Voltage Meter Power Supply

Discussion in 'The Projects Forum' started by fredp, Sep 15, 2010.

  1. fredp

    Thread Starter Member

    Sep 24, 2009
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    Hi,

    I am using a panel mounted digital voltage meter to measure the output of a op amp. The op amp is powered by a 12v linear power supply. When I use a 9v battery to power the voltage meter, everything works fine. When I power the voltage meter with a switch mode 9v power supply, the meter reading goes haywire.

    I think this is related to some common reference between the two power supplies. Is there anything I can do to solve this problem? I don't want to need to use a battery to power the meter.

    Thanks,
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Did u try decoupling the supply lines
     
  3. fredp

    Thread Starter Member

    Sep 24, 2009
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    I don't know what you mean
     
  4. fredp

    Thread Starter Member

    Sep 24, 2009
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    the two power supplies are not connected to each other in any other than through the 110 VAC side and their mutual connection to the voltage meter.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Connect a 100n ceramic caps at the output of the SMPS.
     
  6. fredp

    Thread Starter Member

    Sep 24, 2009
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    I should just connect it across the DC output on the supply? Can you please explain what this does?
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Connect it across the SMPS out put.
    it will ground the HF noise in the circuit. It could be the switching frequency that is coupling into the meter
     
    fredp likes this.
  8. fredp

    Thread Starter Member

    Sep 24, 2009
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    Thanks, I read up on the concept. I will try to connect it tomorrow and let you know how it works out.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    cool.
    IMHO DMM's do not like switching noise in the supply. It interferes with the ADC, the sampling , u know what I mean
     
  10. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Are the 2 PSU grounds commoned? If not they should be. The supplies could be floating. The same would be true for the battery as well but you may have accidently grounded the case of the battery effectively commoming the grounds so it would work fine.
     
  11. fredp

    Thread Starter Member

    Sep 24, 2009
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    I don't think I should connect the grounds on the two supplies. I am fairly sure that the meter's power source cannot share a common ground with the signal that it is measuring. In fact, I suspect that some relationship between the two signals is somehow causing the problem I definitely didn't have the battery grounded--and it worked perfectly.

    Another notable fact is that when I leave the SMPS connected as the source to the meter, and I apply voltage to the meter with a lab bench power supply, it seems to work.

    Would this be consistent with the coupling issue? It doesn't seem like the coupling issue would cause problems if the SMPS was connected regardless of the measured signal. I only have a problem with the combination.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U need a common ground in either cases
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Is this one of those inexpensive eBay meters that requires a separate isolated power supply? If so that's your problem, they can't share a common ground.

    I bought a few by accident, just got a few isolated DC-DC converter modules from Mouser and that solved it.
     
  14. fredp

    Thread Starter Member

    Sep 24, 2009
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    I don't understand. I am using separate power supplies. Do they communicate through their 120 side somehow?

    It is a fairly expensive meter from Newark. I called them and told them that I wanted a meter that could run on a power supply that has a common ground with the signal it was reading--because originally I wanted to do it that way, but they said that they couldn't promise that any of their meters could be used that way.
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Earth and ground are two separate things.

    U need a ground connection to measure the voltage .
    A ground is needed in the same way as you need a negative and a positive to measure the voltage

    An earth is used for protection
     
  16. fredp

    Thread Starter Member

    Sep 24, 2009
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    Clearly, they both have their respective grounds. The problem is that if I measure between the 9v smps and earth, I measure half its voltage. The op amp signal is also referenced to earth.

    Is there a way to isolate the smps from ground? This is happening through the 110 vac side somehow. Maybe I need a different ps.
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Does the opamp use dual supplies?

    why don't u show us ur work
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Could be, your current PS may be one of those transformerless designs that simply reduces the voltage. They're generally sufficiently isolated to be safe and work fine for things like charging cell phone batteries but their output related to actual earth ground may be floating in some manner.

    In theory you should be able to connect everything into a common ground situation but your current PS may not be as noise free nor function as expected in producing a pure DC output.
     
    Last edited: Sep 16, 2010
  19. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Here is a very interesting idea......

    POST A SCHEMATIC.

    That way we can see whats happening. I think you are confusing the grounds. Ignore the mains side of the circuit. Pretend it doesn't exist.

    You have 2 DC power supplies that have a positive and negative output. In MOST circuits you would conect the 2 negatives together. Without seeing your schematic we don't know if there is some strange connections going on.
     
  20. fredp

    Thread Starter Member

    Sep 24, 2009
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    Attached is a block diagram of the circuit. I didn't think it was necessary to include the specifics of the amplifier. The voltage meter clearly states that the measured signal cannot share a ground with the meter's power supply. The negatives of the power supplies are not connected directly--although I can measure a voltage between them, and I assume that this relationship is the cause of the problem.
     
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