Capacitive discharge welder

Discussion in 'The Projects Forum' started by gopalyajur, Aug 2, 2016.

  1. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
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    Hello All,

    I am trying to build a simple capacitive builder. Part of the circuit is shown as below
    upload_2016-8-2_16-3-45.png

    The gate drive gets signal from a microcontroller. The pulse time can be varied between 10 - 200 ms.
    Capacitor bank consists of four 100 F , 2.7 V ultra capacitors connected in series. They are charged with a seperate power supply and is disconnected during welding trials.

    When the gates of MOSFETs (IRLB3813PB) are turned ON I am not able to do any welding. However, I measure around 10 V at the electrode outputs.
    Looks like there is not enough current for welding.

    I tried drving with pair of IGBTs as well, but faced the same problem.

    Can someone help me where the problem could be?

    Thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The Cap value seems very low, what material are you welding? Also what is the storage voltage?
    You can't regulate the welding time after a full discharge.
    Max.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    There is probably no need to control the "off" function to end a weld. It is very quick. Have you considered using a high-current SCR like most of the hobby units use? I use considerably less capacitance (ca. 0.8 F) in my unit but can charge to at least 30 V. I have found 16 to 18 V does what I need to do, which is battery tabs and light fixturing of thin-wall SS tubing.

    A slightly higher charge voltage gives more "headroom" for losses.

    John
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    On your diagram there is no connection between capacitor negative and circuit ground. If it is actually wired like that then it definitely won't work.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The capacity and peak charge voltage is going to be determined by the nature of the application, the industrial ones I have worked on use SCR discharge and very large switch selectable capacitor banks and around 200vdc.
    For small fasteners using a projection on the stud.
    Max.
     
  6. BobaMosfet

    Active Member

    Jul 1, 2009
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    Bingo. A 'Capacitive Welder' usually requires abig Caps :)
     
  7. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    93
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    Thanks everyone for fast response.
    I am trying to weld nickel tabs.

    I connected four 100 F capacitors in parallel , which gives a total capacitance of 25 F!!! Is that not enough.
    The caps are charged to 10 volts.

    I even try ed with 3000F maxwell caps at 2.7V. With mosfet it does not work as well.

    The problem with fast discharge is, it just punctures the metal surfaces.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    4x100μf in parallel is 400μf.
    You mean series?
    I see one guy on utube used a 1farad from an auto radio system, welding battery tabs for a home made CD welder!
    Max.
     
  9. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
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    It's not in micro Farads. But it's 100 Farad caps
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    My mistake, I though that was a tad low.
    Why not go with a SCR and switch in a cap as needed?
    Check what @AlbertHall mentions about grounding.
    Max.
     
  11. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
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    SCR would dump all the energy at once. Cap ground goes through mosfets.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    How do they get charged?
    Max.
     
  13. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
    93
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    Separate power supply. Alberthall is right about gnd. Correct circuit is shown below.

    Nevertheless the circuit does not work the way it should

    upload_2016-8-5_13-41-29.png
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Obviously not.:rolleyes:
    If you were you would only have one capacitor, 500K uF - 1F at most, and a large SCR. :oops:

    And what's wrong with that?
    If you used the correct size of capacitors along with the proper circuit design and layout it's not an issue so unless you're welding 14 ga plating you don't need 25F.

    Probably because you're overcomplicating what is a well known and simple process and using the wrong type of components and layout for the job. :rolleyes:
     
    Last edited: Aug 5, 2016
  15. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Here is a detailed description of a small, CD welder: http://www.philpem.me.uk/elec/welder/

    I built a welder along very similar lines and have been using it for the past 8 years or so. As I mentioned earlier, I used an SCR and a much smaller capacitor bank. Mine was only about 0.4 F and was made from twelve, 33,000 uF capacitors in parallel. That allowed me to use higher voltages. Here View attachment Capacitor_Comparison[1].pdf is a comparison I did on capacitors.

    The other attachment is the completed welder.

    John
     
  16. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
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  17. gopalyajur

    Thread Starter Active Member

    Jan 3, 2010
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    Thanks Japanalt!
     
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