around to finishing your project, let me know. I would also like to convert my 220V AC welder to a tig machine. I understand that for aluminum I do not need DC. Except, a HF arc starter would be nice. Do you know of a quick circuit, maybe using a car ignition coil or a TV flyback transformer and a few transistors for the oscillator ?
 

marshallf3

Joined Jul 26, 2010
2,358
Actually now that I looked into it a little more, I am thinking about using one of those optoisolators with a triac output to drive the main high current triac, or am I still wrong about being able to use a triac to drive the primary of the welders transformer all together?
I'd start that way provided the optoisolator would be able to drive the big triac.

BTW: You're going to end up with a ton of switching harmonics, most of which are going to head directly back into the AC mains. You'll want to filter those out as best you can.
 
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