Grounding of a power supply's output?

Discussion in 'The Projects Forum' started by Lintukori, Sep 20, 2010.

  1. Lintukori

    Thread Starter New Member

    Jan 13, 2010
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    Hi!
    I have a project, where I'm installing a programmable logic (with internet) and some sensors to a coffee machine (large vending machine) to monitor the different substances. I installed a Murr power supply (230 VAC to 24 VDC) to provide power for the sensor and the logic. The question is, can I ground the output (let say the negative terminal) of the power supply to the machine's casing, which is also connected to the power supply's input ground? I want to do this, because I want to connect some sensor from the coffee machine itself to the logic, but as it is, the circuit where the logic and the power supply are connected is "floating" in reference to the coffee machine's circuit. This causes problems, because the logic can't really detect a sensor signal in the other circuit.

    So is it allowed to tie the power supply's input ground to same potential as the negative output terminal? (the input has three terminal's: L,N and GND)
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    It depends on the coffee machine.

    If the common of the coffee machine signals are already grounded inside the coffee machine, then you can ground the common of your 24V power supply, albeit it creating a ground loop because now you have grounding in two separate point. Usually this won't present much problem.

    However, if the common of the coffee machine circuit is not grounded, then you will have a problem as you will be now be introducing a ground to the coffee machine circuit common. This might or might not cause a problem.

    Your problem sounds like you are not taking out the signals with respect to the coffee machine circuit common so the signals are at present floating and you are hoping to tackle the problem by creating a grounding on the 24V power supply to "un-float" the signals.

    More information is needed on the coffee machine signals and if they are referenced to a common that is/isn't grounded.
     
  3. Lintukori

    Thread Starter New Member

    Jan 13, 2010
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    The signal from the coffee machine is referenced to the ground. In other words the voltage between the sensed point and the ground (casing & earth gnd) is 24 V, when the machine's door is closed and 0 V, when the door is open.

    So if I understood your reply correct this means, the coffee machine's signals are grounded, and I can ground the power supply's output. Or did I understood this right? Thanks anyway for the help :)
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    So the coffee machine signal is referenced to a common which is also grounded. In this case, you can ground the negative of the +24V supply to ground without causing problem.

    However, you really need to get both the signal wire and the common of the coffee machine into your sensing circuit input instead of relying on the ground connection as a return path for your signal.

    The main point is, your sensing circuit should work properly, even without a ground connection on the negative of the Murr power supply.
     
  5. Lintukori

    Thread Starter New Member

    Jan 13, 2010
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    0
    Ok. After thinking this through I kind of see the problem. Using ground as a return path for the sensor current is unreliable and probably unsafe (??) as the input impedance for the logic is 3 kOhms and so there would be about 10 mA current going through it. So I just need to wire the common of the coffee machine sensor's to the logic circuit as well as the sensing wire. Thanks.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    So is it allowed to tie the power supply's input ground to same potential as the negative output terminal? (the input has three terminal's: L,N and GND)

    You can't depend on the building's wiring as being anywhere near -0- ohms even between two close outlets and chances are the physical "ground" won't be at the same potential at any two points in the system. It will also tend to be noisy.

    If you've got to run a wire anyway one more won't hurt, and don't forget that unless you've got PoE there are 4 unused wires in a standard CAT5 ethernet cable.
     
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    The problem you'll run into is that the supply from the machine, which you've indicated runs the sensors, will disagree with your controller's input requirements. In such a case, you would want to isolate those signals optically. Your controllers power supply then can be left floating.
     
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