AC welder question

Someone recently gave me a 225 amp AC welder which was made by Lincoln about 25 years ago. It has been sitting for about 20 years and was
previously hardly ever used. It has a switch on the front with positions ranging from 40 - 225 amps. I am an electronics technician by trade but admittedly I don't know much about welding however I have heard that DC welders are much more desirable. So I have some questions: 1. Where or when would it be more desirable to use an AC over a DC welder? 2. (well this ones not a question). I would like to construct on a suitable heatsink, say a 400 amp bridge rectifier circuit and install it between the switch and before the ground and electrode cables? 4. Are the outputs of welders normally protected in some fashion against current overload. Would I need to do this? 5. I understand that I will need a very large choke on the output side. The factory rep tells me that I can use almost any choke from a similar size DC welder of any make. He says these a quite large. Does anyone know what the typical inductance of this choke might be? Thanks very much for any information on this. Lenny Stein, Barlen Electronics
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snipped-for-privacy@yahoo.com wrote:

When you don't have enough money to buy a DC welder, or when you are experiencing arc blow and can't find a way around it other than to go to AC.

They would have to be RUGGED or well snubbed in some way.

Don't know about normally, the smaller welders I've seen have had nothing at all in the way of current limiting. A duty cycle detect circuit would be great.

Sorry, don't know. Miller has their schematics online, might look there, see http://www.millerwelds.com/service/locate_om.php
GWE
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I think that the OP simply can look a little harder. I bought a DC welder for $9.99. Simply thanks to looking for a few months/

I am doing an opposite project, I have a DC welder and I want it to make square wave AC. I talked a lot in sci.electronics.design and bought most components. I suggest that the OP checks out that newsgroup.

I think that he can rely on the overload protection of the AC side...
i
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need a very large choke on the output side. (clip) ^^^^^^^^^^^^^^^^^ Normally, the combination of capacitance and inductance in a DC power supply is there to smooth out ripple. Since you can weld with AC, and since good DC welders often include some kind of pulsation for control purposes, it seems to me that you don't need a choke. The heat input to the arc is averaged out by the heat capacity of the rod and puddle.
In a way it's like feeding AC or rectified DC to a light bulb. The filament doesn't cool much between pulses.
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If your main goal is an electronic project then the answers others have provided should help some but if your goal is mostly to weld you might want to read this first...
http://www.weldwell.co.nz/arcinfo.htm
Scroll down to the last section called "The Welding Current" and read what it says about AC vs. DC.
IMHO you should weld a bit with what you have first before you worry about modifying it. See what you can do with it. If you're truly bitten by the welding bug as many are you'll probably want to upgrade to a better welder rather than try and improve on the one you have. Just don't try and use 7018 rods unless you buy the AC specific version because the standard (DC) version can be really tough with AC. A good general purpose rod for learning is 6013 although others here will probably suggest something different.
Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com
"I'm not grown up enough to be so old!"

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DC will give you a smoother arc, less spatter. Some rods will only run on DC, but there are a good selection of rods that do run on AC. About the only time you WANT to run AC is when you are running into arc blow (magnetic fields building up that distort the arc)
It's common to build the bridge diode circuit using 4 (or more) diodes. Plain diodes need to have some hefty ratings. The open circuit voltage on the Lincoln runs in the 70 volts or so, there tends to be some serious voltage spikes on top of that. Amperage can be a little closer to the ratings on the welder since the current overloads are not as great.
Amperage in the welder is limited by coil and core design. Commonly the core is fully saturated, no more current can be pulled in the secondary. But when you stick the rod to the work, you WILL pull the full secondary current.
The more diodes comes into play by putting them in parallel. You need to add a tiny bit of resistance to each one to load balance. A piece of 18ga or 16ga copper a few inches long will do.
You do not need to put a reactor in the circuit since you have a pretty massive one in front of the diodes. Plus the DC for welding is much more about electrons flowing the same direction than about about a stead flow of them. If you must have a reactor, it will need to be wound with wire suitable for fairly high duty cycles at the max amperage you plan to run. Using the Lincoln as a base, figure 225 amps at 30% to 40% duty cycle. Duty cycle is figured on a 10 minute basis so this would mean running 4 minutes at full output. Most folks do NOT run their welders anywhere near full output so downrating is quite feasible. No clue on what the reactance would be, just wind a couple dozen turns on a suitable core and call it good.
I'd suggest that you just learn to weld using AC before getting all fussed about going to DC. Pick up 10 pounds of 1/8" 6013 rod plus 5 pounds of 1/8" 6011, burn the whole works on practice pieces. If you still need DC at that point, go get 4 big diodes.
snipped-for-privacy@yahoo.com wrote:

