decreasing output amps??

[Joe H]

I've got an Econotig. The dang thing will only turn down to 25amps, this is apparently a little too much to weld 22ga stainless. I can weld it if I lap weld, and have the piece tightly clamped to a chill bar, but they still come out dark, and I can't always get a chill bar where I need to.

I was wondering if it would be possible to connect my work-clamp to a big resistor to drop my output to 20 or maybe 15 amps. I guess I could just try it, but I thought I might ask here before I go hunting giant resistors, and possibly wasting my time.

Anyone think this is feasable?
If so, could one of you electrical-types tell me how to calculate how much resistance I need to drop in increments of say 5 amps?

Thinking about this, I'm wondering if it would interfere with the HF/HV arc start. Could a fine-wire jumper around the resistor help any?

Oh yeah, if this might work, where could I find, or how can I make a resistor that could handle it?

Thanks in advance!

[Washman]

Before everyone jumps in my poop ....

This is definately a Shade tree fix or possible work around if you have no other option.

I'm NOT going to say this is the best idea and NO I HAVE NOT EVER TRIED IT!

But the laws of basic electricity say's yes it can work.

I simply read somewhere on the internet a post from an Old Timer who said he used to put a piece of S/S tig wire in series with the ground clamp to reduce the the current.

Obviously the Diameter and the length of the wire would both effect the amount of the drop.

CAUTION !

The wire will get hotter than a six peckered puppy and can burn you.


Washman

[riley mcmillan]

Something else you may try in series is a heating element from an electric clothes drier or series of high wattage light bulbs. I actually used a light bulb in series with a battery charger on the 12 volt range for charging an 8 volt battery on my 8n ford tractor.

[awright]

I'm going to engage in wild speculation and free-association here, since I don't actually KNOW the answer to your question. I have, however, thought about this issue before, having an old AC TIG/stick welder with saturable reactor current control but without an external current control pedal.

One thing to consider is that the whole purpose of a constant current control welder is to supply the preset current relatively independent of the resistance in the welding loop, that is, independent of the arc resistance. So, simplistically speaking, the welder will try to push the preset current through whatever external resistance exists, including any external add-on resistor. There will obviously be some resistance that the current control cannot overcome and at that point the current will decrease. (Bear in mind that my understanding of this may be all wet.)

Look at the volt-ampere curves on page 11 of the op manual (http://www.millerwelds.com/om/o303t_mil.pdf). The left-hand curve on the DC graph on the right is the curve along which you are operating at minimum DC current setting. Its nearly vertical slope in its mid-range indicates that it is nearly a pure current source and will not be significantly effected by practical values of external resistance. That is, the machine wants to put out about 20 amps for welding loop voltages between about 20 and 40. I do not know without researching it what a typical TIG arc voltage is, but I assume it is somewhere in that mid range. Of course, a lot depends on your technique and arc length. An external resistor WOULD have a strong influence on the high current (up to 50 amps at short circuit) at low voltage (short arc) by pushing the voltage seen by the welder up to at least 20 volts, but that may not be of any practical use to you. That would take about a 1 Ohm, 400 Watt resistor.

As usual, the control board is a blank rectangle on the schematic with no hint of the control technology used. Looking at the very sparse schematic (page 19) and the drawing of the main transformer (item # 21 on page 22), it looks like the control technology is saturation of the center, magnetic shunt leg of the main transformer by feeding back control voltage to the upper winding marked "BE," (presumably Beginning and End of the control winding). This is somewhat akin to the older type of current control that pushes a magnetic shunt into the core of the main transformer of a welder with a big lever or crank, except it is done electro-magnetically instead of mechanically.

The minimum current setting would be with zero control voltage fed back to upper winding BE from the control board since that results in maximum shunting of the magnetic flux via the middle leg of the transformer. Flux that is shunted through the middle leg does not couple with the output winding, weakening the output current. The significance of this is that normally the minimum presettable current is set by the magnetic design of the main transformer, since you can't reduce the feedback control signal any further. It's like having the magnetic shunt in a buzz-box shoved all the way in.

There does not appear to be any sensing of the output current external to the main transformer that would, in a more sophisticated machine, be fed back to maintain the preset current. Thus, it appears that the control system operates "open loop," that is, without knowing what the actual welding current is except as indicated by the magnetic flux in the main transformer. This might be good, since whatever is done externally to reduce current will not be fighting a smart feedback control circuit. (It could, however, be fighting the feedback inherent in the main transformer design.)

So, after all that, I don't know whether external resistance will give you the ability to reduce welding current below design minimum while maintaining good arc characteristics. One thing to keep in mind is that it is normally not considered desirable to use resistive current limiting because the resistance does not give you the beneficial current smoothing effect that you get from a "stabilizer coil," such as Z1 in the Econotig circuit diagram. In fact, the resistance might actually swamp out the beneficial effect of the stabilizer coil.

