How to isolate 2 different power supplies

Discussion in 'General Electronics Chat' started by danny.w.ferguson, Oct 12, 2015.

  1. danny.w.ferguson

    Thread Starter New Member

    Oct 12, 2015
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    Ok, I need some help here. I am designing a pH control system for a small waste treatment facility where I work. I have two +24VDC Power supplies that power individual OPLC's. The problem that I am having is that the two power supplies are seeing each other. That is, that I can measure voltage between the two. The only thing linking these two power supplies is the AC in feeds. I need these power supplies to be completely isolted form one another. Any ideas?

    And I have considered a current isolator for the probes themselves, but this will not work for the fact that the power supplies are still seeing each other, which makes the current isolator null and void.

    Thank you in advance for your assistance.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    By "seeing" each other, do you mean they have a common ground? Are the power supply ground outputs connected to earth ground?
     
  3. danny.w.ferguson

    Thread Starter New Member

    Oct 12, 2015
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    They are earth grounded on the AC side. They are not on the DC side. By seeing each other, I mean that if I test between the +VDC on one power supply and the 0VDC on the other I see voltage. and vise versa. In order for this to work properly, I cannot have this.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What type of supplies? SMPS? Linear? they normally would be isolated from each other, are you sure you are not getting any high impedance reading due to electronic VOM?.
    Do you still get the reading when the DC supplies are unconnected to a load?
    Max.
     
  5. danny.w.ferguson

    Thread Starter New Member

    Oct 12, 2015
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    When I disconnect the loads, I am not seeing the voltage, or at least it is minuscule. But when the loads are connected, and I have verified that nothing is connected to cross paths for the power supplies, I see the voltage.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    Are the grounds in the loads connected to earth ground?
     
  7. danny.w.ferguson

    Thread Starter New Member

    Oct 12, 2015
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    No, I have ensured that everything physical that could tie them together has been eliminated. But I just performed a continuity test from my common AC line to the both of the power supplies, and one of them doesn't ohm at all. The other one ohms at 52 ohms. Does this mean I have a bad power supply? I have never had a situation that I have had to worry about this, let alone check it.
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Many power supplies use a transformer to provide isolation between the input and output. That isolation can be circumvented by connecting the supply ground to earth ground, or having the load do that.

    If you're measuring a low resistance from the supply output to the neutral prong on the power cord, and ground isn't connected to earth ground, there could be a problem. If you're measuring with respect to neutral on the power outlet, earth ground is introduced (in the USA) because neutral is connect to earth ground at the power panel.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Obviously something in the PLC's are referencing the DC to ground, is the 24v for PLC DC supply or is it also for the inputs?
    Again, what is the nature of the DC supplies?
    Check continuity from one PLC pwr input to the other, with the DC unconnected.
    Max.
     
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