panel mount voltmeter

Discussion in 'The Projects Forum' started by servo, Dec 7, 2011.

  1. servo

    Thread Starter Member

    Aug 11, 2011
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    Hey, everyone. I have some questions about hooking up these voltmeters. http://www.cd4power.com/data/meters/30lcd.pdf

    I have two of them hooked up in the single-ended input configuration because I have pin 3 grounded the the negative side of the 24v power supply as well as to the negative V out pin on the 5v converter. I have them set up for +/- 20v, and the inputs are in the range of +/- 10v on one meter and 2-13v on the other. The problem is, I am getting a constant negative overrange indication on one and the other takes a while to become visible and sometimes fluctuates wildly. I checked the inputs with a multimeter and nothing is looking strange. The power supply is at about 5.5v and the inputs are what they should be, but the voltmeters are not giving me the proper voltage. I'm not sure where to go from here.
     
    Last edited: Dec 7, 2011
  2. Wendy

    Moderator

    Mar 24, 2008
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    It is an idiosyncrasy with these kind of meters that requires they have a totally isolated power supply. One of our other members came up with a simple solution for the problem, which I bookmarked.

    http://forum.allaboutcircuits.com/showpost.php?p=285808&postcount=23

    If you want to prove this is the problem you could use a battery short term, and see if it works.
     
  3. servo

    Thread Starter Member

    Aug 11, 2011
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    The converter is isolated. Does that not count?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Thanks Bill, I forgot where I put that.

    Servo,
    Each of your panel meters needs to have its' own isolated supply.
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    A few years ago I built a test fixture that included 3 of these meters that all required isolated power supplies. My answer was to use 9V batteries (one for each meter) and using opto isolators. DC power from the unit under test supplied power to the LED's in the opto's wired in series and the transistors in the optos acted as switchs for the batteries powering the meters. When the UUT is powered up, the meters turn on. When not testing, meters are off. Battery life is usually over a year with usage averaging about 4 hours a week.

    (ED) My meters DO NOT have backlighting.
     
  6. servo

    Thread Starter Member

    Aug 11, 2011
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    So an isolated DC converter does not count as an isolated supply? Or does that mean I need one converter for each meter?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Like I said, you could test this with batteries. Use one battery for all 3 meters, and see what happens. I don't claim to understand what is happening, but knowing it does happen is more than half the battle.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, it does, if it is just connected to a single meter and nothing else.
    Yes, that is what is needed.

    Try powering just one of them with the isolated DC-DC converter, and power the other(s) (one) with (a) battery/batteries. If the problem goes away, you have then verified that you need two separate isolated supplies. If the problem remains the same, you have something else going on.

    [eta]
    Whoops, Bill beat me to it.
     
  9. servo

    Thread Starter Member

    Aug 11, 2011
    31
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    Thanks, guys. I'll try it and post back with the results.
     
  10. servo

    Thread Starter Member

    Aug 11, 2011
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    Okay. Unhooked the other meter and ungrounded the negative supply. The meter works, but is still fluctuating between 0.4v and -1v. Measurements from the inputs are steady at close to 0V. Any idea why it's doing that?

    Edit: Actually, going back to it again, it is fluctuating even more than that. +/- 2v approx.
     
    Last edited: Dec 8, 2011
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Fluctuating between 0.4v and -1v is what it was doing before?

    Can you check the voltage with a handheld DMM or analog meter while you are also measuring using the panel meter? Better yet, do you have an O-scope to look at it with?
     
  12. servo

    Thread Starter Member

    Aug 11, 2011
    31
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    The voltages are steady when measured with my DMM. So the issue must be with the voltmeter or the wiring.
     
    Last edited: Dec 9, 2011
  13. servo

    Thread Starter Member

    Aug 11, 2011
    31
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    Are there any panel meters out there that aren't so darn picky?
     
  14. servo

    Thread Starter Member

    Aug 11, 2011
    31
    0
    I have solved the problem by purchasing the dc/dc converter kit from Murata. Thanks for all of your help!
     
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