However, it's not too difficult or expensive to try it out. Log onto the website of C&H Sales in Pasadena, CA (http://www.candhsales.biz/CandH/index.htm) and look for or ask about their large, (I MEAN LARGE!) power resistors. These babies are from a couple of inches to about 5 inches in diameter and about a foot long with power ratings up to a few kilowatts in air. Last time I was there, I wrote down the following items on their shelves:

0.069 Ohms, 170 amps.
0.171 Ohms, 100 amps (est.)
0.185 Ohms, 100 amps (est.)
0.218 Ohms, 68 amps.
0.405 Ohms, 70 amps. (est.)

This is not a complete list - just the ones that looked interesting for MY welder (and for testing fork lift batteries). They also have many smaller (but not small) ceramic power resistors in higher resistance values that may be more suited for your situation.

By Ohms law, the voltage drop at, say, 10 amps through the 0.405 Ohm resistor would be 4.05 volts, which would be too low to be of much use in reducing welding current on your Econotig. Notice that your machine has an open circuit voltage of about 75 volts.

If you are interested in their resistors, I suggest checking them out promptly. Like so many other surplus sources, this one appears to be drifting toward oblivion, having allowed their store stock to dwindle noticeably and, I just noticed, cutting down on their store hours.

I wish I had a better grasp of the effect of external resistance on your welding current and could make a more specific recommendation instead of making your eyes glaze over with these ruminations, but I am sure you are grateful this is all I can say.

awright

[Sandy]

You are looking at some dinosaur technology here :). Big ole thermal resistors aren't a common item any more. You'd have to get one that not only reliably and consistantly passes the current desired but dissapate the heat from from the current you want blocked. If you picked a fixed value resistor based on the machines present lower amp output you'd be limited to zero tunability. If I were to go to the effort I would start with something that would deal with my machine set, let's say, at 30 amps or so and give my self a little tunablility anyway.

[halbritt]

It's questionable to me how the CC circuitry would react to a resistor in series, but I suppose it'd be easy to test. I see big power resistors in the surplus stores around here. I wouldn't want to pay full retail for one, however.

You could give DCEP a whirl and see if that helps at all. You'd need something like a 300amp torch to handle 30 amps on DCEP, but it might help.

[elvergon]

I´ve heard of people dismantling their welder and taking out some loops of the transformer of the welder. This will allow you to have less amps. However I haven´t done it.

Maybe you should ask that instead =)

[Joe H]

Wow, thanks guys for all the interest! Awright, thank you very much for your detailed reply. I know that took some time, was mighty nice of you. When my head stops spinning I'll read it about 5 more times, maybe I'll understand then ;). I think what your saying is that it might work if I'm lucky. Would it be ok for me to use a big carbon rod for testing, or maybe a water resistor, or a big homemade inductor? The price of those big resistors is a little more than I want to spend to find out that it won't work.

Thanks again fella's!

[awright]

Joe H, I was actually saying I DIDN'T think it would work, but I'm not sure and it wouldn't be too hard or costly to give it a try and do a little experimenting. I do think you can attenuate that tail out to 50 amps at short circuit with the 1 Ohm resistor I mentioned. I don't know if that would help your welding or not.

I'm surprised at your comment about the cost of the big resistors. Did you visit the C&H website? I forgot to mention that the price for all the big resistors was $25, which I think is a real bargain. Smaller ones are cheaper.

Since you are working at low voltage, I suppose a water resistor would be reasonably safe, but can you get an ohm or two out of them? I suppose using enough plates and small spacing and saturated solution of what, salt (?), could get down to a few ohms. Never tried that. If you are using DC, as I think you would be if you are welding SS, a homemade inductor will not drop voltage unless it is upstream of the rectifier bridge. If it is in the DC loop, it's affect is only to smoth the current flow like a "stabilzer coil," and add only the DC resistance of the wire.

Sandy, the large resistors I was describing dissipate about 2 KW in free air, I believe. Putting a blower on it would increase power rating even further. These are coils of about 20 turns of resistance metal conductors about 1/2" x 1/8", wound edgewise around ceramic cores. They are the type of resistors used under transit vehicles for dynamic breaking (among many other industrial applications). No reason you couldn't put them in a bucket of water and greatly increase power capability. And, don't forget the benefit of the limited duty cycle of real, manual welding, even by experts.

No reason a big carbon rod couldn't be used, but where do you get one that is big and that has resistance down around a few Ohms? By the way, you can purchase a higher resistance wound resistor and tap it down by clamping your wire to an intermediate point of the resistance coil. Of course, you lose power rating that way proportional to the percentage of the coil you are not using. You can also tap the center and short the ends of a higher resistance wound resistor, thereby dividing the rated resistance by four. At 5 or 10 amps and 20 volts drop, you only need 100 or 200 watts power rating, less if you cool the resistor extermally with air or water.

