CAMtool V3.3 vs. The Fat Fingers of Death

As is my custom, the day before showtime I talked my way through a final full-up dress rehearsal, with the HP 7475A plotter and the CNC 3018XL running their demo plots. As if to justify my attention to detail, the 3018 refused to home, with its X axis motor grinding in a manner suggesting something had gone terribly wrong with its driver.

OK, I can fix that™.

Turn off the power, verify the leadscrew turns smoothly by hand, check all the connections & connectors, then pull the DRV8825 PCB to see if anything looks obviously wrong. It didn’t, so I carefully re-plugged the driver and moved the whole affair to the Electronics Workbench for further study.

I turned on the scope and Tek current probes, then turned on the 3018 power supplies, whereupon a great cloud of Magic Smoke emerged from the CAMtool board and filled the Basement Laboratory with the acrid smell of Electrical Death.

It seems I carefully and meticulously re-plugged the DRV8825 PCB into its socket exactly one pin too high, which, among other Bad Things, connects the +24 V motor power supply to the driver GND pin.

Obviously, this did not end well:

CAMtool V3.3 - blown stepper fuse
CAMtool V3.3 – blown stepper fuse

The fuse, put under considerable stress, vented smoke & debris in all directions across the board; note the jets above the white motor connector. Surprisingly, the 1 kΩ resistor just below it is in fine shape, as is the rather blackened electrolytic cap.

The fuse measures the same 150-ish mΩ as the fuses in the other two axes, but I doubt it’s actually a fuse any more.

Astonishingly, the Arduino clone on the board worked fine, so I could extract the GRBL configuration.

Memo to Self: Never plug things in with your head upside down!

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  1. #1 by RCPete on 2020-01-30 - 12:24

    One of the times I replaced the two 12V batteries in the UPS, I touched the PCB with the steel cover. Fortunately, it was only a fried ground lead, and a jumper fixed that. There’s now a piece of plastic that goes on top of the board, along with a note teing me when I changed batteries.

    • #2 by Ed on 2020-01-30 - 14:42

      Then there was the time I dropped a headset’s steel band across a 12 V SLA battery in the back seat of the car. I didn’t recognize the red-orange-yellow ohmygod thing … then managed to not sear my hand batting it out the door.

  1. CNC 3018XL: Arduino + Protoneer CNC | The Smell of Molten Projects in the Morning

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