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Just wondering why to spend time and money on all this. Just go buy the right machine and be done with it. And have something that is going to run, and that you can get service and parts for.
Oh, I understand about the fun of building it and all that.
Just seems like a waste to me.
YMMV
STeve
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I built a DC rectifier for my 225A Miller stick welder. I used 12 35A bridges in parallel. To compensate for device variations I used about 14" of #18 wire (to the best of my memory) for each of the leads to the bridge. This was equivalent to 0.01 ohms so each bridge had the equivalent of 0.04 ohms in series (4 leads) which at 20A was 0.8V drop. For my normal welding range, around 120A, the drop is 0.4V.
The bridges are mounted on an aluminum heatsink so they are all at the same temperature and I also have a couple of muffin fans blowing air across the sink. I don't think it ever gets very hot, probably not even very warm since welding usually is a very low duty-cycle operation especially for beginners. You run a bead then spend time chipping slag, examining, setting up for next bead, etc.
Another thing is that you have to consider the maximum current the welder can but out on a dead short (when you stick the rod down). For my 225A Miller that can be as much as 280A! You need to check the VA curve for the welder. At lower output current settings the short-circuit current will be lower.
I also put 0.01uf caps across each diode in each bridge and a 120V varistor across the +/- leads of each bridge. I think this was overkill but you can't argue with success.
The overcurrent protection is provided by the design of an arc-welding transformer. It has magnetic shunts in it such that it saturates when too much current is drawn if you stick the rod down or limits it when welding if the "resistance" of the arc would want it to draw more current than the setting.
I used the cores from microwave oven transformers to build two inductors for current smoothing (or at least to prevent the arc voltage from going to zero). They are not quality transformers which means the E laminations and the I laminations aren't staggered. You can file/grind the weld holding the transformer together and pop the I pieces of the Es. Remove the microwave coils. I wound as many turns as I could of #4 welding cable around the E (probably around 4 or 5). You can't put the I pieces back on without an airgap or the inductor will saturate. I used a piece of around 1/8" thick wood panelling for a spacer. I made 2 of these and put them in series. I did not measure anything and it seems to work fine. Could it be better?Maybe but I don't want to bother fooling with it. I have read that the inductance in the long welding leads also helps.
Billh
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With an AC welder the current goes through zero and the arc could stop. Welding rods made for AC welders have something in the flux covering that helps keep the arc going as the current goes thru zero. Some rods are made that do not work well on AC. So your choice of rods is less, but it is not all that significant.
With a DC welder you can pick whether the rod is positive or negative. This makes for more or less penetration. An AC welder does not have this choice. It usually does not matter.
The current is limited in welders by the leakage inductance of the transformer. The transformer is designed to have a large and often variable leakage inductance. On a stick welder there is usually no separate inductor or any capacitors. You really don't care about how smooth the current is, you just don't want it to drop to zero and lose the ions that support the arc.
Dan
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I use 6013, 7018AC and sometimes 6011, and can't really tell the difference between AC and DC.

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I didn't know some of those big oil-cooled welders were AC. The arc was so smooth and the weld so good I thought this must be DC. Not so. It was the quality of the machine which is doing it. here in UK, oil-cooled transformer iron-and-copper AC arc welders are twice the price of a DC-only inverter welder. Hence lot of use of inverter. But the AC machine is known for being sweet.
Some rods will only run on DC. That isn't much of a restriction for most weldings, as there are AC rods to cover most needs. xx11/6011/cellulosics (potassium in binder) -- gives penetration for first run. xx13/6013/rutile gives a smooth weld finish for capping runs and can be used for first run in most circumstances Other posters mention a "7018AC" which I take it is low hiydrogen, good strength and runs well on AC
Perhaps weld with this machine, try using a DC machine - go take a project to one - and see on that basis whether there is any call to spend a single extra dollar on welding machines.
Richard Smith
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On 14 Oct 2005 16:58:28 -0700, snipped-for-privacy@yahoo.com wrote:

Unlike the consumer electronics industry, where anything and everything is obsolete before is taken out of the box and worth less than zero by the time you bring it home from the retailer, welders are industrial grade machines built to last. Welders are bought and sold according to make, model number, and condition of machine, not choronological age of machine.
Wear on the switch contacts is what counts. A basic "hardly ever used" Lincoln AC stick welding machine built 25 years ago is as good as one built today and worth about the same amount of money -- transformer technology did not change between then and now.

