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Grounding question

Discussion in 'General Electronics Discussion' started by c131frdave, Oct 8, 2013.

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  1. c131frdave

    c131frdave

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    Oct 4, 2013
    Okay, let's say I have a 12V battery powering a computer. The computer has a 5V output to a sensor. The sensor generates a signal by varying the voltage back to the computer.

    So, the sensor has three wires, 5V in, GND, and Signal Voltage out.


    Now, lets say we need to artificially change the sensor's scale by sending it 6 volts from a source other than the computer. I'd rather not get into whether this is safe or effective or how it would work or anything else. Just that we now have an outside source sending the sensor 6 volts and have cut the 5V signal from the computer.

    How does it need to be grounded? The regulator that generates the 6V has a positive and negative terminal on the output side, and a 12V and ground on the input side, but the sensor is already grounded back to the computer (and presumably to the battery ground). Do you need to cut the ground wire at the sensor, which goes back to the computer, and run it back to the regulator instead? Or, can I leave the sensor ground as is, and leave output side of the regulator ground terminal open? Or maybe ground the output side to battery ground? Is ground ground no matter what voltage is coming out of what?

    I hope I'm clear. I know it's confusing.. Thanks!
     
    Last edited: Oct 8, 2013
  2. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    Ground is ground is ground.

    You should avoid ground loops since the loop can pick up electromagnetic energy and give different voltages around the loop. This is a common problem in audio equipment where the voltages are low so giving mains hum.

    To avoid this, select a central common point to connect all grounds.
     
  3. c131frdave

    c131frdave

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    Oct 4, 2013
    Well, it's a car, so that's sort of tough. But I can ground the signal voltage to the chassis then, even though the battery is 12V (and is the common ground)?

    Thanks for the reply! :D
     
    Last edited: Oct 8, 2013
  4. shrtrnd

    shrtrnd

    3,734
    478
    Jan 15, 2010
    I'd get into all the circuits and find-out how they're wired for ground. I don't know a thing
    about car computers, maybe you can't get into those.
    I'd worry about floating the grounds, and having enough of a potential difference between
    circuits large enough to cause damage (because of whatever sensor, and how IT'S
    connected to circuit ground).
    It sounds like you're leery about specifics in what you actually want to connect, but
    watch-out for differences in potential between whatever circuits are involved here.
    I've seen a lot of guys in the pre-car-computer days, fry a lot of 'accessories' they install
    when the battery amperage somehow finds it's way though their accessory.
     
  5. c131frdave

    c131frdave

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    Oct 4, 2013
    Thanks. Changing the voltage to your car's manifold air pressure sensor is actually an old fuel economy trick people have been using for years. Voltage from the computer is 5V, but if you lower it, you'll get a little better full throttle power, and if you raise it, you'll get better mpg. Or visa versa (I forget). I'm just doing it to rescale my sensor to "trick" the computer into thinking it's a 2 bar sensor instead of a 2.5 bar.

    Anyway, I don't think it will fry the computer, but I wasn't sure you could ground the regulator to the chassis like any other ground due to the fact that the voltages are different. The battery is 12V, and the signal from the computer (which obviously has its own voltage regulator) is 5V.
     
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