Learning to weld - difficulty with DC

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I have bought a welder and am teaching myself to weld. Don't worry! Eventually, I will get instruction, but for now I'm just practicing making beads. Really basic stuff. My welder is a Montgomery Ward / Power Kraft AC/DC unit. I have been practicing with some E6011 rods that came with the unit. I have gotten okay at striking the arc using AC, but whenever I switch to DC, I cannot get an arc to strike no matter what I do. I have tried various amperage settings, up to and including max output on the welder; DCEN vs. DCEP, etc... The electrode mostly just sticks to the work piece. Finally, with the most careful of careful motions, I was able to strike a very precarious arc (at recommended amperage for the rod). The pool moved very slowly and the hot metal wanted to actually bead up and roll along the surface of the work piece. When I was done, there was very little penetration and lots of tall build-up of weld metal, some of which cracked off when I brushed off the slag. See the attached image for an example.I can't for the life of me figure out what's going on here. Maybe I should spring for a box of fresh rods just to rule that out? Or is this just lack of technique? DC is supposed to be easier, but so far, I cannot even really get started with it. For the very-brief time that I actually had a DC arc going, I thought I got a sense of its benefits. The pool was steady and not spattering and jumping around. I want more of that!Thanks very much for any assistance you can offer. Attached Images
Reply:my knee jerk reaction is to tell you to crank up the amps, but that begs the question of what amps you are running at now and what diameter rod... and what thickness is your steel... and are you cleaning your steel... and do you have a good ground... etc.  Answer those questions and let's see.
Reply:Originally Posted by RodJmy knee jerk reaction is to tell you to crank up the amps, but that begs the question of what amps you are running at now and what diameter rod... and what thickness is your steel... and are you cleaning your steel... and do you have a good ground... etc.  Answer those questions and let's see.
Reply:I'm wondering if the DC rectifier diodes are shot. That would be my 1st guess if you can get decent results on AC, but not on DC. The machine makes AC power and then converts it to DC thru the rectifier. If one or more of the diodes are gone, that would cause the issue you describe.I'm sure Duane can tel you how to check the diodes. I know there have been several threads here in the past that described this problem and how to check the diodes..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks. I will do a search for that topic. I have a multimeter with a diode check function, so I can test them.
Reply:I think DSW hit the nail on the head as DC is easier to start/maintain than AC due to the arc voltage doesn't drop to 0V 60 times a second on DC as it does on AC if everything is as it should be.One or more failed output rectifier diode(s) can result in total loss of (-) and/or (+) every half cycle.MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:I did a little more testing with a multimeter. The rectifier bridge has four diodes as near as I can tell. Two of them show 3.3 MOhms through them and 0.44 volts drop using the multimeter's diode test function. The other two show 2.5 MOhms and 0.41 volts drop. I also measured voltage across the DC terminals of the welder with it plugged in. I measured 55 volts DC. On a hunch, I also measured AC volts and found 22 volts AC. Is that normal? Should any AC voltage at all be measured on the DC terminals?Also: I did some searching on this forum, but didn't find any threads with a diagnostic procedure for a rectifier, so if you have any suggestions on search terms, I would welcome them.
Reply:In case it's relevant, I did not disconnect the diodes from the heat sink or wires before testing them because it would require desoldering. I read that may cause spurious readings. But I found a thread over on a Miller forum that says that resistance over about 1 MOhm should be treated as "open", so it sounds like I may have four bad diodes here. However, further research suggests that multimeters that put out a very weak ohm-testing current may read very high resistance values when testing diodes. So... not sure where to go from here.Last edited by joshuabardwell; 09-23-2012 at 01:46 AM.
Reply:If you just got the machine can't you take it back?Should be under some kind of warranty...From the desk of Kevin CaronTrying to be the best me I can!www.kevincaron.comAHP Alpha Tig 200x MillerMatic 251Miller Syncrowave 200
Reply:Originally Posted by joshuabardwellAlso: I did some searching on this forum, but didn't find any threads with a diagnostic procedure for a rectifier, so if you have any suggestions on search terms, I would welcome them.
Reply:I see I missed the part where you do have a diode test but 0.4Volts doesnt sound right for Si. Did you disconnect the bridge when testing? And a bridge is four diodes.
Reply:Originally Posted by kevinacaronIf you just got the machine can't you take it back? Should be under some kind of warranty...
Reply:You do know that you are supposed to test a diode in both directions?You should have good conduction in one direction and little conduction when you reverse you meter leads on a diode.Rick V 1 Airco Heliwelder 3A/DDR3 CTC 70/90 amp Stick/Tig Inverters in Parallel1 Lincoln MIG PAK 151 Oxy-Acet
Reply:Originally Posted by joshuabardwellI also measured voltage across the DC terminals of the welder with it plugged in. I measured 55 volts DC.
Reply:Originally Posted by Rick VYou do know that you are supposed to test a diode in both directions?
Reply:Originally Posted by papaharley03It certainly sounds about right for that class of welder. You should inspect and/or clean the polarity switch if it has one, or the separate terminal lugs for the DC output if that's what it has. You know your leads are working because you can weld on AC.