Have fun.

awright

[Sandy]

Sandy, the large resistors I was describing dissipate about 2 KW in free air, I believe. Putting a blower on it would increase power rating even further. These are coils of about 20 turns of resistance metal conductors about 1/2" x 1/8", wound edgewise around ceramic cores. They are the type of resistors used under transit vehicles for dynamic breaking (among many other industrial applications). No reason you couldn't put them in a bucket of water and greatly increase power capability. And, don't forget the benefit of the limited duty cycle of real, manual welding, even by experts.

I've got a clear picture in my head of what you're talking about. :) I've also got a couple of the smaller wire wrapped resistors inset in a ceramic tube with the sliding center tap laying around here somewhere. With the painted on writing long gone, you can only guess at the heat dissipation values. I'd guess no more than 500 to a 1000 watts.

[awright]

Joe H, what ever happened to your current reduction experiments? I was just down in the Los Angeles area for a week of baby sitting (my wife does the baby sitting for her daughter's kids while I roam the LA area hunting and gathering), but I wasn't able to get to C&H Sales when they were open to jot down their entire high-power resistor collection values.

I'd be very interested to hear how things worked out.

Sandy, unless your ceramic adjustable resistors are quite large, they are probably rated at one or two hundred watts or so. Power dissipation in free air is pretty directly related to bulk and surface area. I'm looking at one CLAROSTAT matt green, ceramic 10 ohm adjustable resistor about 8.5" long, 1.25" diameter that is rated at 140 watts, and another OHMITE brown "DIVIDOHM Vitreous Enameled Resistor" clearly marked 10 ohms, 4.000 amps and 160 watts, same dimensions.

As I mentioned in some other thread, one cheap, readily available form of high power resistor (at somewhat high resistances for our purposes) is water heater elements. I'm looking at an 8.5 ohm (cold), 1650 watt 120 volt element that costs about $10. Of course, they must be submerged in water to avoid rapid burnout if used anywhere near rated power, but you would not get anywhere near rated power using one as a dropping resistor in a welder current loop. For example, if we assume you are running 10 amps through two of these 8.5 ohm resistors in parallel, each one is dissipating only 212 watts, which it MIGHT be able to handle in free air due to large surface area. If you try this at home, watch for discoloration from overheating (or just put the elements in front of a fan or in a pail of water). This random example would give a voltage drop of 42.5 volts.

Higher power rated elements at low voltage (120 volts instead of 240 volts) will have lower resistances. Paralleling two or more identical elements will divide the resistance by the number of paralleled elements. Seems easier, tidier, and safer than "water resistors."

Figuring out voltage/current/resistance relationships is merely an application of ohm's law E (voltage) = I (current in amps) x R (resistance in ohms). Also, power dissipation is P (power in watts) = I x E = I^2 x R = (E^2)/R. (For those of you familiar with these relationships, please don't be insulted.)

awright

[drivesector]

If its a 230V unit could he just get a buck boost transformer and try to buck the voltage down toward 208v reducing the output amps of the machine.
Just a thought
Drivesector

[Joe H]

I haven't gotten around to it yet. I've just been busy lately, no time for pulsejets. Things are going to slow down now, and I'm single again as of a few days ago :), so that will help ALOT. I'll get on it and let you know how it works out.

[awright]

drivesector, whether the input voltage reduction will work in reducing minimum output current depends upon the effectiveness and sophistication of the welder's built-in current regulation function. Don't forget that the control board is trying to put out a constant, preset current and in a high-quality machine, line voltage variations would be conpensated for in the control system so the user is not trying to adjust for line voltage changes caused by other loads on the line. That said, my guess is that the Econotig is not that sophisticated and, at least when operating at minimum current setting (that is, with no feedback current to winding BE), reducing input voltage WILL result in reduced output current.

Another unknown is that the control circuit may get confused or become non-functional if it's supply voltage is below that for which the electronics were designed. We don't know how they will react to low supply voltage, but could ask Miller Service guys.

Worth a try.

awright

[yeller_twin]

I have the schematics for the EconoTIG's inner workings. My EconoTIG decided it no longer wanted to function. I exhausted my troubleshooting abilities with their limited schematics in the manual and ended up taking it to the dealer. They worked on it for a few days and narrowed it down to the main card. $600 later I have a functioning welder. As I was leaving I started asking the Tech what tests he preformed. He showed me and looked around the shop, no one was there. He could tell I was not impressed with his explinations and said, "I have the schematics if you'd like to look them over?" I said sure, he dug through his pile of papers and handed me quite a bundle of information.

I will dig them out this week and scan them in. This reminds me, I need to troubleshoot the old card. I just know it's a simple component failure and everything else works just find.

I'll post up the schematics soon. This is a very useful thread, as I too have a lot of trouble welding thing walled stainless tubing with my EconoTIG and was thinking about this exact topic last night.

[MotoFab]

I have the schematics for the EconoTIG's inner workings.

I'll post up the schematics soon. This is a very useful thread, as I too have a lot of trouble welding thing walled stainless tubing with my EconoTIG and was thinking about this exact topic last night.

Yea YT, if you have the pcb circuit schemas and overall diagrams I'll take a look.


