Blown my welder up.

  1. Retired Member

    Messages:
    171
    Location:
    Huddersfield. UK.
    Hi,

    My TIG welder is a Parweld XTT 202P. I'm completely new to TIG but have been arc welding for the last 55 years. I finally gathered all the kit and materials so this afternoon I was keen to finally have a go.

    I was trying to weld 1mm thick aluminium as a lap joint and tried various amps; I don't have a foot pedal but after a few attempts started to make progress at 40A then suddenly a loud bang followed by a dense cloud of smoke.

    The welder is brand new but sold to me by the buyer who had stored it under his bench for about a year; the warranty isn't transferrable. I came up to the bungalow to allow the smoke cloud to disperse whilst I enjoyed a mug of tea.

    Having now checked and found it wasn't covered by warranty I had nothing to lose; I've stripped the welder pulling out a circuit board containing a bank of 6 capacitors; Jianghai CD294 200V 1200uF two of these had expired big style; I'm now trying to locate capacitors of the same physical dimensions; I can easily buy the 200V 1200uF via eBay. I restored vintage valve radios for ten years so changing a couple of caps isn't going to upsert me; normally I'd just upgrade the voltage but given the many electronics on this machine I'll stick to original spec.

    I'm just posting this for interest because I'm used to such problems; I wondered if any other member had suffered such a problem; as I say I'm new to TIG welding but I expected the machine to be idiot proof which it obviously isn't.

    Yesterday I bought a new Optrel Crystal 2.0 helmet and at least this is absolutely brilliant as the Americans would say it's awesome.

    Kind regards, Colin.

    Blown welder_0006.JPG
    Two blown caps; easy job to replace them but more difficult to remove the circuit board.

    Blown welder_0007.JPG
    A smaller circuit board had to be removed to gain access to one of the mounting screws. I took a lot of care not to touch any of the component connections because I'm well aware of static problems.

    Blown welder_0008.JPG

    The circuit board out ready for the caps to be replaced. This is most unexpected.
     
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  2. Hood

    Hood Forum Supporter

    Messages:
    9,102
    Location:
    Carnoustie, Scotland
    Hopefully it is just the capacitors that are the problem and I can not see how there is anything you could do (welding wise) to have caused it.
    Might be worthwhile replacing all of the caps if they are all the same brand/size.
     
  3. Paul.

    Paul. Moderator Staff Member

    Messages:
    6,116
    Location:
    Northampton. UK
    Hope that you fix it but when my little draper inverter went bang the blown caps were only part of the problem, it took a power supply with it, It was repaired by an ex member on here on a fixed price arrangement, I know that he spent far more in time and parts than a new welder would have cost, good luck though.

    Thread here, it was a while ago, I still have the welder and it works a treat, I suspect that the repair was of better quality than the original build.
    https://www.mig-welding.co.uk/forum/threads/draper-inverter-gone-bang.33886/#post-457270
     
  4. WorkshopChris Forum Supporter

    Messages:
    4,372
    Location:
    South East Essex
    Depending on the layout on the reverse of the PCB it might be worth a look at the rectifier diodes.
     
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  5. Retired Member

    Messages:
    171
    Location:
    Huddersfield. UK.
    Hi,

    Many thanks Hood & Paul.

    I've just ordered four caps from RS. Thanks for the tip Chris regarding the diodes; I'll have a look.

    I don't have luck with circuit boards. I once blew up a small Clarke lathe destroying both circuit board and motor; I junked both and upgraded to a countershaft and heavy duty servo motor which sorted the job out. One regular job I do is to replace the 8uF cap on our tumble drier.

    Kind regards, Colin.
     
  6. Ruffian Member

    Messages:
    1,679
    Location:
    Devon UK
    I hope you get this fixed easily.
    Such a shame it has gone pop so shortly after acquiring.
    If it has properly died though please let me know.
     
  7. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    That i would say is a real concern, i pray it is just the caps but one had to wonder if this might be more sinister. What does the board do?
     
  8. eddie49 Member

    As WorkshopChris suggested, I would turn the PCB over and trace the connections around those two failed capacitors.
    Are they in parallel? Are those two connected to the other four? On the component side of the board, to the right, there are two rectangles outlined in white. Are these two bridge rectifier blocks? If they are connected to the caps, a possible shorted diode there would explain what happened to the capacitors.

    The working voltage of only 200 volts is strange. Most inverters rectify the mains straight off, resulting in 325v DC, and the rest of the circuitry runs off this DC bus. This part of the machine is obviously running at a lower voltage ( or the designers were really optimistic when they selected the working voltage of their capacitors...! ). Can you trace where the AC mains input goes, and scout out what components are connected shortly thereafter?
    [ I hope the capacitors have all emptied themselves by now, whether of magic smoke or of electrons.]
     
