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We're having some super weird issue with a CNC TIG welder that we're building.

To start the arc (and complete the circuit for DC TIG welding), bursts of high voltage (making a taser-like sound) at a frequency around 136 Hz are generated by the welding machine (measured by probing a loop of wire next to the torch). These seem to be producing EMI big time, i.e. all steppers stall and start making a sound when the welder is activated, and resume normally when the welder stops pulsing and starts welding.

Usually it takes a fraction of a second of this taser-like pulsing to start the welding process, but even in that fraction of a second, the motherboard we're using (MKS Tinybee) starts performing random operations, and even no longer works after reset and requires being powered off for a while before it works again. It also stops being recognized as a USB device.

Probing the STEP and DIR pins of the stepper drivers with a scope shows no ripples/perturbations besides the normal waveform. Also, and here's the crazy part, it keeps happening even if the welding machine is many meters away, and only stops when it's already in a different room.

Any suggestions on what experiments I should try to diagnose this? Any suggestions on how to fix?

Relevant context:

Steppers are NEMA23 Drivers are UM245-T Motherboard is MKS Tinybee Housing is built out of MDF, no conducting material is touching the electronics. No Earth connections (no Earth wiring in my country)

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    \$\begingroup\$ Welcome! Please show a photo of your driver setup and draw a block diagram of how everything is connected, from the plug all the way to the TIG touch and motors. \$\endgroup\$
    – winny
    Jul 1, 2023 at 10:29
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    \$\begingroup\$ ”No Earth connections (no Earth wiring in my country)” Any water faucet nearby? \$\endgroup\$
    – winny
    Jul 1, 2023 at 10:30

1 Answer 1

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even in that fraction of a second, the motherboard we're using (MKS Tinybee) starts performing random operations, and even no longer works after reset and requires being powered off for a while before it works again. It also stops being recognized as a USB device.

This is a problem that most EEs will face from time to time. My approach would be this: -

  • Shield the motherboard inside an aluminium box

  • Has the problem improved (it may not be perfect but you need to make this assessment)

  • If there are still problems, ground the metal case to the nearest reliable electrical wiring point on the system you could call 0 volts.

  • If there are still problems then assess the power feed to the motherboard

    • Try improving power supply decouplers (capacitors)
    • Maybe there are spikes on your power rail that could be suppressed by TVS diodes
    • Try adding ferrite cores on the power supply leads
    • Try running the motherboard from an isolated supply such as from people like Traco
  • Try adding ferrite cores to other wires going to the motherboard

  • Try optical coupling of motherboard IO

For the list of things mentioned above, add them cumulatively because you may need several of the things mentioned to fix your welder.

If after fixing the welder you want to try removing some of the things I've mentioned then do it one by one and assess the situation to see that the welder still operates correctly.

Also, and here's the crazy part, it keeps happening even if the welding machine is many meters away, and only stops when it's already in a different room.

This could mean radiated emissions (air-borne) or, the longer inter-connecting cable reduces conducted emissions back to your motherboard. Either way, the same approach as detailed above may help you find a fix.

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