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9V battery grounding issues

Discussion in 'General Electronics' started by Sean Cronlund, Apr 25, 2004.

  1. I am using a 9V battery to power 6 LM324s (Quad Op-Amps), which
    requires a ground connection, not a -VCC. When I tested the circuit
    using an electronics kit to provide the voltage, it works fine, but
    when I replace the electronics kit voltage with the positive and
    negative leads of a 9V battery, the grounding is somehow not correct.
    It seems as if the 9V battery is sending a negative voltage from its
    ground lead. I have it connected with all the grounds meeting at one
    strip on a breadboard. Is there some kind of component I can purchase
    to act as the ground? Or is there some kind of suggestion on how to
    remedy the problem? Can anyone help me? Thanks!
     
  2. Karl Uppiano

    Karl Uppiano Guest

    Opamps are designed to work on a "bipolar" power supply, meaning that the
    supply has three terminals, +Vcc, -Vcc, and ground. A single battery can
    only supply +Vcc and ground, or -Vcc and ground (depending on how you have
    connected it). You can build a bipolar supply using two batteries connected
    in series. The common (middle) terminal should be considered ground. The two
    ends then become +Vcc and -Vcc. This is probably the simplest and most
    reliable solution, if your application could use two 9V batteries.

    As for running opamps on a single 9V battery, that becomes problematic,
    although I once converted a floating (ungrounded), single-ended supply into
    a bipolar supply by using an (otherwise) unused opamp in a quad opamp
    package. I biased the opamp's (+) input halfway between the (+) and (-)
    power supply rails using a resistor voltage divider (e.g,. two 100K
    resistors in series). Then, I connected the opamp's (-) input to the opamp's
    output terminal, and used the resulting output as a "virtual" ground
    reference. If the power supply starts to become unbalanced, the opamp will
    detect this, and pull the ground back to center.

    This only works for low power applications involving one or two ICs. Since
    all four opamps in a single package will have similar current drive
    capability, the remaining opamps can't be used to drive more than about 1/4
    of their rated output, since the virtual ground would become overloaded. You
    could add an external current buffer for more power. Since most power
    supplies are bypassed by capacitors, the virtual ground opamp might not like
    this capacitive loading. If that is the case, you can put a small (less than
    100 ohms) resistor or inductor between the circuit ground and opamp output
    (the (-) input must remain connected directly to the opamp output). Finally,
    if your power supply is already grounded at one end (like a 12V negative
    ground automotive system), you'll have to pick your own internal ground
    reference, and avoid confusing it (or connecting it) with the external
    ground system.
     
  3. Sean,

    As you write your application runs good on a mains powered supply but not on
    a battery I guess you forgot the internal resistance of the battery. The
    last one tend to be much higher than the internal resitance of a power
    supply. That's why a quite large electrolytic capacitor is used parallel to
    the battery in most battery powered applications.

    petrus
     
  4. dont know

    dont know Guest

    are you sure you disconnected the kit-power from your
    quad op-amps before you connected the 9 volt battery?
    completly disconnected? use the neg terminal of the 9 volt
    battery as reference when you make and measurements.
     
  5. Gary Lecomte

    Gary Lecomte Guest

    An Actual Ground shound make any difference. But depending on how long
    your battery leads are, a Bypass Capacitor (a 1uF tantalum or 10uF
    Electrolytic) on the breadboard would be a good idea.

    Gary
     
  6. Karl Uppiano

    Karl Uppiano Guest

    Since a picture is worth a thousand words, here's what I'm referring to
    (best viewed with a monospace font):

    +-----------o------------------o---------------------o-----o +4.5
    | | | |
    | | | |
    | | +----------------------+ +|
    | .-. | | | --- 22uf
    | | | | | | ---
    | | |100K | |\| | |
    | '-' +-----|-\ ___ | |
    - | | >----|___|----o------o-----o GND
    --- 9V o----------------|+/ |
    | | |/| 100 +|
    | .-. | ---
    | | | | --- 22uf
    | | |100K | |
    | '-' | |
    | | | |
    +-----------o------------------o---------------------o-----o -4.5


    Single-Ended to Bipolar Converter
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
     
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