This could be a barrier to your becoming a good weldor due to your propensity to tinker with the machine rather than use the machine as it was designed to be used.

You are trying to tinker with a machine not built for tinkering -- its built for welding. Your next step should be to take a welding course. After taking a welding course, you will be in a much better position to decide whether or not a DC capable welder is something for you to condsider.

This is akin to asking, "When would it be more desirable to use a BMW over a FORD to get from point A to point B?" Both will get you there but, for some, the BMW offers more amusement along the way.
One must remember that, in welding, the objective, ALWAYS, is to "glue" two pieces of metal together. Farmers, with $millions invested farm machinery, get by with a basic AC stick welder just fine when it comes to maintenance and repair.
Those who earn their living from welding for others, prefer a welder with a DC option because:
a) Under some conditions, one can make welds look more pretty (having less splatter) by using DC, thus making one's welding skills look more "professional". The appearance of a weld might be important if one is trying to convince a reluctant customer to pay one's inflated fee for a "professional weld" and has nothing to do with the quality or strength of the weld that could just as easily have been done with AC.
b) Secondary to a) above, when one welds for others, one has no control over the materials being welded. DC allows the use of non-ferrous and stainless steel electrodes in case that one-in-a-million customer should show up on one's doorstep with something other than mild steel to weld. This assumes that one has enough experience to recognize what the material is before touching it with the wrong electrode and enough knowledge to select an appropriate non-ferrous electrode.

This might be a "fun" electronics project if one finds amusement in bolting large rectifiers to large heatsinks, however; if you have no interest in welding, your welder is worth more money in unmodified factory condition then it would be after your proposed modifications. People in the business of welding would be reluctant to purchase a machine that has been tinkered with. Simply sell your machine on e-Bay, as is, unmodified, in factory condition, and be done with it. The fact that you want to modify it is a strong indication that this machine is not for you.

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I am in a similar situation to the OP. I have a DC TIG welder that i bought for $9.99 (sic).
I am now working on modifying it to do advanced square wave AC.
I cannot really worry about "reducing value" of this welder, since I bought it for a song anyway. I hope that it will be worth more (for me) after I modify it, unless I blow it up or some such.
I think that doing what I am setting out to do, is quite worthwhile.
i
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I am not a weldor by trade and also have an electronics background. Overall, I agree with what you say. If you are really interested in welding take a course and learn how to do it with the least amount of agony. An AC stick machine will certainly glue metal. I made a rectifier for my Miller and like the results but do I do anything I couldn't do with AC given the selection of AC rods - no. My rectifier is in a separate box so my Miller is unmodified.
Why did I make the rectifier then? Well, I thought there might be a big advantage to DC over AC which might not have been the case if I had taken a course. Other equal reason was that I was looking for an electronics project since I hadn't done any "building" for a long, long time. So I got a bee in my bonnet about doing it and nothing was going to stop me.
I don't totally discourage people from experimenting but you have to know just what your primary goal is and aim for it and try not to get sidetracked. To be a good weldor is not easy and you have to spend the time learning how to walk (getting the basics down) before you try to run (doing the exotic things).
Billh
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Lenny, you've gotten a variety of responses already, but I thought I'd share my experience. About two years ago I bought an ancient AC-only welder (and when I say ancient I mean at least 50 years old) that still welds like a dream. I too thought I would be building a rectifier circuit very quickly to make up for the deficiencies of AC. As it turns out, two years later I'm still welding happily and haven't yet run into a situation where I need the DC.
I took a class at the community college, and was especially looking forward to welding with DC so that I could see what I was missing. I found out that I wasn't missing much of anything, at least not with my old monster. Could be that some newer/cheaper AC welders fall short, but I can hardly tell the difference between using my machine and using the industrial-duty DC machines at the school. OTOH, when I tried the AC setting on those machines at school, I *could* tell a significant difference, so obviously not all AC welding is the same.
If you want to build the circuit, check out past posts by billh on this newsgroup, and google around -- I found plenty of information to allow me to build the circuit, and from all that I could see it would be dead easy ... but as I said, I just haven't bothered. I run 6011, 6013, and 7018 routinely; I've also run some "mystery rod" that turned out to be for stainless steel. Someday I may run into a situation where I need a rod that can only be run on DC, but until then I'm not going bother to build the circuit.
I've never used a Lincoln "tombstone" (I assume that's what you have -- so called because of its shape), but by all accounts they are rugged and dependable. As others have said, I would encourage you to start welding with it and see how it performs for your needs. If and when you discover any shortcomings, you can always go ahead and build the circuit.
Andy