Reply:Okay. Well, I've got good news. I did as a friend suggested and used a tail-light and a 12 volt battery to make sure the diodes were actually carrying current during the meter test. I disconnected the post-end of each diode from the array and ran the bulb through it, then used the diode-check function of my multimeter. The results were different from before! Three of the diodes, the tester showed 0.702 volts. The third one, it showed 0.695. As I understand it, that's dead-on right for silicon diodes. I also checked the resistance of the diodes while the 12 volt bulb was running, and they showed 0.00 ohms. This seems to indicate that all the diodes are working correctly.I also ran the bulb through the whole rectifier array, with the hot lead clamped on the wire leading into the array and the negative lead plugged into the + lug on the welder, and the bulb lit up brightly.So... where does that leave us? Cleaning the lugs, of course. We know the leads are good because I can weld AC. It seems like the diodes are good too. What else could cause cold welds?Thanks for the assistance, folks. This has been really educational and fun for me. One thing I've learned is that my cheap little meter's diode check function is unreliable in the absence of an external source of current.
Reply:A silicon diode like that will always fail shorted or open.  So the ohm function on a $2 meter is all you need to check them (for future reference).  So.  If the transformer is putting out and the rectifiers are working (seems so) then the only thing that could stop enough current from flowing would be a high resistance somewhere in the circuit.  Or more to the point, an open circuit.  If you stick the rod into the material, it should not only weld itself to the material, it should sit there and buzz until the rod turns red hot (that's how I weld, furiously twisting the rod to get it unstuck, lol).  If you can't even strike an arc, then there is a very bad connection somewhere.  Prime candidates are if the thing has crappy connectors for the cables, a more likely candidate is the ground clamp.  Maybe even the connection at the stinger.  Just take it all apart, use a scrubby pad to clean the connection surfaces bright and shiny, and re-assemble tightly.  When you get to the part where there is evidence of heat, that will be your smoking gun.  Any high resistance point in the circuit will show up as heat -- lots of it -- and some heat should have been generated the one time when you got a partial weld started there.If you can't find anything that way, stick the rod onto the material and let it buzz for 10 seconds or so.  Then shut it all off, unplug it, and carefully feel around for the hot spot.You did scrape the flux off the end of the stick, right? Last edited by Niner7Tango; 09-24-2012 at 01:06 AM.
Reply:Thanks for the feedback. I am about to be away from the Internet until a week from today (Monday), so don't think I've forgotten about y'all. I will try some more testing when I get back and update accordingly.
Reply:Hi there, folks. I'm back from my trip and am troubleshooting my welder again. You may recall that the last step was that I troubleshot the diodes and concluded that they were probably okay. On another suggestion from a friend, I measured the voltage put out by the welder, both with and without a load (120V incandescent bulb) in circuit. Here are the results.With the welder set on the lowest amperage setting (20 amps DC), I measure 57 volts DC and 22 volts AC with the light bulb not in circuit--in other words, just measured between the leads. With the light bulb in circuit, I measure about 27 volts DC and about 39 volts AC. The numbers are basically the same with the welder set on its highest setting (140 amps DC).It's not completely out of the ordinary that the DC voltage drops when a load is added, although I wouldn't expect that from something as beefy as the welder pushing through a load as relatively small as a 120V light bulb. It is really surprising that the AC voltage increases, though.Another interesting fact that may be relevant is that a couple times, when I turned the welder on in preparation for running a test, it would show just 1 volt AC, until I touched the leads to the light bulb, after which it would jump to the above readings and not return to 1 volt AC until I turned the welder off and back on again.I performed the same test on the AC side of the welder, in the interest of science. It showed about 63 volts, dead steady whether the bulb was in or out. It showed zero DC volts, as you would expect.If anybody has any insight into what this might mean, I'd welcome it. I'm no expert, but I'm surprised at the results I'm getting on the DC side, and I think they are probably not right. But what they might indicate as far as troubleshooting, I don't know.
Reply:Well, I got no responses to my last post, but what the heck, here's some more data.This is a diagram of my welder's DC rectifier array. The little black rectangles are diodes. Do any you have enough experience reading electrical schematics to tell me what, if anything, is wrong here? The thing that stands out to me is that, on one side of the circuit, there is a consistent 27 volts across each of the two diodes. On the other side of the circuit, one of the diodes is at 3.7 volts and the other is at 80 mV. This seems anomalous because it's inconsistent, but I don't have enough knowledge about electrical circuits to say for sure. It would be great if somebody out there had enough experience to look at it and see in an instant what, if anything, was wrong.Regarding my last post, I believe that the AC voltage I measured is just the normal "ripple" that occurs in an un-regulated, rectified DC output, so I don't think it is an issue. Also, some research shows that around 25 volts is the expected DC output of my welder--at least, I think--so seeing that voltage under load doesn't seem anomalous.
Reply:Well, folks, I thought I would give y'all this update.Earlier in the test procedure, I did a full diode check with a 12 volt battery and a tail light as a load, and all diodes checked out, but due to the high likelihood that a bad diode is the cause, I went and did it again tonight. Sure enough, I found a problem with the same diode that had the 80 mV reading in my voltage tests. Once I disconnected its post from the plate it was bolted to, I noticed that it had a bad internal connection to its own lead. If I held it just right, I could get the bulb to light and the meter to read a good values in the diode test function. But if I wiggled it another way, it would show open both directions. Also, I can twist it 360 degrees while the soldered lead just sits there and doesn't twist at all, so that's clearly not right. It must have been marginally connected, and I must have finished breaking it internally when I did this second round of diode tests.So, new diodes it is!