Old man new hobby.

Discussion in 'General Electronics Chat' started by ukmxer, Jul 10, 2006.

  1. ukmxer

    Thread Starter New Member

    Jul 10, 2006
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    0
    Hi , I have a dead tig welder and I would like to salvage the transformers and the IGBT from it, I have found a similar idea on the net But they are not descriptive enough on what is what. I need to know how to make the igbt work and also how does the large current get through the IGBT. All this is new to me I did not know what an IGBT was last week so I am trying to learn what is a totally new technology to ME. PLease please please, can anyone supply a block diagram of a basic TIG inverter and a brief description of what is happening at each block in the diagram.
    I dont understand what the inverter is doing and how does it give me my square wave 150amp welding current.

    Thank you.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Are you up on power mosfets? An IGBT looks a lot like one in circuit. In essence, it is a transistor that has a field-effect gate. It goes into conduction when a couple of volts appear on the gate, and will saturate when the voltage hits about 10 volts. The drive circuit is essentially pushing charge onto a small capacitance. The device does not multiply base current as a transistor does, so you can run them from a low power drive. You can switch 600 volts at 200 amps with just a logic gate drive, although more power is a good idea to decrease turn-on and -off times. The on and off times are in the nanosecond region, by the way.

    Originally, IGBT's were all PNP, but I have just seen some stuff that indicates that there are NPN devices available. International Rectifier has data sheets and aplication guides if you want to dig in farther.

    I can't help with the welder circuit. Never got into one.
     
  3. ukmxer

    Thread Starter New Member

    Jul 10, 2006
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    0
    Thx beenthere, I will do my best to learn what you are talking about. If an IGBT has a 600V 200A rating and I put 80V 200A through it, will it be comfortable or is there not enough voltage going through. I think my welder will have approx 80 OCV and 200 A . But I am not sure until I have finished researching how I even get the power to the switching device. My initial problem as a noob is how do I create a max load of 200A. Do I use a transformer and then switch the transformer power on and off with the IGBT to create a pulse. But I want a pulsed square wave of power upto 200A. How do I create that. Any suggestions on any of these problems will be happily received.
    Many thanks
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Do some research on the IGBTs first. The thing to worry about is dissipation in the device. No matter what the IGBT is rated to handle in terms of voltage and current, it's the internal losses that cause it to heat up and fail. The dodge is to make the IGBT switch on and off as quickly as possible so as to minimize time in the ohmic region where the device heats up.

    The question is more like "how do I prevent the load from drawing in excess of 200 amps?" Old resistance welders placed resistors in circuit to hold current to some maximum. A tig welder may need to control pulse width on the output to similarly hold down current. That would be my approach, but I don't work with welders, so I may be speaking in ignorance.
     
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