Miller 180SD Tig Question (technical)

Greetings All,
I'm just curious, How does the electronics on my Miller 180SD Tig machine Actually control the amperage?
In my 'good old days' I took apart an old A/C Stick Buzz Box. Its internals were fairly simple. When you cranked the Amperage range selection handle, it appeared to mechanically move parts of the large transformer. Thus I assumed the old buzz box was nothing more than a huge variable transformer.
I guess my question is, since I know the Miller 180 is a transformer based machine, how exactly are they controlling the transformer output?
I've built a 2KW "TOY" Induction heater, so I'm pretty familiar with general analog / digital electronics,
Thanks.
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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got plans for that induction heater?
would like to build one with a 8 inch id loop for heating cylinder barrels and rod nuts?
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On Sun, 19 Dec 2004 20:15:40 -0600, "williamhenry"

I built mine based on the 2kw IGBT schematics found on this website: http://dave.golfbuddys.com/hvguy/new/indheat.htm
Here's a direct link to a Gif file of His Schematic:
http://dave.golfbuddys.com/hvguy/new/indheat/igbtindheat.gif
Of course, the one I built was a little different..
I used 3 parallel IRFZ44 mosfets for each of the 4 corners of the H-Bridge, and used a large battery bank for the power source. Other than that the driver is the same.
One trick not noted on the HVguy web-site is you NEED to add capacitors across the work coil. The trick is to put the right amount of capacitance across the coil, so the coil makes a nice sinusoidal wave form at your selected frequency.
The problem I ran into was you really need those special water cooled capacitors, as I kept melting the legs off mine due to the insane amount of current.. On my next effort, I plan on using a water cooled induction coil, and solder the capacitors directly to the copper tubing. Hopefully this will keep the legs from melting off.
Here's a link to some pictures of my rig in various states:
http://lerch.no-ip.com/atm/Projects/Induction_Heater/page_01.htm
http://lerch.no-ip.com/atm/Projects/Not_Silly/page_01.htm
BTW, check out this website, it's a great source of ideas and makes for some excellent reading material: (Check out the video library under the 'Learn' link!)
http://www.ameritherm.com /
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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James -
Yes - The 180SD has a transformer, it isn't a transformer only welder.
If you bought the 180SD you have a manual - if not it would be useful someday.
On page 25 - (I have every page (2 per) in plastic page protectors in a 3 ring) Coming from the top is where the supply voltage comes into a transformer. Out the other side - (can be traced back from the 'work' cable connection.
Notice the four SCR devices (act like Thyratrons) or control elements for high current. The control element is the diagonal line and notice it goes to the "Main Board" where the magic control signals are generated. The Capacitors across the SCR devices are 'snubbers' that shunt RF power pulses that are coupled or transmitted back to the SCR. Post flow of the SCR's is a current loop that measures the current (likely AC, DC, AC+DC) and passes it back to the "Main Board".
So the Magic is in the Main board while the gating of the high current is in the MOD1 and MOD2 SCR's.
Martin [ has a 180SD myself ]
James Lerch wrote:

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Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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Hi Martin,
Yes, I have the manual, and I think I'm starting to get it now.
In its most simplest form, the SCR's are working similar to household electronic dimmer switches.. ( Or maybe its operation is closer to the output side of a switch mode power supply)
The reason I was curious about this stemmed from my curiosity on how the 'Clean / Penetrate' knob functioned while in A/C Tig mode on Aluminum.
I wonder how much it costs to take the training courses to become a Miller authorized repairman?
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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