Joe, there might be some very simple you could try out after a cheap trip to Radio Shack or eq.

Changing min or max resistor values of a control circuit or two may do the trick. Or there may be trimpots that can be adjusted.

We'll have a look.

- Jim

[MotoFab]

In tripping through the EconoTIG manual Joe, it looks like the 20% AC Low output is as low as 18A. Maybe that'll work for the thin gauge SS.

- Jim

[Joe H]

Can I weld SS with AC????????? I had thought about trying that, figuring it would reduce the heat apllied to the work a little. I guess I should quit thinking so much and actually try.

[yeller_twin]

Here are the schematics and wiring diagram. :blob2: I have a few more sheets, like: troubleshooting and test points, but these are the most important.

http://img228.imageshack.us/img228/8637/pc1gv5.jpg (http://img228.imageshack.us/my.php?image=pc1gv5.jpg)

http://img169.imageshack.us/img169/4347/schematiccj1.jpg (http://img169.imageshack.us/my.php?image=schematiccj1.jpg)

http://img131.imageshack.us/img131/8514/wiringdiagrampc9.jpg (http://img131.imageshack.us/my.php?image=wiringdiagrampc9.jpg)

[MotoFab]

Ok Yeller -

Lessee. It looks like there are two trimmers to adjust the voltage input to the amp control pot.

It's hard to make out, but maybe R66 and R67, located at 6 o'clock low on pc1gv5.jpg

One or the other trimmer is selected to supply the amp control pot, based on the mode. Either TIG or Stick.

But, there is something questionable about the labeling of trimmers R66 and R67. They appear to be designated INT (internal?) and EXT (external?).

Though the surrounding circuit says that either one or the other trimmer will supply the voltage to the amp control pot, based on the TIG or Stick Mode.

And either trimmer is selected based on whether the transistor connected to Mode input (9 o'clock far left on pc1gv5.jpg) is conducting. (I cannot read the Q# but its base is R22)

So I am wondering about the INT and EXT labels. :confused: I mean it is obvious that the NPN controlled by R22 also stops the Post Flow Timer and holds the HF arc starter Pulse Generator in reset. That goes along with selecting the TIG or Stick mode, not the INT or EXT amp control.

Can you make a clearer scan? In multiple overlapping pieces maybe? Or higher resolution? If the file size is too big, you can set the scanner to 8bit monochrome or grayscale if you want a smaller file size. You co not have to worry about lining anything up, I'll piece it together here with Photoshop.

- Jim

[awright]

I concur with MotoFab that higher resolution images, primarily of the pc1gv5 control board schematic, would be very helpful. I'm not very skilled or knowledgeable about image manipulation, but I did finally manage to crop the pc1gv5 image into left and right halves and print them separately in portrait orientation, ending up with an effectively 11x17 image. However, due to the native resolution of the posted images, the labels on the components are still mostly unreadable and it is not likely that further enlargement would improve the readability.

Thanks

awright

[yeller_twin]

My version was printed with an ink jet printer and that's the reason it's hard to read. I scanned it in at 1000 dpi (3 meg file size - 11,500 x 8,000 pixels it was huge and took forever to open on any program), then reduced the file size down. I cannot read the printed images that I have, the scanned image is actually easier to read due to it's size.

Looking at my board - the two trim pots in question are R56 and R57. Glue has been applied to the tops of each trimmer to stop tampering.

My thoughts in the INT and EXT labels - The ext portion of the circuit connects to your pedal which has a simple pot with a lever. The int portion of the circuit is the welder output on the front of the machine (current output). It's just a large knob attached to a small pot.

If it helps any - RC4 and RC5 are the control wires for the shunt portion of the main transformer. When my welder quit, it would only strike an arc but no current could be brought up. In the end, the test that showed if my card was damaged was by shorting the wires for RC4 and RC5 together. The welder went to full output when shorted. I am almost willing to bet that Q13 is the damaged component on my card. It's a large transistor with a huge heat sink attached.

If it will help any, I will take my card to work tonight and take a picture with a decent camera. I will post that up tonight.

Can someone explain to me how the shunt portion of the transformer works? I've tried internet searches and nothing really gives me a definative answer on how transformer output is controlled.

[awright]

OK, here's a wild thought.

What is the possibility of shunting some of the minimum current that your machine will not go below away from the arc via an external resistor to ground?

NO! NO! Don't shoot or call the men in the white coats. We know that we will be fighting the inherent constant current design of the machine if we try to reduce arc current by inserting a resistor in series with the arc. This makes the use of a series resistor of dubious value in controlling arc current.

In electronics there are two types of voltage regulators - series and shunt. Shunt regulators are used much less than series regulators for a variety of reasons, one of which is that you are wasting power via the shunt current path. Another is that the energy supply must be current limited internally or by use of a series resistor. Our TIG sources are current limited. The zener diode is the most common application of a shunt voltage regulator. You can also buy sophisticated ICs for shunt voltage regulation.

