Newbie needs help asap

Discussion in 'General Electronics Chat' started by dfullersnap, Jun 19, 2011.

  1. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    My work has a welder that the control board that controls the wire feed is no longer functioning.

    I have tested motor with drill battery and works fine.

    As per tech support I replaced a transistor Q8 on board which they said usually is the one that goes bad, but still not getting power out to the wire feed motor when trigger is pressed.

    The wire feed motor is 4vdc to 24vdc.

    The gas control valve is 24vac, which is activated when trigger is pulled and also used to send dc voltage to motor from the control board.


    I now have a separate dc power supply for the motor.

    I want to use the 24vac from gas control valve when trigger is pulled to turn a switch or relay that will allow dc power to flow to motor.

    Can anyone help me, I am hoping that this is a simple circuit to build.

    I have attached the control board schematic for the welder in pdf format.

    thank you

    Douglas
     
  2. Jotto

    Member

    Apr 1, 2011
    159
    17
    is the board beyond repair? the schematic is kinda hard to read. why reinvent the wheel, there is more to this unit then just turning on the gas, you have safety features built in also.

    If you can use a voltmeter first thing to check is voltage to the board.
     
  3. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    Jotto,

    thanks for the response. The company who made it still supports this unit, but no longer has parts for it. I spoke to them a few times and once they had no other suggestions, they told me that in the past something like this has been done successfully.

    I am still hoping to find a very simple circuit that will use 24vac to turn on a relay or switch which will complete the dc circuit between the dc power supply I have and the motor. Also, I appologies that I left out that I also have to have a potentiometer which needs to be between dc power and motor so that the speed of the wire can be adjusted.

    The support guys at Hobart said there should be no issues with this. It all comes down to using the 24vac when the trigger is pulled it also turns on a simple circuit to allow the motor to get dc power.

    I will eventually figure something out. But would be nice if someone could point the way.

    Thanks

    Douglas
     
  4. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    I did check the voltages to the board, 24vac going in and there are several locations that have a voltage value like +5v and +15v. I tested the ones I could find and they were good.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,523
    What ever you end up doing, don't forget that the gas solenoid has to come on slightly before the wire feed. If not, the weld isn't shielded at the very start and won't meet specification if it is a inspected weld.
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Why wouldn't you fix or replace the board? That has to be much cheaper than you spending a bunch of time coming up with new circuitry.
     
  7. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Normaly in a MIG welder the wire feed motor speed needs to be adjustable you cant just feed a voltage to the motor. The circuit is hard to read but asuming Q8 is the motor drive, then IC U1 is the motor speed control & probably PWM. Board repair is the better way to go. Good clear pics of both sides of the board would probably be a help.
     
  8. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    Here is a picture of the board.

    I need to use the trigger of course and today I went through and tested each of the TP on board, both with trigger engaged and not and found that TP5 is 15vdc then drops to zero when the trigger is pulled. So I took a relay and used TP5 and connected to the motor using a drill battery and it worked, I pulled trigger and wire feed worked fine. But of course, when I turn off the welder TP5 loses power of course which closed the relay and the wire feed motor started to run again. So I thought about adding a second relay to the circuit so that the power to the wire feed motor is only closed when the welder is on.

    But, if anyone has any suggestions as to what I should test on the board that would be great. I would rather fix the board, but if not then ok, because my test today actually worked with only a small hitch.

    I will place the potentiometer into the final circuit so that I can adjust the wire speed.
     
  9. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Which conector is the wire feed motor conected to on the board? If TP5 15V is going to 0V when triger is activated, then there must either be exess current draw on that or the regulator U3 is faulty. What is the voltage on the unreg side of U3, & what hapens to that voltage when the trigger is activated?
     
  10. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    The positive side is connected to J5 and negative is connected to J6 on the right side of board towards the bottom.

    I will check the voltage of U3 tomorrow while at work.

    thank you

    Douglas
     
  11. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    To be honest not sure what side would be the unreg side of U3. So I tested each lead starting with the top lead which the board has the letters "ADJ" and I was getting approx 13vdc, off the center lead I was getting approx 15vdc, and off the bottom lead which the board has letters "IN" I was getting 31vdc. No change in any of those voltages when the trigger is pressed.

    I do appreciate the help and advice.

    Douglas
     
  12. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Can you post a pic of the otherside of the board, needs to be as good or beter quality than the other one, use macro. Looking for poor solder joints. Just a posability as there would be a fair amount of viabration in these units.
     
  13. dfullersnap

    Thread Starter New Member

    Jun 19, 2011
    9
    0
    I will take some more pictures today. The first thing I did was check for bad solder joints and even touched up a few contacts that looked like they needed it.

    thank you
     
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