Parallel resistor question

Discussion in 'General Electronics Chat' started by cszym001, Mar 2, 2014.

  1. cszym001

    Thread Starter New Member

    Feb 26, 2013
    2
    0
    I have the following question that I am having trouble working through.

    The input to both circuits (attached) is 120V 60 hz. In normal mode R1 and R2 form a voltage divider there is a capacitor C7 in parallel with R2. The voltage divider connects to an op amp non inverting input.

    In test mode resistor R3 is connected in parallel to the entire circuit.

    What are the equations I would use to determine how switching in R3 up stream of R1 and R2 affects the op amp input?

    I dont see how the current would be affected though R2 as it is connected in parallel.

    https://lh3.googleusercontent.com/-...pBWCk/w489-h231-no/GFD+Circuit+%28Test%29.jpg

    https://lh5.googleusercontent.com/-...Eb0/w382-h218-no/GFD+Circuit+%28Normal%29.png
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi cs,
    The effect that R3 has upon the circuit depends upon the Source impedance of the 120Vrms.

    If the source impedance is low, R3 will have no effect.

    Is this a Homework question.?

    E
     
  3. cszym001

    Thread Starter New Member

    Feb 26, 2013
    2
    0
    Unfortunately this is not a homework question. The source impedance is 3.9% it is coming from a isolation transformer.
     
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