`Scoping SCR trigger signals? Single phase TIG welder repair

[Chris Wilson]

Sorry to have to return to this subject again. I borrowed a supposedly good and working control box from the Migatronic guy, and although the machine now welds in AC and DC I believe there is still an issue. I renewed the unobtainable R1 on the "schematic.jpg" link 100 Watt 10 Ohm centre tapped rheostat above the fan, that blew when the choke shorted, with 2 off 10 Ohm metal clad 100W resistors in parallel, with another pair similarly connected in series. I then centre tapped the two pairs, giving me again 5 Ohms per side, centre tapped. These big boys are also on big cast heat sinks, below the fan, so should be able to dissipate far more heat than the original. There's a label on the machine saying the original rheostat should be set to give an equal 5 Ohms per side, centre tapped. My metal clads measure nears as damn it 5 Ohms per side, too, measured properly with Kelvin clips on a decent low ohmage meter.  In DC welding or just squeezing the trigger and not striking an arc, these stay cool. But in AC mode actually welding the half connected to M3 pins 10 and 13 gets mad hot. If I stay in AC mode and just pull the gun trigger but DO NOT strike an arc, both get mad hot. They then get very mad hot, fast, if I don't actually strike an arc. I really,, really need someone to tell me what this rheostat does and how it interacts in the grand scheme of things. When actually welding in AC mode it's almost as if the arc is splashy, and it seems to pulse  rapidly.  The Migatronic guy has gone to earth, and does not seem keen to involve himself further, but it's not through my unwillingness to pay, him or anyone else for that matter. Other TIG specialists do not seem to want to know at all. I now have a lot of money tied up in this damned thing, what with a new choke and whatever Mr Migatronic will want for his supposedly good control box...

I will link to the schematics again, unfortunately they span different documents, thanks, if anyone can find the time and will to have a look I'd be eternally grateful:

 http://www.gatesgarth.com/71616531.pdf

and


http://www.gatesgarth.com/schematic.jpg

The original rheostat that blew when the choke shorted is at:

http://www.gatesgarth.com/R1.jpg

R1 is on the "schematic.jpg"

Thanks again!

[megajocke]

You may have to redraw the schematic of the parts around that resistor to make sense of it without going crazy. From what I can see it's part of the circuit involving C1 which is described in the functional description.

The functional description says the open circuit voltage should be DC even when in AC mode. Is this the case? It seems at least one of the main thyristors shouldn't be receiving triggering pulses until the arc is struck. If there is AC even when unloaded, this could explain excessive dissipation in the resistor.

But anyways, how hot do they get and what kind of heatsink do they have? The aluminium clad style resistor power rating is usually given when mounted to an infinite heatsink, while the ceramic ones are usually rated for natural convection. Also remember that the resistors aren't made of flesh nor silicon so being hot may not be a problem unless excessive. "too hot to touch" wouldn't even make them sweat 

[tautech]

Chris, a few simple measurements can answer your question.
First measure voltage across each 5 Ohm resistor.
V²/R=W
The answer will show if you are within the W rating of the resistor.

Ref:
http://www.electronics-tutorials.ws/resistor/res_7.html

[Chris Wilson]

Thanks for the replies, worse case scenario was 30V AC across one half of the "rheostat", so I make that
30 X 30 = 900      900/5 = 180W which sounds a hell of a lot. This was not a steady voltage though, but seemed to pulse...

I am still unclear what the rheostat does though, and why it has such loadings on it.