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Pulse Width control circuit

Discussion in 'General Electronics Discussion' started by mrmodify, Dec 22, 2013.

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  1. mrmodify


    Feb 13, 2010
    Good Evening All,
    I have a harbor Freight M/N 151 MIG welder that I am making it capable of being also used as a TIG welder.

    I am needing to turn the wire feed motor off while in TIG mode and I was planning on just installing a switch on one of the leads to the motor.

    From what I can gather the motor is driven by a Pulse Width control circuit that runs at 120 Hz.

    My question is will it hurt any of the circuitry by removing the motor from the circuit?

    Or do I need to install some kind of load on the keep from destroying the control circuit?

    If I need some type of load what do I do?

    Merry Christmas or Happy Holidays!!
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I can't see any reason why breaking the connection to the motor should do any damage. Just make sure you use a switch that is appropriately rated and that you have everything appropriately insulated.
  3. KMoffett


    Jan 21, 2009
    Not going to happen. MIG welders are "constant voltage" devices. TIG/Stick welders are "constant current" devices. The internal components are very different. You don't just change cables, make a few tweaks, and the MIG is magically a TIG. Were that the case you would see MIG/TIG/Stick machines all over the place. I'd buy one!

  4. jpanhalt


    Nov 12, 2013
    Another consideration is that the motor control for the wire feed may have feedback. It is likely you can just disconnect the motor, but to be sure, you would want to see the schematic and evaluate the effect that might have on feedback and/or protective shutdown circuits.

    My suspicion is that it would be OK, but it doesn't hurt to be sure.

  5. mursal


    Dec 13, 2013
    Well John,
    Do those TIG units not need an increase in output to get things started? Seem to remember someone talking about "a scratch start machine" as one of the contenders. Think there's a few different types, but I could be wrong.
  6. mrmodify


    Feb 13, 2010
    Thanks for all the replies,

    KMoffett - I never thought of "Were that the case you would see MIG/TIG/Stick machines all over the place. I'd buy one!":eek:

    I should have did some research before posting to find out if it was possible to do. I always thought that a DC machine was a DC machine and all that changed was the amperage by turning the control knob. Now I know.

    Thanks men!!
  7. jpanhalt


    Nov 12, 2013
    I was addressing just the question of disconnecting the motor. Ken's comment addresses the differences between welding processes.

    And finally, I have never seen a TIG unit that increased the "output" (voltage?) to get the arc started. I usually try to keep my electrode out of the puddle. HF is used to start the arc.

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