  9. Retired Member

    Messages:
    171
    Location:
    Huddersfield. UK.
    Hi,

    Many thanks Ruffian; mtt.tr & eddie49 for your replies which I appreciate.

    I'll update any progress Ruffian.

    Good question mtt.tr; at the moment I haven't a clue as to what this board does and it's only one of a number of boards but I hope the problem is isolated to this board only.

    Excellent suggestions and advice eddie49. I only stripped the board out yesterday then knocked off feeling reasonably happy I'd found what exploded it being just electrolytics which I'm familiar with and can easily change; however I like you and others wonder what caused the failure in the first place. The welder was only about a year old when I bought it a short time ago and it was brand new in that it had never been used. As far as I could determine but bearing in mind I'm unfamiliar with TIG welding all seemed to be going well; I was using a 1.6mm gold 1.5% Lanthanated Tungsten with correct collet/holder the aluminium was 1mm thick with a lap joint. Amps at 40.

    Electrolytics are nothing but trouble and I've replaced lots of them; the welder had been under a bench unused for a year and I know Electrolytics deteriorate if unused so if these Electrolytics were calculated on the top end of their spec they could have been pushed over the edge; there must be lots of ripple placed upon them due to the fact they are used in a welder? I don't recall solid state rectifiers ever failing on me; Thermionic valve rectifiers yes and many of these actually went low emission first.

    A circuit diagram would be a huge help but I don't have one; I'll have a good look around the board and do simple tests where I can but I think at the low cost of the capacitors I'll simply replace the two which have blown; if the new capacitors also fail I could take along the circuit board to a welder repair shop and have them check it over rather than drive myself silly chasing my tail and wasting a lot of time; I'm very careful working on electronics because it's so easy to cause lots of collateral damage without even being aware of it.

    Electrolytics fail because their specification has been exceeded whether too high voltage or ripple; of course poor capacitors cannot be discounted either.

    I'm nosy and interested;

    http://www.epanorama.net/newepa/2010/03/23/electrolytic-capacitor-failures/

    The details on the link above are what I'd agree with.

    Good point eddie49 about the capacitors emptying themselves; I always check and then double check all electrolytic capacitors are fully discharged before handling them and not even touching the can; I used to have a heavy duty 100 Ohms resistor coupled to a pair of insulated probes for discharging Electrolytics but an insulated screwdriver across the terminals is a sure fire way of discharging them and this needs doing a couple of times because Electrolytics are nasty blighters to play with especially the bigger ones.

    Initially I thought I had caused the failure due to my inexperience with TIG welders but possibly I'm blaming myself for nothing when it could be down to component failure; I'll stick with it because the welder at the moment is expensive scrap.

    Kind regards, Colin.
     
  10. Seadog

    Seadog Forum Supporter

    Messages:
    4,513
    Location:
    NE London - UK
    That's an interesting page. I think I may have a go at the ESR meter, when I've a Round Tuit going spare:D
     
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  11. sako243

    sako243 Member

    Messages:
    546
    Location:
    My mansion in Wales
    I agree with the others 200V seems a little low - given the value of the caps they're almost certainly part of a smoothing circuit and I'd expect them to be higher rated than 200V. It wouldn't harm if you get higher voltage rated caps.

    The welder isn't meant to be run off 110V by any chance?
     
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  12. Hopefuldave Intergalactic pot-mender

    Messages:
    903
    Location:
    The Shed of Danger, surrey, England
    The 200v rating is *probably* because the two that blew are in series so each only has half the rectified mains applied (340v across the pair, 170 each) - 400v rated electrolytics are a lot more expensive so it's pretty common to do that. There should be bleed resistors across them individually too so they discharge when the power's off, around 100 - 500 ohms per volt would work ok, so something over 20K Ohms and fairly chunky, 5 watts or so each?

    The most common cause of that failure is excess ripple (too small a capacitor for the load) closely followed by a failure of the upstream rectifier, AC kills electrolytics, so definitely check the bridge rectifier(s) feeding them! When one fails, the other will follow very very rapidly...

    Interesting fact: a lot of 400v input VFDs (particularly older ones) have series connected electrolytics in the reservoir circuit, some hackers (Guilty!) have been known to move the neutral of incoming 240v to the common point between them to make the input stage a voltage doubler so they can get 415v or more out from 240 in (I'm pretty sure this is what some well-known VFD sellers do when they hoik the prices up a lot for 240 to 415v inverters).It does shorten the life of the capacitors a bit though!

    Dave H. (the other one)
     
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  13. eSCHEn

    eSCHEn Bit Wrangler

    Messages:
    5,580
    Location:
    SW Scotland
    I had a look around as I knew there was a larger cap here that might have done Colin but it's physically much bigger so I don't think it'd fit.
     
