EconoTig pulse controller

I'm considering building a pulser circuit for my EconoTig. The front panel jack provides provides an external access point that can be used
to pulse the welder on and off or to alternately switch between an amperage programmed on the torch thumbwheel and a setable background amperage.
My goal is to make the welder more friendly and usable to use for butt welding 20 gauge steel, such as auto patch panels. Can anyone who has tinkered with an EconoTig controller board tell me what sort of bandwidth may be available at the front panel control port for pulsing the unit and what sort of pulse rate, duty cycle setting and background current settings would be suitable. The front panel port has an on/off switch and a 1K post that varies amperage from minimum (whatever that is) up to the front panel dial setting which can be as high as 160 amps. It should be a simple matter to configure some cmos analog switches and a multivibrator or two to create a lot of options this older design could benefit from.
I understand the output inductor may be one of the limiting factors for obtaining minimum current settings. It's not clear how running the higher current setting at 50% duty cycle will work out, but I'm hoping it will make heat control more precise when working with thin materials.
Larry
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    --Give me a holler if it works! :-) Option #2 is to upgrade to a Miller Dynasty and I'm reluctant to lay out the dough too!
--
"Steamboat Ed" Haas : Whatever happened
Hacking the Trailing Edge! : to Porgy Tirebiter?
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Yeah, the Dynasty was my first choice too, but I'm already over budget on my used EconoTig.
Miller was kind enough to email a schematic for the control board but without component values or part numbers. As I understand the design now, it consists of a magnetic amplifier transformer and an SCR connected to the control winding adjusts saturation. Miller also commented that the transformer design prevents further current reduction.
So, I'm thinking that adding a pulser circuit to the remote input is not going to further reduce the output current below the minimum range of the mag-amp design. Pulsing would require being able to switch the output voltage off. My goal was to reduce average welding power to make welding 20 gauge sheetmetal easier. It looks like a pulsing approach won't work.
If I understand the design, the higher the DC current circulating in the control winding, the lower the output amperage will be. The DC circulating in this winding is only the current generated the control winding. There is no external DC source feeding this winding to vary saturation, so it cannot be driven harder as it exists. I'm thinking that the current range could be extended lower by fishing additional control winding turns through the core and adding them in series. This could be switched in with an extra range switch.
Am I understanding the theory of operation correctly and would this work?
steamer wrote:

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    --Yeah; I understand you need. I've been trying to weld lap joints of 1/8" aluminum welding rod, for a weird project that required that a dozen idntical wire frames be made as armatures for electroluminescent wire. My "solution" was to pulse the pedal which worked, but it was damn hard on the eyes as mask kept flicking on and off. You're right about the problem of not being able to drop the amperage; IIRC the minimum is something like 30 amps.     --I'm woefully ignorant in all things electronic, but would it be possible to use a second transformer, downstream from the one that's inside the Econotig, to drop power to around 5 amps?
--
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Hacking the Trailing Edge! : to Porgy Tirebiter?
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I've done some more research today on the EconoTig 150 design. The remote controlled transformer is described in United States Patent 5,187,428 and 5,363,035 which explains how the transformer output is remotely varied. The patent is available online at www.uspto.gov in case anyone wants to read about it. You would need to install the free viewer to read the document. The transformer and controller circuit described in this patent form the basis of the EconoTig design.
The transformer output is varied by applying a phase controlled short circuit across the shunt control winding. The transformer produces minimum output current with the shunt open circuit and maximum current when driven to full duty cycle. The patent goes on to explain that the shunt controlled transformer has a nominal range of 30 amps to 180 amps at an output voltage of 80 volts. The gain and control range is set by physical dimensions of the core and the size of the shunt winding air gap. It's not clear that welding current can drop to less than the minimum rated 30 amps in the absence of any drive to the shunt winding. If true, that means pulsing the shunt control winding won't drop output lower.
This welding transformer also powers the controller circuit board through a 24 volt secondary. That makes pulsing the input side of the welding transformer a bit of a problem. I suppose a 24 VAC transformer could be added to power the controller board and a controller could be added to the primary side of the welding transformer to pulse modulate it. This is starting to become more involved than I wanted.
I'm going to measure output current with the shunt winding open circuited and see what maximum control range is available, but I suspect if there was more available range, Miller would have designed it into the product. I don't have a high current ammeter at the moment or I would have that answer already.
I'm hoping someone else following these newsgroups knows about the EconoTig design and will make suggestions.
It is kind of impractical to cycle the contactor on and off with the pedal even if one's auto-dark helmet offers delay. With the torch mounted thumbwheel control I have, I cannot pulse the contactor rapidly anyway.
An added downstream transformer would drop the output voltage to the point the arc could not be maintained. The HF arc start would be similarly scaled to an inadequate level.
Does anyone have suggestions?
steamer wrote:

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    --Well I could imagine a purely mechanical method, maybe using a variable speed motor and a cam to make/break contact, but that's a little hokey. OTOH it's still cheaper than popping for a Dynasty, heh.
--
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If you placed this motor driven cam driven switch in the output lead, the Arc Assist would maintain an arc across the contacts as they opened. It wouldn't work well and wouldn't last very long. If you put this switch in the primary side of the transformer, you would need to add a 24 volt transformer to run the control board. The control board runs on a secondary winding wound on the welding transformer. Pulsing the transformer would pulse the power the control board runs on. The switch would need to be of heavy construction to withstand the power. It would need some sort of snap action for positive closure.
You could insert your switch in series with the foot pedal switch. The foot pedal switch turns off the Arc Assist through functions existing on the control board each time, before the contactor opens. You would be limited to pulse rates that the contactor can keep up with. Maybe that's adequate for your needs since you successfully pulsed the foot pedal manually to do what you wanted.
My goal was for as fast as a ten pulses per second and with adjustable on/off ratio from 10 to 100 percent. That would effectively reduce average output power down to where one could weld aluminum soda cans.
I don't think you would want to operate the contactor faster than two or three pulses per second and even that speed would give the contactor quite a work out. If a slow rate like that is adequate for your use, you could construct a motor driven cam operating a microswitch and wire it in series with the switch section of your foot pedal.
It would be simpler to do with electronics, such as an NE555 timer circuit. It wouldn't take more than $30 worth of parts to build and package as an accessory.
I measured output current today with the control winding open circuited. The output current was not any lower than the lowest setting on the control. Trying to control the welding transformer output through circuit modifications won't do the trick. The control range of the welding transformer design prevents it from being commanded to currents lower than the lowest setting already available.
That leaves electronic control of the transformer input power as the best way to accomplish what I wanted to do. It might be cheaper or simpler to sell this machine and buy something better suited.
steamer wrote:

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replying to Larry, Mark wrote: My econotig won't start and there's no points that I can find on the board so I can relate to what your trying to do.
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replying to Larry, Mark wrote: My econotig won't start and there's no points that I can find on the board so I can relate to what your trying to do.
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