Capacitor question

Discussion in 'The Projects Forum' started by Adam Hill, Jan 27, 2016.

  1. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    I am working with a Small 110v flux core welder. It came set up as an AC machine. Flux core wants dc current. The secondary output is around 26 volts with, I think, 125 amps max. So far I have taken the secondary into a bridge rectifier rated at 1600v and 200a, from there it goes to a 89000mfd cap, and finally out to the ground and lead. With this set up there is still a charge left in the cap after weld is complete. I was thinking of adding a resistor across the cap to discharge it after each weld. Will this work, or would it cause a failure somewhere? If it would work, any idea what size resistor would be best?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can wire a discharge resistor across the cap, but it is generally considered a large series choke/inductor is more common to see that a cap.
    Max.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes, it will work, and is common practice in welders. A 10 watt, 20 ohm resistor will discharge the cap to 6 volts in about 6 seconds.
     
  4. SLK001

    Well-Known Member

    Nov 29, 2011
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    Why do you think that you need to discharge the cap? It is only charged to 26V DC max.
     
  5. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    after a weld if the wire sticking out the end of the gun touches the grounded work it will throw a rather large surprising spark and bond the wire to the work, usually in a spot you did not intend to weld. I would imagine this discharge going through my hand would be most unpleasent.
     
  6. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    Thank you all for your help. I am fairly new to electronics but love learning about it
     
  7. wayneh

    Expert

    Sep 9, 2010
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    26V won't conduct through your hand. Heat and sparks are another matter, but you can grab onto 26VDC with little concern.
     
  8. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    That is comforting to know!
     
  9. wayneh

    Expert

    Sep 9, 2010
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    I said "little" instead of "no", because under unusual circumstances (sweaty hands, large metal surface to grip), you might feel it. You certainly feel a 9V battery on your tongue but it's hard to replicate that outside of your mouth.
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
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    You weld without gloves? Not very smart (but I have to admit, that I will MIG weld in a T-shirt and a glove only on the non-gun hand). I also admit to knowing about the "after-spark" that you are talking about. Better technique is probably a more cost effective approach to your sticking problem. As a welder, both stick and MIG, you get used to being shocked. Sweat will provide a conduction path thru your gloves and clothes as will welding in the rain or in wet conditions. It isn't "most unpleasent" as you imagined, but more like a fly or gnat inside your helmet - just irritating, but doing no real harm. The voltage is just too low for any major concern.
     
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  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I have a pro level Miller welder and it does the after spark too. Always put it down to inductance from the cable liner inductance being discharged. Without tearing into the welder, I'm pretty sure the on-off switching circuit is between the smoothing cap and the output to the stinger. If so a discharge resistor on the filter/smoothing won't stop the after spark. Every mig/wire feed welder I've ever used since ~1966 does the after spark thing.
     
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Do you need the capacitor?
     
  13. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    Well... I'm not sure that I do need the capacitor. I got the inspiration from watching several people on the internet do this mod. They put a cap in, so like a sheep I followed suit and put a cap in. The theory is it will make the weld "smoother." Then again the ripple from the Rectifier would probably be smooth enough for welding. Now you make me want to go take the cap out and try it. Once I get this all worked out, my father has the same welder that we will repeat the best practice on. Shure would be easier fitting just the rectifier in that tight space.
     
  14. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    I thought the same thing about this welder until I took it apart. Granted this thing is anemic compared to Miller and only a fraction of the cost. What I found inside is the trigger switch seemed to operate a relay that energized the primary coil on the transformer. I also thought it odd that the primary coil is the tapped coil to select the "HI/LOW" voltage. I was expecting to see a tapped secondary. By the was this is the cheap flux core 125 welder sold by Northern tool. I got this for Christmas.
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Never welded with one of the small ones like that, but have used many different brands of industrial ones, and they all did the after spark. And another difference is the industrial ones don't switch the primary to start and stop the weld. An inductor/choke is the usual form of output smoothing in a welder not a cap.
     
  16. SLK001

    Well-Known Member

    Nov 29, 2011
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    I don't see how. The 89,ooo uF cap doesn't hold enough energy to "smooth" things out. Especially when you are discharging a continuous 55 or so amps while you weld.

    I converted my 180 amp Lincoln to DC for stick welding. It does quite nicely without any cap.
     
  17. Adam Hill

    Thread Starter New Member

    Jan 26, 2016
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    2
    An update
    I removed the cap and am now running just straight rectified DC. I really cannot tell any difference in the "smoothness" of the weld. What I do notice is much less splatter, more even steady arc, and better penetration. Since then I have converted my fathers welder in the same manner and got the same results. And of course without the cap there is no longer a zap if you touch the tip to the work after a pass.

    Thank you all so much for the information!!
     
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  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Thanks for coming back with the result!
     
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