Problem with a circuit

Discussion in 'Homework Help' started by johnkaz77, Oct 11, 2010.

  1. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    Hi,i have this circuit that sources current when Vin+ and sinks when Vin-, however i don't really understand how it works,can anyone explain?Moreover,I should control Vin- and Vin+ with a pot with its 2 edges connected to a negative and a positive power supply respectivelly,how can this be done?If the 2 inputs of the op amp are the same how will it work??Am i missing something here?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is a Howland current pump. See National's AN1515. Figure 9, on p.7, is the same configuration. If you study this document, and others you can find by using search engines, you may gain some insight.
     
  3. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    I still find it difficult to understand how can this be implemented with only one pot.Something like that in the picture below seems more logical to me,is it correct?
     
  4. Ron H

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    Apr 14, 2005
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    You'll need to buffer the pot with an op amp. See the attachment.
    What is your application? There might be a simpler way to do it. Or not.

    BTW, who makes LTV272C? A Google search brought up nothing.
     
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  5. johnkaz77

    Thread Starter Member

    Sep 20, 2010
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    Oh,so i apply voltage only to the V+ terminal and the pot controls whether the circuit will source current (positive voltage) or sink (negative voltage),right?
    It must be wrong typing,i couldn't find anything either!
    Thanks a lot for your help!!
     
  6. Ron H

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    Apr 14, 2005
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    That's correct. Your other choice would be to ground Vin+ and apply the pot and the voltage follower to Vin-. The circuit would work the same, but pot rotation would be reversed - you would get positive output current when the pot voltage was negative, and vice-versa.
     
  7. SgtWookie

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    Jul 17, 2007
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  8. Ron H

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    Apr 14, 2005
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  9. JoeJester

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    Apr 26, 2005
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    I'd agree with you if they annotated the symbols, but I belive that is usually done with the software.

    What software does the OP use?
     
  10. Ron H

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    Apr 14, 2005
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    Yeah, I was wondering about that. It's like he has a spice model that is incorrectly named.
    Maybe it's just a schematic drawing program.
     
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