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  #46 (permalink)  
Old 01-04-2006, 03:16 PM
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to make this usable and adjustable we use a Potentiomer as a variable voltage divider this is what controls the base voltage on the #1 transistor

a variable voltage divider using a potentiometer looks like this:
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  #47 (permalink)  
Old 01-04-2006, 03:21 PM
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if you look at it as if the middle connector.. known as the wiper is right in the middle we would see half of the output voltage on the wiper connection..

as we move it closer to the negative side we would see less volts down to zero. and if we move it to the positive side we would get more volts all the way up to the 25VDC max +


so with this we have hooked up to our DC power we now have a variable output voltage from 0 to 25VDC.

the problem now is that a potentiometer will only take a small load before it burns up.. this is where the transistor comes in

how are we doing.. you guys learning this stuff??
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  #48 (permalink)  
Old 01-04-2006, 03:26 PM
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I think this is a much better way of figuring it out, rather than just a schematic becuase youll know how it works, and you can change it up or troubleshoot it
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Old 01-04-2006, 03:40 PM
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So,

we now have the variable output from the Pot.

the variable wiper output goes into the Base of a TIP3055 Transistor you can see it just below the rectifer on the aluminum plate.. both the rectifer and the tip3055 have heatsink grease on the back so it will dissapate any heat better.

Here is a pic of a TIP3055 there are only a couple bucks at an electronics store
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Old 01-04-2006, 03:45 PM
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But yet the tip does not have enough current carrying ability to serve the field coil reliably yet, so it controls the base input of two 2N5885 Transistors in parrallel.. you can see them on the right, they have there own heatsinks, and I got them used from the local electril suppy store for about $5 each.
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Old 01-04-2006, 03:49 PM
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the output form the 2n5885's are run through about 12" of 14 AWG wire to act as a small resistance to balance the load between them.. before there are joined and hooked to the field coil wire. you can see those wires as the red wires that are coiled up.

Also when i was inside the alt I ran the negative field coil brush terminal out of the case as well, so it is isolated from the welding output and is connected directly to the 25VDC - output. simply a safety measure to protect the electronics
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  #52 (permalink)  
Old 01-04-2006, 03:53 PM
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with this all hooked up you know have a variable welding supply controlled by a pot.. you can see the main power control pot sticking out of the back of the welder on the right hand side.

Becuase Volatge dividers work on resistance and there is so much wire the 25VDC we started with max's out at the field coil input at about 18VDC wich is great as all it needs is 12VDC. but I like ot be able to crank it up that extra little bit..

But!! if I was hooking this up to a 240VAC wall input I would end up with more power there and might have to limit it a bit so not to have a melt down.. I am at 208VAC input so we are good to go.


next up foot pedal control
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  #53 (permalink)  
Old 01-04-2006, 03:54 PM
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Crazy Larry how you makin out??
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Old 01-04-2006, 05:33 PM
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Travis,
you are doing good :-)

Maybe, an addition of a needle voltemeter 0-50V to your setup will be of some benefit to get a 'feel' of what is needed (voltage-wise) to work with different thickness steel?

Where would you install it to protect its circuit?

Thanks,
Denis
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Old 01-04-2006, 06:28 PM
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Well yah, I would use a digital one though. You can put them anywhere really they are wired in paralell. I would get one that goes up to 150 v or so and put it right on the welder output. but its not a big deal anyway. its easy to just get a feel for it. with tig your using your foot on demand. so its all feel.

I got the schematic done and here it is...
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Last edited by Travis Rankin; 01-04-2006 at 06:36 PM.
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Old 01-04-2006, 06:44 PM
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VR1 is the Max power control and VR2 is the foot pedal control


VR2 is a sliding pot I canabalized from a sliding light dimer switch.

are a couple shots of the foot pedal
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Old 01-04-2006, 07:27 PM
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you could easliy swap out T1 and BR1 for a car battery.

It would make for a nice setup for an onboard tig welder on your truck
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Old 01-04-2006, 11:34 PM
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I'm with you.

do the resistors in the voltage dividers have to be any specific resistance? or any as long as they are identical?.....n/m, i see you spec'd 0.1 ohm.

if we need to step down to 12 vdc, is it necessary to start with the 240 vac? Is 120 vac insufficient?


Travis, thanks for your time. I do appreciate it.

Last edited by crazy larry; 01-05-2006 at 12:05 AM.
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  #59 (permalink)  
Old 01-04-2006, 11:38 PM
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Damn good job! I am even understanding it! mostly...
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Old 01-05-2006, 12:59 AM
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Super!!!!!!!!
Everyone just have patience. It looks worth the wait. When you get it all done, just post the grocery list, all the drawings you can handle, and the tech info to go with it. That would be sweet. And some alternate info on building other rigs, like 110v supply models, 220v models ..etc. And i will be building a cover over mine, (fully inclosed using polished diamond plate ) so perhaps a small cooling fan would be in order? how would that wire up?
For some excellent electrical readin in the meantime, look at http://www.madelectrical.com/electrical-tech.shtml
i did these header with a mig. (they leak like crazy)
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