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Yeahhh -- get to technical college on an evening course. You might be opening up a whole new interest, and, regarding your electronics skills, you know what they say -- a change is as good as a rest.
You will fall in with a whole diverse bunch of people, all captivated by their new interest. The machines go on and the crackling sounds of welding start the second the clock hits "course start" time and at the end, with minutes to go, the instructor will be coming around saying "Come on ladies and gentlemen; time to pack up. Finish the run you are on." and having to deter people from having "one last try".
And you'll be able to make really decent welds when you actually need to make something. Shelves. A trolley. A case for a machine. A weird jig or holding arrangement. Stuff which makes work good.
Richard S.

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This is more information than I had ever expected to receive. Thank you to everyone for all your time and responses. I have actually been posting this for my son who in fact is currently enrolled in a night welding course at the local tech college. Someday I may be able to find the time to go myself as well, but for now I thought that I would try to experience the the hands on part vicariously. As for the electronics part, that was what interested me. After reading all this I may never attempt the modification, but we'll see. Before I do anything though, as suggested I am going to let him try the rods that were recommended here for AC and see what he thinks. However the post by Billh which mentions his external modification does interest me. So I would address this last part to him. I can pick up all the microwave ovens I would want at the dump, but I'm having a hard time visualising what it was exactly that you did to the OEM transformer to wind the choke. Lincoln told me theirs uses about 25 to 50 turns of a substantial guage wire, (the tech didn't know what size) wound on a core. I could wind that many turns of # 14 or perhaps #10, which I'm guessing he might have been referring to on a core from a microwave transformer, but at some point I would think that the added resistance of the wire in series would decrease my current output significantly. Lincoln told me that the modification which they made to their 225 amp AC welder to convert it to AC/DC actually makes it a 125 amp welder when it is in DC mode. Lenny
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You can see a picture of my convertor at (click on the pictures for a slightly larger view): http://www.howatt.org/welder_rectifier.htm
Like so many things I had planned to do a write up and circuit diagram but never finished it.
Now that I see it again I think I used the heaver cable rather than #4 to wind the coils. If I were to do it again I would use lighter wire maybe 4,6,8. #8 is easy to find since it is typically used for stove wiring and it is heavy enough that it wouldn't overheat quickly - the welding duty cycle allowing for cooling periods. Since I haven't measured anything and it seems to work there may be no reason to go lighter than #4 which would be easier to wind than the stuff I used.
I can't speak for the Lincoln specifically because I don't have a circuit diagram but on other AC/DC welders I've seen they only use the low-current/high open-circuit voltage (OCV) winding to power the rectifier circuit which is why the lower current rating. (Welders often have 2 ranges, one is low-current typically up to 150A or so at about 80V OCV and the other is 225A at about 55V OCV. This is done to keep the power requirements under control and at high currents a lower OCV doesn't cause a problem to strike an arc easily.) Note that with the 80V you have a lot of voltage to play with in terms of dropping voltage since the arc voltage is probably around, very roughly, the 25V region. It varies by arc length mostly. The constant current transformer ensures the current doesn't go through the roof when the arc sticks and regulates the current for varying arc length. Lincoln may have put their DC circuit in such a manner that the current remains below 125A to reduce the current requirements of the rectifier diodes etc however, this is speculation on my part.
For mild steel in the 1/8 region the 125A is usually enough and if it isn't, and if you feel you need DC, then you can use a different electrode which has a lower current requirement. In my box, it is connected to the Miller by plugs so I can plug it into the high-current range if I want to. I ran a rod with it set to 225A and it worked but my work only requires the low range which on the Miller is anything under 160A AC.
Another point, if you look at the inductor in terms of keeping the arc alive as the valleys go to zero volts you only need to shift the phase of the current enough to keep a few volts across an established arc so it won't go out.
HTH, Billh
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