A voltage divider is a way to use less than the maximum output voltage.
if you take the negative of a DC power supply and the positive of the same
DC powers supply and put two equal resistances between them in series, and then attach a wire to the connection between the resistors you will get half the voltage of the main output. like this
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
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.
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
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.
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.
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)