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MDQ5 fuses on power supply blowing

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  • MDQ5 fuses on power supply blowing


    I'm having a problem. Machine has been working perfectly fine until yesterday. I went to initialize and got the "x servos tripped" error message. I looked on the power supply and the 2 MDQ5 fuses were blown. I thought it might be a freak thing and went ahead and replaced the fuses but they blew right away the next time I powered up the machine. There was no power surge during operation and I keep the main breakers flipped off when the machine is not in use. I checked every connection I can think of and the only thing I could find was a loose ground wire by the breaker box, it's one of those clamps that holds the ground wire to a grounded pole. It's cracked so the ground wire is a little loose. I'm going to replace that but can this be causing the fuses to blow? If not, what is going on? At $5 per fuse I really don't want to keep experimenting. Someone please help I have work backing up!!!!
    Peter Piedra

    South Florida
    Cobra CR-408
    5 hp Spindle
    Hurricane vacuum
    V Carve Pro
  • #2

    Peter,
    A lot of (but not all) modern power supplies are "crowbar" protected which means if you have a dead short, the supply shuts down and the mains fuse may not even blow. To dig deeper, can you post the manufacturer and part number of the supply and I will try to look it up.
    I would check for pinched wires on the DC line. It could be something external shorting to ground. For example, the 24VDC feed to a limit switch may be touching ground. If this happens, the power supply would "crowbar" or blow the mains fuse to it.
    Another quick check would be to remove the output wires from the power supply + and put an ohm meter between all 24 vdc feed wires twisted together and machine ground (on my machine there is a jumper that ties 24 vdc common to chassis ground). If you have a 2 amp (output) 24 vdc power supply, you should measure no less than 12 ohms total. If you have a 5 amp power supply, it should be no less than 4.8 ohms. (0 ohms is a dead short)Your reading should be much higher (by 10 ohms at least) and that would mean you don't have any wires pinched to ground or devices shorted to ground and possibly your power supply has gone bad. A missing ground to the electric cabinet may have caused voltage hit to not go to ground which means it could go through a component but that probably did not happen.
    John Stanley Stinger II 3'x4', FTC, Laser, 1.7 Kw HSD w/ AB 523 SVC drive, V Carve Pro 8, WinCNC

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    • #3

      Peter...
      There is a possibility that a loose connection in the ground circuit could allow high voltage potential on the ground circuit. This is similar to what often happens during a lightning storm. Most CNC controls that are damaged had voltage on the ground circuit.

      You should have your power distribution system checked out by a qualified electrician.
      Gary Campbell
      Servo Control Upgrades
      GCnC411@gmail.com
      https://www.youtube.com/user/Islaww1/videos

      "There are 10 kinds of people in the world. Those that understand binary logic, and those who don't"

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      • #4

        Went through everything on Friday morning with one of the techs and he told me I had a bad power supply. Sent it back and waiting for a replacement. I went ahead and changed that ground clamp but it wasn't loose enough to be causing a problem. Thanks for the feedback.
        Peter Piedra

        South Florida
        Cobra CR-408
        5 hp Spindle
        Hurricane vacuum
        V Carve Pro

        Comment

        • #5

          Blowing MDQ 5A fuses on machine start

          Originally posted by hefty View Post
          Peter,
          I would check for pinched wires on the DC line. It could be something external shorting to ground. For example, the 24VDC feed to a limit switch may be touching ground. If this happens, the power supply would "crowbar" or blow the mains fuse to it.
          This is interesting. I just bought a used 2013 Cobra 510 and have been having what I think is a related issue. The first time I turned on the machine, it worked as expected and started to initialize, but something was wrong with the x-axis limit stop switch and the machine crashed into the bracket that triggers the switch. This broke the switch and bent the bracket.

          No big deal, I thought. Got a new switch and replaced it. Only now, the power supply (Teknic EMF-75) is blowing the MDQ 5A fuses immediately on machine start.

          Support recommended I just replace the power supply but since that is discontinued it'll be about four weeks just to get a new one.

          I think Hefty's comment is interesting because my machine was working until I crashed it and had to replace the limit switch. Is it possible I screwed up the replacement and something is touching ground?
          Last edited by cncdood; 04-23-2019, 11:48 AM. Reason: added supply model
          Sam
          Austin, TX

          2013 Cobra 510 w/ ATC

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          • #6

            I have just bought a used Cobra X3 and I am having the same issue! Did this get resolved by buying a new power supply?

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