Puzzling Voltages

Discussion in 'The Projects Forum' started by bertz, Sep 13, 2014.

  1. bertz

    Thread Starter Member

    Nov 11, 2013
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    I have the dual battery arrangement shown in the attached diagram in my 50cc RC plane. B1 and B2 are Lithium Iron Phosphate (LiFe) batteries. The third circuit shown in the diagram is a little battery sharing circuit configured on a piece of stripboard.
    The diodes prevent current flowing back into the batteries should one fail. The LEDs are just reminders to shut the switches OFF. Battery voltage is 6.6 volts.
    So here is what’s happening:
    When SW1 is closed, I read 6.36 volts at TP1 and 1.65 volts at TP3
    When SW2 is closed I read 6.43 volts at TP1 and 1.65 volts at TP2
    When BOTH switches are closed I read 6.53 volts at TP1 and 6.6 volts at TP2 and TP3. So far so good.
    Now here is the really odd thing – when both switches are open I read 0.16 volts at TP1, even when the batteries are disconnected!
    I have two questions: Why am I reading voltage (1.65V) at TP2 and TP3 when only one battery is on line?
    The second, and most puzzling question is why am I reading 0.16 volts at TP1 in the OFF state? I have used two different DMM and get the same result. Any ideas?
    dual_battery_setup.jpg
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Some small leakage that can be read by a modern meter, if you decrease the impedance of your meter by placing a resistor, 1k-5k, across the leads the voltage most likely will collapse.
    I would be very surprised if this test showed any support for a current flow.
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
    16,257
    6,762
    0.000000165 amps of leakage can cause a reading of 1.65 volts on a meter with a 10 meg input impedance.
    Add a 1K shunt resistor to your meter leads and see if the voltage drops to .000165 volts (almost unmeasurable).
    If you don't like that amount of leakage, you are about to enter the world of immaculate cleanliness and using a hair dryer to drive humidity out of the circuits. (That still won't fix leakage if it's inside the switches.)
     
  4. bertz

    Thread Starter Member

    Nov 11, 2013
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    Using a 1k shunt, this completely collapsed the voltage. Once again, I was getting a 0.16 reading with SW1 and SW2 in the OFF position.
     
  5. bertz

    Thread Starter Member

    Nov 11, 2013
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    This is not leakage, I am getting a 4.78 voltage drop across D2 when SW1 is closed and SW2 is open. This results in a 1.65 volt reading at TP3. I thought the diode would block current flow. LED2 is not lighting up. I need to take some voltage measurements across R2 and LED2 to get a better picture of what is going on here.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As expected.
    One of the anomalies when using a high impedance meter, you get used to interpreting odd readings and why the occur after a while.
    They often occur especially when measuring AC due to the capacitive/inductive coupling etc.
    Max.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Did you put your meter directly across D2 or did you take two measurements to conclude this drop?
     
  8. bertz

    Thread Starter Member

    Nov 11, 2013
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    I concluded that the voltage drop across D2 was 4.78 volts because I was reading 1.65 at TP3 - but I was wrong! There would have to be a current flow in order to see a voltage drop.
    In order to determine the current in the D2-R2-LED2 loop, I measured the voltage drop across R2. Much to my surprise, there was none! Therefore, there could be no current flowing in this branch.
    At this point, I can only conclude that my circuit is working properly and that the voltage reading at TP3 is somehow due to the characteristic of a Schottky diode in a no-load state. Am I missing something here?
     
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