Murex tradesmig 160

  1. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    Having removed the cover panels on the welder today to give it a bit of a clean out I found this only connected at one end?
    Im not sure what I need to look for as a replacement.
    Any help would be appreciated.
    Ive attached a picture of offending item
    Thanks 2020-05-19 12.47.41.jpg
     
  2. Bobbes Member

    Messages:
    158
    Location:
    Maarssen / Netherlands
    That is a resistor. I can reed 16 Watt. Turn the resistor a littlebit. So we can se the value of the resistor.
     
  3. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    Hi Bob, whats on the paper is whats on the resistor... thats all there is
    the next bit underneath is ..79 2
     
  4. Bobbes Member

    Messages:
    158
    Location:
    Maarssen / Netherlands
    I din't reed the papier. Sorry. The resistor is 24 Ohm and 16 Watt. There must be an empty place on the board I think.
     
  5. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    It wasnt on the board its attached to the switching gear that operates when you close the trigger
     
  6. eddie49 Member

    It is a bleeder resistor, used to drain the voltage from the DC capacitors.
    There may be two thin wires attached to the capacitor bank ( as opposed to the heavy bus bars there ) which go to a pair of normally-closed ( NC ) contacts on the main contactor ( heavy-duty relay ). The resistor should be connected on the other side of that pair of contacts. When you press the torch trigger, the contactor pulls in, this pair of contacts opens, taking the bleeder resistor out of the circuit, and the capacitors charge up. When the trigger is released, the contactor lets go, so the NC contacts close, putting the resistor back across the capacitors and draining them down. This ensures that there is no residual voltage, which would otherwise cause a "live torch" effect, and give a small spark at the end of the MIG wire before you press the trigger.
    The vibration of the contactor clacking in and out often causes the solid copper resistor leads to fracture. If you look carefully under the screws on the contactor, you should be able to find the broken resistor leg still clamped in there.
     
  7. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    Hi Eddie
    Yep I think you hit the nail on the head with that, I did find the other end of the wire at the time, so next question.. what is its full title so I can search for a replacement?
     
  8. Bobbes Member

    Messages:
    158
    Location:
    Maarssen / Netherlands
    In this case you can allso use a standaard value of 27 Ohm and at least 16 Watt or more.
     
  9. eddie49 Member

    As Bobbes said, the standardised resistance value that is the nearest replacement to this component is 27 Ohms, rather than the 24 Ohms that your machine was fitted with. Either one will work, it is just there to drain the charge from the capacitors.
    This type of component is called a high-wattage wirewound resistor. Basically a ceramic tube wound with high-resistance nichrome wire.
    Often mounted inside a finned aluminium heatsink, then it's called a "cased wirewound resistor".
    That version has the additional benefit of an aluminium case with mounting lugs, so it can be rigidly mounted somewhere close to the original location at the Contactor, and connected to the screw terminals on the Contactor with flexible wire. The resistor will not be vibrated, and the flexible wires will not fracture, so it will be better than the original design.

    You could replace the damaged resistor with a 24 Ohm 25 Watt cased wirewound resistor from here:
    https://www.ebay.co.uk/itm/0-01-5k-...e-Wirewound-25-50-100W-Resistor/183603453219?
    Select 24R and 25W, price £0.99 plus postage.
     
  10. eddie49 Member

    Actually, there may be two pairs of contacts, so that both ends of the resistor are switched on or off from the capacitor bank when the Contactor operates.
     
  11. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    Hi Eddie

    Cheers for that, I will mount it on the frame and not on the switch where it can vibrate, I hope the delivery isn't to long as I need to get welding asap.

    Many thanks. Clive
     
  12. eddie49 Member

    You could still use the welder, if you have to, it's just that when you first touch the end of the wire against the workpiece you will see a spark, which is due to the remaining charge in the capacitors. This will be just before you press the torch trigger, it can be a bit disconcerting, and long-term it is better for the capacitors to be discharged through a resistor rather than with a dead short.
    If you scrape the paint off the end of the old resistor you may find a brass end-cap, to which you could temporarily solder a wire.
     
  13. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    Ive been just tacking up my repair plates as it is a bit of a pain when it sparks and sticks..
     
  14. Welder Wilk New Member

    Messages:
    13
    Location:
    Lincoln UK
    It took a bit of doing as there really wasnt a lot to solder too but its done and working, will get the new one fitted as soon as it arrives
     
    eddie49 likes this.
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