Now, if we can start the arc from your Econotig normally and immediately activate a shunt path to divert some of the current away from the arc, we can get down to any desired minimum arc current that will sustain the arc. The activation of the shunt could be via an SCR as long as we are using DC in the welding circuit. In fact, if the SCR was inside a rectifier bridge, it could automatically handle straight and reverse polarity and AC welding current. At the currents we are considering, power MOSFETS would also be adequate and cheap.

Arc power at, let's say 20 amps and 25 volts is 500 watts, so we would have to dissipate up to a few hundred watts in our shunt circuit - not a big deal. Let's say we want an arc current of 10 amps but our machine won't go below 20 amps, and that our arc voltage is about 25 volts (wild guess without researching it). Then a shunt resistor of 25/10 = 2.5 ohms, with a power rating of 25*10 = 250 watts, would be about right. Sounds feasible.

I haven't given this idea more than about ten minutes thought while typing this up, so there may be big hookers that I haven't thought of. One is the question of whether a manually set shunt resistor could do the job or if an active circuit is required to dynamically adjust the shunt current to compensate for or aid operator technique.

Is this reinventing the wheel? Is it crazy? Any thoughts? If it could work without snuffing the arc or ruining arc handling characteristics, it sound fairly simple to implement. Have to protect the shunt switch against HF, though.

awright

[awright]

yeller_twin, these saturable magnetic devices are very confusing to understand. In fact, I think I got confused in my description of functioning in the 5th paragraph of my post of 7/28/06. I confused saturable reactor control with this kind of transformer with a saturable shunt leg on the core.

Let me try to understand what is happening as I simultaneously try to explain it to you. (Any Mag-Amp designers out there to help us out?)

Think of the welders with a big crank or lever for current control. Moving the crank or lever either moved a laminated magnetic bar into or out of the main transformer core between the primary winding and the secondary winding, or it physically moved the primary and secondary windings closer together or further apart. I am going to ignore the movable winding type of control here because it is not related to the Econotig type of control.

With the magnetic shunt fully withdrawn from the transformer core, the magnetic circuit was essentially a circle linking the primary and the secondary, with most of the magnetic flux induced by the primary passing through the secondary (except for the leakage flux that didn't get to the secondary because of the relatively large distance between the two windings). This is the way a normal transformer works.

Now, as you move the magnetic shunt into the transformer core with your crank or lever, you start to divert flux away from the main magnetic circuit passing through both the primary and the secondary windings, shunting it instead across the diameter of the core through the bar you are inserting. When the bar is fully inserted, you are diverting the maximum amount of magnetic flux through the shunt, taking that diverted flux away from the secondary winding and thereby reducing the output current to the minimum that the transformer is designed for.

That minimum current is built into the electrical and magnetic design of the transformer. You can't reduce the current any further (for a given external welding circuit condition) because the magnetic shunt is fully inserted into the core. This is important in figuring out how to reduce minimum output from the Econotig.

Now build a transformer with a magnetic shunt permanently built into the core of the main transformer, as in the Econotig. In the absence of some other controlling factor, it will produce the minimum output current because the shunt is, in effect, fully inserted into the core. How do we increase the output current above that inherent minimum? We magnetically saturate the shunt leg so it cannot carry any more magnetic flux, that is, it cannot divert more flux away from the secondary coil. When the magnetic shunt is saturated, it essentially disappears from the magnetic circuit. Saturation occurs when all the magnetic domains in the steel are lined up with the magnetic flux and a further increase in flux does not result in more domains moving into alignment - there are no more domains to align. We saturate the shunt leg by passing control current from the control board through the "shunt coil" on the shunt leg. This is where the control circuit from in the Econotig gets confusing to me, (HELP,out there?) and where it would be very helpful to be able to read the schematic.

The circuit block at 2 o'clock on the control board schematic supplies the control current to the shunt coil. What we CAN see is that the main control is by an SCR directly across the shunt winding. So two things might be happening. (1) the SCR fires and creates a short circuit across the shunt coil and high current induced in the shunt coil saturates the shunt leg, or (2) external power is supplied to the shunt coil from the power input to the control circuit (the arrow above the opto-coupled SCR) and the firing of the main control SCR shunts control current AWAY from the shunt coil. Hard to say which is actually happening without being able to read connections and component values. I think it is (1) above. Can anybody see the values of the two resistors feeding the SCR anode or what the arrow connects to? Haven't figured out how you get full-wave control out of this arrangement.

Hope this answers more questions than it raises. More knowledgeable comments are welcome.

awright

[MotoFab]

Looking at my board - the two trim pots in question are R56 and R57. Glue has been applied to the tops of each trimmer to stop tampering.

My thoughts in the INT and EXT labels - The ext portion of the circuit connects to your pedal which has a simple pot with a lever. The int portion of the circuit is the welder output on the front of the machine (current output). It's just a large knob attached to a small pot.

Thanks for explaining that INT and EXT refer to the 'panel amp pot' and the 'pedal amp pot'. I understand that part Yeller.