  14. TigTigger Forum Supporter

    Messages:
    95
    Location:
    UK South Gloucestershire
    Another problem with caps in series is voltage sharing. Due to the often wide tolerances on cheap caps it is difficult to ensure that the voltage is shared equally between the two. As they are both already working close to their limit it doesn't take much imbalance between them for one of them to exceed the max voltage.
     
  15. Retired Member

    Messages:
    171
    Location:
    Huddersfield. UK.
    Hi,

    Thanks sako243; yes it's 240V; it was supplied with a new 16A plug. Good question though.

    Interesting information Dave H; thanks. it never surprises me how people get around problems to obtain the result they want. During my radio restorating days many small sets had big wire wound dropper resistors and these used to get so hot they could damage the cabinet; if they went open circuit bridging resistors from RS used to be available; I used to convert to Watless Droppers using Class X2 caps; I used to wind lots of transformers too of various types.

    Thanks for taking the time to check eSCHEn; as you say higher value/voltage capacitors are available but space on the board is limited; they could of course be remotely mounted using point to point wiring.

    Thanks TigTigger; I've checked new Electrolytics and they've been miles off rated value; they can read 50% off rated value; I'm sure they also have a shelf life; I don't like the things because of the troubles associated with them but unfortunately they are a necessary evil.

    It's time I had something go right for a change so I've just been into the workshop to test the full wave bridge rectifiers (2) and they read perfect at .556 nothing open circuit or shorted; the nearby resistors also look unstressed; I've not checked these because I'm colour blind so can't work out the colour bands and I don't have a circuit diagram to work from but I've seen lots of stressed resistors; obviously not meter tested but a good indication; testing resistors whilst they remain in circuit can be misleading and at this stage I've no intention of removing them. It looks as if just the caps have blown so my fingers are crossed; I'll know as soon as the new capacitors are installed.

    Out of interest; when I first started restoring vintage radios I quickly found I needed three hands for replacing chassis components like caps and resistors; one hand to hold the soldering iron; one hand to hold the solder a third hand to hold the component. I dreamt up a mini wire winder; I designed and made the winder and it worked a treat; there are many of these now in use; I must have given over a dozen away as gifts. In use I could now snip out the original component leaving two small tails at about 1/4" long each; these indicated where the component had been which was a bonus but with a craft knife I could scrape clean the tails and having added a coil each end of the new component it was slipped into place locating it then only two hands were needed to solder it in position; a very simple solution.

    Mini coils_0001.JPG

    A pair of my miniature wire winders.

    Mini coils_0002..JPG

    Components with coils added ready for installation; soldering was rapid without causing collateral damage; lots of these are now in use.

    As I say it's just for interest but I enjoy pottering in the workshop trying out ideas.

    Circuit board_0001.JPG
    Here's the top side of the board again.

    Circuit board_0002.JPG

    Here's the underside; the two rectifiers are seen on the left.

    Kind regards, Colin.
     
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  16. Hopefuldave Intergalactic pot-mender

    Messages:
    903
    Location:
    The Shed of Danger, surrey, England
    Red/green colour-blind or totally ?achromatic? - Postman Pat the CB rig doctor near me was red/green, , had a pair of glasses with red and green filters in the two lenses like the old 3D ones at the pictures, with a printed sheet with the colours labelled for a reference he could identify most pretty well?

    I like those little coil winders, lateral thinking that!

    Dave H. (the other one)
     
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  17. eddie49 Member

    So under those silk-screened rectangles there are indeed two bridge rectifiers. There's also some beefy power transistors, so this is the main input power section for the inverter.
    Thinking about the fact that this is an AC/DC TIG welder with square-wave pulse capability, it must of course have TWO power buses - one positive and one negative. That explains the two bridge rectifiers. There appear to be three large bolted connections, for zero, plus, and minus outputs?
    Two 1200 uF capacitors in series, the total capacity is halved... would they really do that?
     
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  18. WorkshopChris Forum Supporter

    Messages:
    4,372
    Location:
    South East Essex
    I keep looking at this I can't work out what I am missing, the board looks to be multi-layer but if all 6 caps are common to the centre section in the underside photo then should all the caps be orientated to either positive or negative to share this common point?
    Might just be me missing something due to meds.:mad:
     
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  19. eddie49 Member

    I don't think it is just you.... the only way that I can reverse-engineer a PCB circuit diagram is to hold the board in my hand and turn it over and back again, whilst sketching each step.
    I would still recommend tracing the AC mains input wires, I'm sure they end up on this board, and then measuring the DC ( hopefully DC ) that is going to get applied to those new capacitors before soldering them in. I would not want to have them go bang again in front of my nose.... but then again, 200 or 300 v DC needs to be respected too.
     
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  20. eddie49 Member

    Yes, from the component side photo, the stripes on the caps are positioned left and right from the common rail.
     
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