Polarity/Negative voltage applied to grbl V5b

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  • March 3, 2015 at 11:27 pm #7500
    LPE656
    Member

    I recently purchased a grblShield V5b for a personal hobby CNC project. I connected this to a new 12VDC power supply and to my Arduino R3. When I attempted to open the serial monitor I received an error, so I unhooked the USB going in to my arduino. When I did this the USB cable (from the arduino to the computer) was hot to the touch!

    After some research and some voltage measurements I may have found my problem. My power supply takes AC input and outputs 12VDC via V+ and V- terminals. When I measure voltage between these terminals (red multimeter lead on V+, black multimeter lead on V-) I get a +12VDC differential as expected. It gets strange when I reference either of these to the power supply ground, when the V- terminal is reference to ground I get -12VDC (red multimeter lead on V-, black multimeter lead on GND). Referencing V+ to ground results in a 0VDC differential. It looks like I have -12VDC from my V- and 0VDC from my V+ terminal.

    I’m afraid having -12VDC on the arduino USB_gnd terminal somehow resulted in too much current through my arduino (raising the temperature of my USB cable). I am no longer able to communicate with my arduino.

    Has anyone witnessed this before? Any chance that I also ruined the grblShield?

    Any help is appreciated.

    March 5, 2015 at 10:25 pm #7505
    glassTransition
    Member

    When you say ground on the power supply, is it the ground from the AC plug (earth ground) or is there another terminal? Often the DC side is “floating” with respect to the AC side and you have to reference it by connecting it to something e.g. the chassis ground.

    It would be useful to get more information about the power supply if possible. You may try disconnecting it from the AC mains and testing for continuity from each of the DC outputs to the chassis ground. You should also test for continuity from the earth ground of the AC cord to the chassis of the power supply. The results of these tests should explain what you’re seeing.

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