op amp single supply

Discussion in 'The Projects Forum' started by tech1, Oct 22, 2012.

  1. tech1

    Thread Starter New Member

    Oct 22, 2012
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    Op amps used : lm358
    Image 'a' is inserted for reference.
    In image 'b', a second op amp is introduced but with a single positive supply in contrast with the first dual supply op amp. The measurements taken with multimeter shown in image 'b' indicate that the voltage of the batteries seems to drop with time.
    Image's 'c' measurements were taken after about an hour and they show that Vout has reached a negative voltage value. How is this possible with single positive supply voltage and why do the batteries seem to drop voltage?
    Thanks for your time
     
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    • c.pdf
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  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Where are you measuring from the Gnd or the Batt-ve terminal?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    I don't know for sure what's going on, but your second op-amp won't like having a voltage on its input that is below its negative power rail voltage. I could speculate that's causing an unexpected current flow.
     
  4. tech1

    Thread Starter New Member

    Oct 22, 2012
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    All measurements are taken from the Gnd.
     
    Last edited: Oct 22, 2012
  5. Audioguru

    New Member

    Dec 20, 2007
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    Why don't most NOObs not read the datasheet?
    The datasheet for the LM358 says that its maximum allowed input voltage is 0.3V more negative than its negative supply voltage. They say that if the voltage is more negative then input current will flow.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Speculation confirmed. Thanks AG
     
  7. tech1

    Thread Starter New Member

    Oct 22, 2012
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    0
    I forgot to mention that the voltage drop of the batteries was not battery drain, when op amp 2 was removed both batteries rose to their initial voltage values with only some millivolt permanent drain. The same thing happens when connecting a battery with e.g. a 220 Ohm resistor for some time. Thanks for checking the datasheet for the input current flow, should have noticed that myself, hasty post.
     
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