But pertaining to the schematic, more accurate labels might read 'SMAW/INT ONLY' for R56, and 'GTAW/BOTH INT AND EXT' for R57.

And, adjusting isn't tampering :)

If the results are satisfactory, a simple switch can be added to the front panel to select LOW AMP range.

- - - - -

Hey, is Joe still with us? I bet he just switched to AC and is done with it.

- - - - -

Yes, it is nice to have the reference designator #'s on the drawing, but the circuit is understandable without them. The R and C values, and silicon part numbers, are important to determine exact levels and waveforms. But they are not necessary to determine the general circuit function. And from that, which components are operating the shunt control.

Take a look at something. The wipers of R56 and R57 are in the same 'net', (jargon for connected together).

Pots R56 and R57 'trim' the supply voltage, which then controls the shunt circuit. The way this circuit is designed, the 'trim' levels are different for TIG and Stick.

The waveforms or levels appearing on nets X, Y, and W, control the shunt circuit.

The waveforms or levels on X, Y, and W, may or may not be different for GTAW and SMAW.

What is certain - R57 is selected as the supply voltage for GTAW amp control, and R56 is selected as the supply voltage for SMAW amp control.

The supply voltages from the trimmers might be different because for GTAW, the supply voltage from R57 passes through both INT and EXT pots by way of Q14.

- - - - -

I am not trying to confuse anyone.

Depending on which process is selected, one or the other of the trimpots adjust the shunt control.

Placing the process switch in GTAW, and adjusting R57 CCW, occurs as the most logical step.

- - - - -

And please, my friends call me . . .

- Jim

[Joe H]

I'm still here! I can't honestly say that I understand anything that you guys have said, but I read it anyway :). Actually, what Awright said about the saturable core sparked a flashback from HS physics class, leading me to think that it may be hopeless.

I've been working too much lately, and haven't had much time to play. Tomorrow though, I'm going to try AC welding 22ga SS and see what happens. I'm sure it isn't going to be as pleasant as DC TIG, but maybe it'll work at least. I'll let you know how it turns out.

Thanks again guys!

[MotoFab]

Error.

Last post reads:
The supply voltages from the trimmers might be different because for GTAW, the supply voltage from R57 passes through both INT and EXT pots by way of Q14.

Should read:
The supply voltages from the trimmers might be different because for GTAW, the supply voltage from R57 passes through both INT and EXT pots bypassing Q14.

I put the wrong sentence halves together

- Jim

[yeller_twin]

Here are some pics of the card.

http://img237.imageshack.us/img237/2347/pc1picch1.jpg

http://img156.imageshack.us/img156/4698/pc1backsq9.jpg


I rescanned the orignal schematic for the board. It's high res and file size is around 25 meg. I do not have a place to host it, but I can email it to who ever wants it. I do not have any information on the component vaules, but I can read them off the card if you would like to know.

The arrow above the opti-SCR is RC9-2 - It's path is (from the board) RC9 to OC1-2 (7 o'clock on the schematic), then it goes to OC2-5 (shunt driver area), then on to R53 (1000 ohm, too me it looks like an inductor - hollow tube with FR5 label), then to R52 (same as R53), then to the Anode of Q13.

Q13 Anode - to Sup 1 and R52 (1000 ohm), R53 (1000 ohm), OC2-5 -> back to RC9-2. Might want to check my color band readings.

I'm with you on the shunt controls, but I do not follow the schematic for the trim pots and how they relate to the shunt control portion. I've found X,Y, and W and how they get to the shunt control. But I do not understand the concept of internal and external pots and their influence on the shunt.

What happens when you get your +35V on your mode (RC1-8) switch? How does that eliminate the TIG portion of the controls?

I really need some more learning on component function and circuitry or dig out my note books from school. That makes my brain hurt just thinking about it :cry:

[MotoFab]

Here are some pics of the card.

I rescanned the original schematic for the board. It's high res and file size is around 25 meg.

I'm with you on the shunt controls, but I do not follow the schematic for the trim pots and how they relate to the shunt control portion.

What happens when you get your +35V on your mode (RC1-8) switch? How does that eliminate the TIG portion of the controls?

The populated PCB pictures will be useful.

And for sure you do not need to send a 25MB scan.

We can do this in pieces. Here is a step. Take a look at the image attached to this post. The snip is from the PC1 schematic located at 7 o'clock.

What does header RC9 (? blur) connect to? Is it in use in your machine?

Re: Tig/Stick mode and trimmer/shunt relationship, I'll markup a part of the PC1 schematic and post it. Maybe you'll get the idea. Dusting off cobwebs is a good thing :)

- Jim

[awright]

yeller-twin, just think of the millions of people who spend an hour a day doing crossword puzzles instead of learning how to read schematics. In my opinion, reading schematics is much more fun. I'd like very much to see the readable schematic. I'll send my address by PM.

