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  1. #1
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    Default DIY spot welder questions

    A long time ago I picked up an old stuffed 240/480V (two phase) 160A or more arc welder.
    It had two coils wound around two opposite sides of a square core (if that makes sense). Each coil had a primary inner coil an a secondary outer coil wound around the primary. The primaries could be connected in parallel for 240 V and in series for 480. The secondaries were in parallel. There was a separate choke coil.
    When I pulled it apart, I found one of the primaries burnt out (melted) and the choke coked and I put them in the scrap back then.
    I unwound the secondary of the good coil the other day and found it to have 30 turns. So now I have a bucket of core laminations and a good primary coil.
    I want to reassemble this lot and add a secondary suitable for decent spot welding. If I remember my high school electrics right, I should get somewhere up to 800A at something like 2.8V (on the assumption that the original coil made 28V and 80A each…
    As it happens, I also have a short length of 95mm2 flexible copper welding cable (from a big industrial welder). Let’s say it has a current carrying capacity of 250A, then two in parallel would carry 500A. I intend winding them on the 'vacant' side of the core, separate from the primary altogether.
    I’m thinking two turns would be a starting point, but I don’t know what open circuit voltage I should aim for. I take it connecting the primary once the core is packed back in, and winding a few turns of any old cable around my proposed secondary space should give me the open circuit voltage I can extrapolate for the fat cable?
    Secondly, would the transformer be significantly more efficient if I wound the new fat cable secondary around the primary instead (downside is a lot longer length of fat cable required)?
    Last question: is the low duty cycle of a manual spot welder going to change anything? e.g. if I draw a current beyond the capacity of the seconday cables, what would happen? Will the voltage and current drop or will the cable just get warm?
    Thoughts and constructive suggestions will be greatly appreciated!

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    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  2. #2
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    Default

    Hi Joe,

    I wouldn't get too hung up on sizing wire to suit maximum amps, the welding time is pretty short ( think milliseconds) . So heat isn't the problem, but voltage drop might be.

    How are you going to control the amps?

    Regards
    Ray

    EDIT: Here's a table of some parameters.. Spot Welding Parameters

  3. #3
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    Default

    Thanks Ray. That was interesting reading. I had not intended to control the weld current, since that will be below optimum anyway. The table in your reference confirms that: the lowest current listed (for the thinnest material listed) is 8500A, well above what my tranformer is likely to provide...
    I will have to determine timer settings experimentally for my welder when I start using it. My guess is that my current flow periods will be measured in half seconds (25cycles) rather than single digit cycle numbers...
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  4. #4
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    Default

    Hi Joe,

    Capacitors should do the trick for getting the current you want. I recall Harty69 built a spot welder, you might PM him for some tips.

    Regards
    Ray

  5. #5
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    Default

    You could use smaller tips also Joe. What are you planning to weld? The spot welders I have used have been capable of much longer weld times. 6 seconds and more and they were real welders*, so while a couple of cycles of weld time might be needed if you are banging a few 100 spot welds on a car, for a couple of spots here and there longer times arent such a bad thing. Make control easier for a start.

    *going by Rays link the welder would be up around the 11k amps at a guess

    Quote Originally Posted by RayG View Post
    Capacitors should do the trick for getting the current you want.
    Please explain?
    Are you using the caps to up the primarys RMS voltage?

    Stuart

  6. #6
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    Hi Stuart,

    You charge the capacitor bank and then zap when you switch the electrodes on... and that lets you have a low current for a longish period to charge the caps, then a high current for a short period of time.

    Like this design... Zero Emission Vehicles Australia

    Regards
    Ray

  7. #7
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    Default

    Thanks for your input. I'll have a chat to Harty as well.
    I intend to spotweld sheetmetal (steel and gal) up to maybe 0.8 or 1.0mm max.
    A long time ago I watched someone using and then used myself a portable spotwelder. It had not electronics or timing at all. I feel pretty sure it had no compnents other than a transformer and a push button switch.
    From memory it took several seconds to do each weld and you watched the spots get hot and when the surface started sparking you let go of the push button.
    It looked similar to this one: BRAND NEW Portable 240V Spot Welder + 2 FREE Replacement Tips | eBay
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  8. #8
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    Default

    hi Joe
    mine is just made out of 2 microwave transformers
    and a bit of welding earth lead
    i would post some pics but its far from what you would call up to australian standards
    i can email you some details if you like


    cheers
    Harty

  9. #9
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    Default

    Thanks Harty!
    My main question is really the open circuit voltage to aim for. Do you use any controls to set or limit the current?
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  10. #10
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    Hi Joe

    voltage at the electrodes is 2 volts and there is no control exept the power point to turn it on and off
    it will weld 2 pieces of 1mm thick plate together dont use it much but it is handy when i do

    cheers
    Harty

  11. #11
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    Default

    Sounds good to me, Harty
    Do you know that the welding current is - or an estimate of it?
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  12. #12
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    Default

    i think 1200 amps but it was a while ago that i worked it out

    cheers
    Harty

  13. #13
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    Default

    I have a couple of boxes of 'new old stock' spots for use in spot welders, I think as yours is an older model they will be ideal as they are imperial sizes.



    I couldn't help this, and I am amazed no-one else got in first....

  14. #14
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    Default

    love it....
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

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