measuring voltage of cells in series

Discussion in 'General Electronics Chat' started by fredp, Sep 24, 2009.

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  1. fredp

    Thread Starter Active Member

    Sep 24, 2009
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    Hi,

    I am using a voltage scaling circuit to measure the voltages of a bunch of voltaic cells in series. For some reason, the circuit below doesn't work. Each individual meter and resistor set works if connected to a single battery, buy when I connect the next one in the chain, weird stuff starts to happen, like a negative voltage is displayed on the meter.

    Thanks for any advice
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Consider parallel paths. Does this sketch help?
     
  3. fredp

    Thread Starter Active Member

    Sep 24, 2009
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    No, I'm not really sure what you are trying to point out. Does the circuit I posted seem like it should work? It seems fine to me one paper. I can't figure out what the problem could be.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Fredp,
    I don't see anything "wrong" with your schematic.
    Are you certain that you connected the cells as shown in your schematic?

    Sometimes if cells become too deeply discharged, they can reverse polarities. However, with 2 megohms (total) across each cell, that should take quite a while.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The 1.8M resistor is in series with the parallel of 200K and the meter, so there will be ahuge voltage drop across the 1.8M (90% drop). Are you scaling up to compensate for that?

    I don't see anything that could cause negative readings.
     
    Last edited: Sep 26, 2009
  6. Tesla23

    Active Member

    May 10, 2009
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    Are all the CX101B's all fed from a common power supply? I suspect that this device doesn't have an input truly floating from the supply. From a quick search on CX101B:

    The CX101A & CX101B are designed to be used with an independent (isolated) ground. The signal being measured cannot share a common ground with the supply voltage for the panel meter.
     
  7. fredp

    Thread Starter Active Member

    Sep 24, 2009
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    Yes, the cells are powered by an independent 5 V supply, and the measured signals are D-cell batteries at the moment. I can connect 2 in series, but connecting the third makes the first one read incorrectly.

    The strangest part is that if I measure the voltage across the scaling resistor, it is correct. This leads me to believe that the meters are somehow affecting each other.

    The meters are wired as illustrated on the right side of the second page of this pdf:

    http://www.circuitspecialists.com/images/CX101-1.pdf
     
  8. fredp

    Thread Starter Active Member

    Sep 24, 2009
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    Well, the resistors scale the 1.5 V signal to 0.150 V--since the meters have a range of 0-200 mV. The decimal place on the meter is adjusted accordingly. The meters and circuits work perfectly when I only measure individual cell voltages. Things fall apart when measuring multiple cells in series.
     
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