Your photos are great and very helpful. Those two brown tubes near Q13 are definitely power resistors - probably 5 watt or so. Which does raise an interesting question about my analysis of what the relationship of the SCR (Q13), the resistors, and the shunt coil is. If those resistors are dissipating, say, 2 watts, that would imply average current of about (2/1000)^.5, or about 0.045 amps. While that's not a lot, it does suggest that the resistors are passing some power to the shunt coil, not merely providing bias for the control circuitry to the left of Q13. Still a mystery to me. I suppose it's possible that the designer felt that that much current was necessary to reliably trigger Q13, but it seems like a lot. What are the values of the three resistors between OC2 and Q13? Is the H11C1 OC2?

Jim, I see that we have a basic difference in approach to the problem of reducing minimum current. I have my doubts whether diddling with the trimpots can do much to reduce minimum output current from the welder. As I see it, the trimpots may affect the bias points of the SCR trigger circuit and change the trigger point relative to knob or foot pot position (I haven't actually tried to trace these out), but if you are at one or the other extreme of SCR trigger point (max on or max off, whichever results in minimum current out), I don't think the trimpots are not going to have much influence. That is, when the shunt coil has no current flowing in it, the machine is at it's minimum output, no? And I presume that they didn't provide a trigger circuit that would not bring the shunt coil current to zero.

By the way, what are the components inside the empty circles at the collectors of Q1 and Q(2?)?

awright

Oh, yeah. The stick/TIG input at RC1-8 controls, among other things, the HF generator in the upper right corner via the transistor (Q?) and pin 2 of U1, the Schmitt NAND gate at 12 o'clock. My guess is that the "VOLTAGE LEVEL SENSE" block at the upper left senses voltage at the tungsten and turns off the HF generator via pin 1 of the NAND when the arc is struck. Just a guess, since I can't see what RC1-4&5 connect to.

[MotoFab]

Here's something edited from one of Yeller's post attachments.

- Jim

[MotoFab]

Jim, I see that we have a basic difference in approach to the problem of reducing minimum current.

As I see it, the trimpots may affect the bias points of the SCR trigger circuit and change the trigger point relative to knob or foot pot position

(I haven't actually tried to trace these out), but . . I don't think the trimpots are not going to have much influence. That is, when the shunt coil has no current flowing in it, the machine is at it's minimum output, no? And I presume that they didn't provide a trigger circuit that would not bring the shunt coil current to zero.

I hear ya AW, but I have never had any luck with my approach changing how a circuit operated :)

It isn't so much the trigger level of the SCR, but the phase. Not what triggers it, but when.

I understand you haven't looked at it yet, maybe take a look which components form the 'top rung' in the divider network formed with the foot pedal and/or front panel potentiometers. Maybe consider it as, where the divider network gets its 'high side' supply voltage.

- Jim

[yeller_twin]

What does header RC9 (? blur) connect to? Is it in use in your machine?

I will be around my welder this weekend and I will pull the cover and trace the wiring. For some reason I remember this being hooked up, but I cannot remember to what and I cannot find it on any of my prints.

Re: Tig/Stick mode and trimmer/shunt relationship, I'll markup a part of the PC1 schematic and post it. Maybe you'll get the idea. Dusting off cobwebs is a good thing :)

- Jim

I would really appriciate something like this. Though I understand the effort you are putting forth for something that is not yours. If you are going to spend a lot of time, then I won't mind if you decline.

One question for EconoTIG owners. Can you TIG on DC positive? When I was welding some really thin (made a little figure out of filler rod) I ran DC+. This kept the heat out of the work and allowed me much better control. I never thought about using this on SS. However, something tells me you cannot run DC+ on this machine. I can't remember if there is a warning or if the machine won't function under these settings. I'll have to check it out.

[awright]

Jim, I agree that the two transistors at 6 o'clock with inductors in their collector circuits are switches that switch on one of the two voltage supplies for the internal or external current setting pots. Is that what you were describing in your last post? No problem there. My problem is figuring out where the wipers from those two pots are connected. Spent a lot of time this evening trying to decipher the notations on the schematic, but had to admit defeat after I started falling asleep. Have you figured out where the pot wipers return to the control board?

At the risk of repeating myself ad nauseam, can't we assume that one of the extremes of the pot wiper position would be where the output current is at the minimum that the machine was capable of? Can you think of a reason they would trim the minimum up to some higher value than the main transformer could go to? If you agree with that premise, are we likely to find an adjustment that allows us to reduce the current below the present minimum? Do you have an opinion about whether the output current is at a minimum with SCR Q13 off or max on? That is, do you think Q13 is controlling the output current by shorting the shunt coil or by diverting bias current supplied to the shunt coil via the two 1K power resistors away from the shunt coil? I haven't figured that out yet.

I'm going to wait to see a schematic I can read before going further with this.

awright

[MotoFab]

I will be around my welder this weekend and I will pull the cover and trace the wiring [for RC9].

I would really appreciate something like this [schematic description].

Can you TIG on DC positive? I'll have to check it out.

Good deal.

