HF Spotwelder Control

Discussion in 'The Projects Forum' started by smorgas, Jul 21, 2013.

  1. smorgas

    Thread Starter New Member

    Jul 18, 2013
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    I am building a controller for my 110V Harbor Freight spotwelder and was looking for some advice. The controller will consist of a trigger/timing circuit, a current control circuit, and the spotwelder itself. I plan to put all the controller on the primary side of the spotwelder transformer because of the lower current values (110VAC, 15-20A house current) vice the higher current values on the secondary side. I have the timer circuit figured out but need advice on the best design for a dual opposing SCR or TRIAC controller circuit. I found this but the SCRs are rated at 16A.

    http://www.electroschematics.com/wp-content/uploads/2013/02/SCR-Phase-Control-Dimmer-Schematic.jpg

    I am concerned about the max current rating of the SCR, and would prefer a buffer for any transients and to lessen wear and tear on the components. Also, what kind of power values should I use for the components if I use this circuit?

    I know that I could probably use a simple R/C circuit to control the gates of the SCRs with the resistor being variable, but am not sure how to design it myself. That why I am leaning towards the circuit I found. Any advice would be greatly appreciated.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Commercial systems have traditionally used either special purpose contactors or mercury vapour device.
    I would tend to use a much higher rated SCR, you can still use the same circuit, just increase the current capability of the switching devices.
    That would be my first choice.
    Max.
     
    Last edited: Jul 21, 2013
  3. Johann

    Senior Member

    Nov 27, 2006
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    Have a look at the 40A triac in the attachment.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am hesitant using NTE due to the cost, they are traditionally a Substitute brand and usually charge at least 3x for the same product, maybe look for the OEM equivalent?
    Max.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  6. smorgas

    Thread Starter New Member

    Jul 18, 2013
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    Thanks for the replies. I got the idea for the timing circuit from 5bears. I had to use another timing relay but it is pretty simple. The timing relay triggers an SSR to energize the circuit for the time that I set on the relay.

    For the current controller, I would rather use two SCRs like in the schematic because of the symmetry in the AC waveform they provide over a TRIAC. If I replace the SCRs with higher power ones, wouldn't I have to change the R/C circuit values in order to get the trigger current correct for the device?
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    All of the spot welders I've seen only use time controls. When I took my current job, they had a 2.5KW/220VAC/60Hz Miller LMSP50 spot welder on a stand and foot control. Since I work with mostly thin stuff, I couldn't get my foot on and off the foot peddle fast enough. Miller wanted more for their timer than the cost of the welder and stand together. I looked at the 5bears timer, but desided to build my own from scratch. My timer has a minimum on-time of 16mS...one 60hZ cycle. The thumb wheels are 0-99. That gives me 16mS to 1.65 seconds range on on-time. Since I work with light weight steel and stainless, that and 2.5KW is enough. I drew up a design for a 0-999 (16mS to 59.9 seconds) timer, but never needed anything longer than one and a half seconds with that much power.

    Ken
     
    Last edited: Jul 23, 2013
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Don't know how I missed the repost to this thread. Like KMofet said, any I've used or seen only use a timer. No need to adjust current.
     
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