tig current current control question..

[mtpturbo]

http://www.mtpturbo.com/fairmont/dta250schematic.jpg

I can hope those of you who are more electrically inclined can give me a hand with this one. I recently bought an old ESAB DTA 250 UM tig machine and pulled the covers off to clean it out and discovered some butchered wiring. I purchased a schematic and have since figured out the bulk of it but have a couple of questions. (btw, there should be a link to the schematic above if I am doing this correctly.) Anyway, as received the machine had two the triacs (nodes 64-10 and 7-10) removed and the current control pot on the front panel connected to 64 and 10 and the "surge suppressor" (between nodes 64 and 7) disconnected. Anyway, in this configuration, the machine will run but there is no current adjustment via either the foot control or the front panel. I have since replaced and properly wired the triacs and reconnected the surge suppressor but am not sure where the current control pot (measures approx 0-1800 ohms) needs to be connected. There is a symbol on the remote/panel control board on the schematic but I believe that is in reference to a smaller pot (only adjustable with the side panel removed) which resides on the remote/panel board. That board is all 20 ga wiring while the big current contol pot is 14-16 ga and everything appears to be accounted for (no obvious connection point for the pot wires). I am at a loss here, do any of you have any ideas?

Also, the surge suppressor was disconnected but doesn't appear to be cooked of anything. Is there any way to verify its function before reconnecting it? Can I test the machine with it disconnected (it ran fine before, but I had no current control, or triacs...)

Any help would be greatly appreciated.

-Chris

[slamdvw]

If I follow the description correctly... It looks like they bypassed the 'variable rectifier' - one leg of it... with that pot.

The 'triacs' are actually SCR's... 12.5 amp ( just by a quick google of the part number ). So... it's possible that the pot isn't carrying the current, which will cause the output current to be in the lowest setting of either High or Low ( am I correct..? )

Looking at the schematic, where that goofy looking transformer "high / Low" is, that's the output current control. Feed it more DC current on the 'right' side of it, allows more current to flow on the 'left' windings. ( saturable core reactor ).

Are the triacs ( scr's ) still there? They may be bad. Or another part of that rectifier bridge may be toast.

As far as testing the surge supressor... I have to step away, they're out of my league.

Do you have access to an oscilloscope? That'd help greatly in checking the SCR's.

What they basically did, is make a hefty Light Dimmer switch. Looks like they take 48V ac, from the main transformer, rectify it and feed it to the control reactor. Control that voltage ( the 48 volt ) and you control the output of the machine.

12.5 a scr

[mtpturbo]

Thanks for the info slamdvw.

As best I could tell the machine was running at 100% (full current, not minimum) as configured less the triacs. I installed two new triacs and wired them back in according to the schematic. I am still not sure where to break in to the circuit with the main current control pot (64-10 is where it was, but that doesn't look like the way it was from the factory as there were new crimp connectors on the wires that didn't match any of the original equipment connectors).

I shoud be able to put 2 and 2 together based on your reply. I too understood it to be a dimmer switch like arrangement.

As explained in the operator's manual, the front panel current control sets the max current and the pedal is used to regulate from very low current up to the max as set by the front panel control pot (when the panel/remote toggle switch is set to remote). I checked the pot on the front panel again last night and it looks like 0 ohms at minimum current and close to 800 ohms (I originally measured closer to 1800) at max current. The pedal shows high resistance (20k+) at rest and goes down near zero as you depress it..(there are three legs on the potentiometer, I need to confirm that I am measuring the correct two..)

Thanks again, if anyone else can shed light on the arrangement, I 'm all ears..

Chris S.

[MotoFab]

Pretty cool that you are going for getting the supply working again Chris.

A surge suppressor functions as two zener diodes connected back-to-back. Based on whatever the 'threshold' voltage is, the suppressor will 'truncate' both peaks from the 48VAC transformer connected to the bridge input. Cuts the tops off the hills as it were.

One way to test it is to remove the suppressor from the circuit and connect a 100K 1/4W resistor in series with either terminal. Then connect the little two-component series circuit across a 117VAC supply and look at the result on a scope.

Or maybe a more practical solution is to use a meter to measure the AC output of the 48VAC transformer, without the suppressor, and with the suppressor.

But maybe some questions to ask first are:

How did the SCRs fail? Or, did they fail? Or, is their control signal present?

Maybe check these:

Is the output from the CT2 secondary correct?

The actual circuit of the front panel switch isn't shown. Does 60/61 switch to 62/63?

If so, does 60/61 appear at 62/63, and also at 67/66?

- Jim

[mtpturbo]

***UPDATE***

I spoke with Dennis at ARCO welder repair who seems to know these machines better than anyone and think I have it figured out... It turns out that removing the pot currently mounted on the front panel and rotating the PC board with the panel/remote switch etc lines up the board mounted pot with the current control pot mounting hole on the front panel..someone did some creative engineering and added a pot which wasn't needed and re-oriented the board..hence the confusion. It looks like putting a 1K 2 watt pot back on the PC board (the original has been removed along with the current control SCRs) will get me back up & running..

Thanks for the info on checking the Surge Suppresor Jim, I'll give it a whirl..

Chris S.
www.mtpturbo.com