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Bidirectional Current Source

Discussion in 'General Electronics Discussion' started by Shehroz, Jun 7, 2016.

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  1. Shehroz

    Shehroz

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    0
    Apr 29, 2015
    Hi
    I am trying to build a bi-directional current source. The simulation I made using Proteus isn't working: no current flows when either polarity of input is applied. Can anyone point out what am I doing wrong?
     

    Attached Files:

  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Is R3 supposed to be the load? What it is connected to? You show a connection to pin 5 of the opamp, what is that? I would think it should be connected to ground.

    Bob
     
  3. Shehroz

    Shehroz

    15
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    Apr 29, 2015
    yes R3 is the load through which current is supposed to change direction when input polarity changes.
    Also, grounding pin 5 doesn't change anything.
     
  4. Shehroz

    Shehroz

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    Apr 29, 2015
    oh, pin 5 and 1 of the op-amp are both offset nulls. I grounded both and it solved the problem. Thank you.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Great!

    Bob
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Did it? I see no feedback to close the control loop across the opamp. The opamp is operated in an open loop. As I see it, the 741 will immediately go into saturation. Either positive or negative, depending on the input signal.

    What am I missing?
     
  7. TedA

    TedA

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    16
    Sep 26, 2011
    Shehroz,

    I'm with Harald, I'm sure we are missing a few things here.

    Any chance you could explain a bit more about how this circuit is supposed to work, and what it will be used for?

    For instance, do you need an output current that changes amplitude with input voltage changes? Some sort of linear operation?

    There is a pretty good 741 data sheet here:

    http://www.datasheetarchive.com/dl/Datasheet-02/DSA0032521.pdf

    On the last page it shows the intended connections for the offset null pins. I'm not sure what will happen when these pins are grounded on an actual device. For sure, the device's input characteristics will not be improved!

    Ted
     
  8. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Harald,

    It is supposed to do that (go into saturation). Look at how the transistor bases are connected. They sense the current from the power supply into or out of the op amp and multiply it, much like the pass transistor used on a 3-terminal regulator.

    I am not saying that the design is good or will work well, but I have see audio amps that use this same method.

    Bob
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,310
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    Nov 17, 2011
    Bob, I know that design is being used to boost the output currrent of an opamp. Still for controlling the current some feedback is required. See e.g. here, page 2 (original page # 132)
     
  10. Shehroz

    Shehroz

    15
    0
    Apr 29, 2015
    The idea was to check if the current flows bidirectionally, which may not be a variable source yet.
    It looked like the grounding worked but it didn't, and I forgot to update on the forum.
     
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