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Ground Isolation

Discussion in 'General Electronics Discussion' started by enonu, Jul 3, 2011.

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  1. enonu


    Jul 3, 2011
    First post for this account. I'm a beginner at electronics. Thanks for your patience.

    I just bought a PC-based logic analyzer, the Logic 16 by Saleae, which describes not connecting it to a "device under test" w/o the device being isolated from MAINS power. I'm thinking they mean that the device needs to be isolated from ground isolation to prevent a ground loop w/ the PC powering the Logic16. Is this correct?

    What I'm mostly testing are breadboard prototypes powered by a DC bench power supply with variable voltage and amperage. I've tried using a multimeter to measure resistance between the circuit leads and the ground plug, and there is no path. Does this mean that the DC bench power supply itself provides power isolated from ground to my breadboard prototypes?

    The laptop is powered using a somewhat similar setup, but with a 19V DC power brick. Same testing applied. My guess is that while connected to the power brick, that the laptop is isolated as well?

    What's an example of a modern device with a digital circuit powered by a supply w/o ground isolation that I would have to be wary about testing?

    Thanks in advance!
  2. jackorocko


    Apr 4, 2010
  3. enonu


    Jul 3, 2011
  4. daddles


    Jun 10, 2011
    What they're warning you against is assuming that both of the logic analyzer's inputs are floating (like e.g. a battery-powered hand-held multimeter) and then connecting the inputs to something like a shunt resistor in the high side of an AC circuit. This is a definite no-no, as it would also be with an oscilloscope, because one of the two input wires of the scope and logic analyzer are connected to Earth ground through the power line. Trust me, once you make this mistake, you never make it again :p. A similar thread is here; check out the referenced PDF from Tektronix, as it explores the topic well.
  5. enonu


    Jul 3, 2011
    What about the discussion in the manual about isolating the device under test from ground? That's about creating a ground loop, no?

    Oh, and I rested my laptop PS, and I must have initially tested it incorrectly. In this case, common is connected to ground, and so it's not isolated.
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