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wire feed not working

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  • #16
    One thing I forgot to ask, did the gas solenoid open when you pull the trigger?


    • #17
      Originally posted by jjohn76 View Post
      The only thing I can add is my frame of reference when troubleshooting a MM 211 Autoset and working through an S-22P12 wire feeder that I received for an understandably good price (it would only feed at full speed, regardless of the trigger pull). Hopefully the next eplanation doesn't confuse too much but follows aeronica's block chart with what I saw on the two machines' PCBs. Pretty much the way those two feeders worked is a single MOSFET (big three legged chip on the heatsink in the upper left PCB of the MM211, on the right in the S22) PWM motor driver with current feedback. The negative lead of the motor connects in series with a shunt (two white cement resistors in the upper left corner of the MM211 PCB, one resistor in the lower right of S-22 PCB), to ground. The positive lead connects to the mosfet (I think it is the center leg of the Mosfet), with the mosfet gate lead connected to controller (network or transistors, capacitors, and resistors between), the other Mosfet lead connects to whatever the positive voltage rail is (there is a big electrolytic capacitor connected between here and ground to help keep the motor running at the right speed). The controller chip senses the voltage across the shunt, which it compares to the reference voltage from the wire feed dial (through a buffer and filter networks of operational amplifiers, resistors, and capacitors to protect the controller as well as provide the right compensation to keep the controller's time response as fast as safely possible) and determines how long to turn on the MOSFET for each pulse. The MM211 uses a microcontroller for the control chip, the S22 uses a specific chip designed for motor control. The only reason for mentioning this is because that network of transistors, capacitors, and resistors between the dial and controller and MOSFET is very different between the two. There is also a protection diode (two legged component, may be on a heatsink) across the Mosfet leads the protects the Mosfet from voltage spikes when it turns off the motor.
      Good stuff, jjohn. Takes it to the next step from my conceptual diagram, and is a great illustration of how you can do the same thing in different ways. The MOSFET drive system you describe is pretty much what I expected for an implementation method. I have an old S22P12 also that I can run with my Trailblazer, and it's nice to have someone already in there describe how it works without figuring it out completely from scratch. So far I haven't cared because I don't use it much, and it works fine, but if it ever breaks, you've already "broken the ice" in the troubleshooting process.
      Last edited by Aeronca41; 02-19-2019, 03:06 PM.


      • #18
        Thank you both. Really and truly, I appreciate your efforts and explanations.
        That's a lot to know, absorb, and it's going to take me some time to comprehend it as my brain struggles to learn more. Honestly, I'm going to need some time to digest this because it's over whelming and over my head. But to my benefit, and that of others, you both have watered the seed allowing it to grow, can't thank you enough, but thanks again.


        • #19
          Glad this helped. My S22-12P has a different problem altogether, which is related to the Mosfet driven by the jog button.
          Noel, a Google search for PWM motor control helped me understand the wire feed. Basically, the Mosfet is just a switch that turns on and off power to the motor. Because there is a capacitor across the motor leads (and the motor is inductive) the motor and capacitor create a low pass filter, which smooths out all of the fast on/off transitions from the switching MOSFET to just a steady average voltage (and motor speed, so long as the motor doesn't experience any change in resistance). The average voltage is equal to the max voltage from the power supply times the percentage of MOSFET on time (this is duty cycle). The problem is the variability in motor resistance from kinks in the wire, bending in the liner, or whatever, that require a higher/lower duty cycle to provide an accurate/constant wire speed. This is the reason for measuring the current through the motor as well. An easy way to measure current through a circuit is to add a resistor in that circuit and, knowing the resistance value, just measuring voltage across the resistor (the shunt is this resistor). This controller measures this voltage, takes the input voltage from the wire feed speed dial (potentiometer that varies voltage output based on position of the dial) and uses an algorithm to figure out the correct duty cycle, then signals to the Mosfet when to turn on and off. Microcontrollers have algorithms in written programs coded to memory, whereas the specific motor controllers use onboard comparators/etc and a bunch of resistors and capacitors connected to them for the algorithms. This explanation left out some of the components unique to the welder that isolate/protect the controller or provide more oomph to the controller to properly switch the Mosfet on and off. Hopefully that made sense...
          Last edited by jjohn76; 02-19-2019, 05:46 PM.


          • #20
            My kid called late yesterday afternoon, a daughter, single mom...her truck won't start. My son, having called not 5 minutes before, needs a ride to pick up his kid from day care. No winter tires on the car and the drive way to steep to get traction.
            One to the west, other to the east, stuck in the middle is me.

            Learning as you get older is harder for a reason?

            I don't want you gentleman to think your efforts are in vain? They're not. I'm learning things (I'm sure others as well) , gleaning a much needed understanding of operation and function, and I'm getting it a bit where it's starting to make some sense.

            Thanks again to you both with your explanations. Understandable, adaptable knowledge. I'm sure those following along are thinking heavily as well.

            Stinson108's MM135 issue. I referenced my problems around a HH135. What I need to do is now is bring it to the wiring diagram, peel back the covers of the machine, identify and more clearly recognize components, and make sense of the path currents takes. I have both machines and manuals. I need to find the time.

            I don't mind doing the homework. This MOSFET, IGBT, capacitors, diodes, resisters and board circuits, interesting stuff. I might yet own a Dynasty? While I'm trying hard to simplify the complex, I'm also grasping how the component, it's rating as called for in operational function plays into things. Being a visual learner, I need to see my own picture, and tell myself the story to go with it.

            I remind myself slowly forward is still a forward progression.

            Thanks again!