Digital panel meters

Discussion in 'The Projects Forum' started by Tazcat, Aug 10, 2010.

  1. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    I have a digital panel meter that I want to wire into a 30VDC power supply that can produce 2.5 amps. The meter is rated at 5 amps and has a 5VDC + - terminal which supplies the MP and lights the display and it also has a terminal block marked IN + - of which I am not sure is wired in series or whether a shunt should be across these terminals and I doubt that it would be wired across the supply. Has anyone had experience with these meters and point me in the right direction
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I've got experience with them and I hate them. In essence you can't common ground the meter power supply with the meter measurement input signal terminal negative.

    They're fine if you've got a separate battery to power them with but otherwise you need a small DC to DC converter/isolator.
     
  3. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    The 5VDC is on a seperate supply that powers up the display on both the volt & ammeter but the point you raise about a common ground is a good one and I need to check it out before I stuff a good meter. :(
     
  4. marshallf3

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    Jul 26, 2010
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    I had to learn from experience and adding the isolation supply cost me more than if I had just paid a little extra for the meters.

    I still haven't understood why they designed them that way unless the divider network enters into it somewhere.
     
  5. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    I managed to find a circuit of something similar showng two meters cobbled together as a voltage and amp display and it shows a shunt across the IN + - terminal sharing a common ground. Just got to work out the value of the shunt,

    Cheers
     
  6. Tazcat

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    Aug 10, 2010
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  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I think it has to do with a negative supply, so you can read "0". At least, that is what the difference seems to be between the common supply models at a MPJA and the ones that need a separate supply.

    John
     
  8. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    I fitted a single supply digital volt meter to my van when I was having alternater problems and it was most user friendly. I am damned if I know why I bought these, other than being cheaper, but seems I have paid the price for ignorance.
     
  9. marshallf3

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    Jul 26, 2010
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    They're still very useful devices. Since most are simple LCD meters they draw so little current that using a simple battery as the isolated supply will often run one for ages.
     
  10. Tazcat

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    Aug 10, 2010
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  11. marshallf3

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    I've got some LED ones too. I think the self-contained isolator module cost about $6 from Mouser and it will run several of them.
     
  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Here is a circuit for allowing a common supply ground, setting the decimal point, range (voltage divider) for the Martel_V1 panel meter.

    [​IMG]

    Of the few meters I have looked at closely, all of them have similar controls. The version shown here fits right on the back of and attaches to the pins of the Martel meter. So, if you are using that meter, I would be happy to post the Eagle schematic and board files.

    John
     
  13. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    These were AUD9.00 including shipping from Taiwan. Don't know how they do it.
     
  14. Tazcat

    Thread Starter New Member

    Aug 10, 2010
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    Thanks John I printed it out and filed it. Never know I might need it one day
     
  15. Tazcat

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    Aug 10, 2010
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