You bet. I'm sure I'll get to a good part of it.

Check out TIG on AC too. The EconoTIG minimum current setting is a little lower on AC than DC, and also the work would be a little cooler with AC. Double your pleasure so to speak.

- Jim

p.s.: Hey Yeller, take a look at the image a coupla posts up. Do you have the waveform views referred to in Section 5-3? I KNOW WE ARE NOT TRYING TO GO THIS FAR BUT . . The waveforms in Section 5-3, and the silicon component datasheets, are needed for a full analysis. With the transformer drawing, it isn't possible to know what AC signal appears on the shunt winding from the transformer primary. (The transformer isn't an off-the-shelf item, you know :) ) I can make some estimation based on the surrounding circuitry.

[MotoFab]

.

awright says: "My problem is figuring out where the wipers from those two pots are connected. Have you figured out where the pot wipers return to the control board?"

Do you mean the trim pots on the PCB? Or the front panel and foot pedal pots?




awright says: " . . . can't we assume that one of the extremes of the pot wiper position would be where the output current is at the minimum that the machine was capable of?"

If you mean the panel and foot pedal pots, then no, there is no logical construction as to machine capacity possible. But we can know that the resistance of the foot pedal and knob does adjust the machine output to the extent that those resistances can. And we can know that without knowing anthing about capacity.




awright says: "Can you think of a reason they would trim the minimum up to some higher value than the main transformer could go to? If you agree with that premise, are we likely to find an adjustment that allows us to reduce the current below the present minimum?"

I'm not trying to screw around AW, but the first question assumes facts not in evidence.

As to the second question; an estimation of the likelihood of the pcb trimmers providing a worthwhile amperage adjustment range can be made without agreeing to any premise.

My estimation is that the likelihood is high.

There is a high likelihood that the designers followed the nearly universal practice of selecting components and component values in the surrounding circuit based on trimmers R56 and R57 set to their middle value. After final assembly, and during calibration, the trimmers compensate for component and transformer tolerance. Without knowing the value or sign of the needed adjustment, starting with the trimmers at their middle value allows for the maximum adjustment flexibility. Get that? High likelihood :)

- Jim

[yeller_twin]

I do not have any more information on paper for this welder. I could take a trip by the shop that worked on it and see if the very helpful fellow is still around. He might be able to help me out if I ask nicely. Atleast let him know what we are trying to do and that I am not trying to make money. I'd say they owe me something after the money I spent there.

I'll take some pic of the welder with it's hood off and see if that helps any.

[slamdvw]

As I read on a newsgroup a while back " never underestimate the power of a 6 pack to the guys in the shop " I had this problem with my welder a NON solidstate Hobart TR250. I doubt it would work on the input of your machine, but how about on the output... a SCR rectifier bridge? 2 scr's, 2 regular rectifiers, and making a variable rectifier. Granted your usual filtering, smoothing after that would apply. Come off the AC side of the machine, to a variable rectifier ( controlled by light dimmer ) thru filters, to your torch.

Just my .02

robert

[MotoFab]

Yeller, I used your schematic scan as the raw material for these.

Some edits are made to the nomenclature for clarity.

The components are moved around, but all circuit connections are the same. No changes to the circuit.

The panel potentiometer and foot pedal have been added.

I hope this makes the input to the shunt control easier to understand.

- Jim

[MotoFab]

Stick mode.

Front panel potentiometer R1 is in parallel with R58.

In stick mode, the command signal does not pass thru the foot pedal pot RC1

[MotoFab]

TIG mode.

Front panel potentiometer R1 remains in parallel with R58.

R1/R58 are in series with the foot pedal pot RC1.

The control signal passes thru both R1 and RC1.

In TIG mode, the front panel pot R1 sets the maximum amp limit.

And the foot pedal pot RC1 sweeps between 'zero amps' and that front panel set maximum.

(The colors do not signify anything. They are just colors.)

[Joe H]

Well, I tried welding 22ga 304SS with AC today. It worked way better than I had expected! I had to turn the machine up to about 15% on the dial. It made some really pretty shiney little fusion welds about 1/8" wide. This should work out well for my needs. Using AC is kind of annoying, but I can get over that.

Thanks guys! I love this forum!

I've got to get a new USB cable for my camera, I'd like to show these little welds off. Maybe I can get out and find one tomorrow...

[yeller_twin]

Thank you very much for the labeled diagrams. That helps tremendously.

I did a little welding today, both AC and DC. I tried some DC+, but had a hard time controlling the arc with such low settings. AC worked ten times better and I could heat the work up without blowing through it. Normally, the tubing I was welding, would burn through the second I struck an arc on the lowest setting. AC low allowed me to turn it up to 100% and use the full pedal range without burning through.

Here are the pics I took today. Nothing is connected to RC9. They must have updated the board, as this one is a lot different than the one I have at home.

http://img145.imageshack.us/img145/1741/boardconnectionsti5.jpg

http://img145.imageshack.us/img145/7048/transformeruo6.jpg