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I Not sure what I can make into equivalent resistors

  1. Mar 16, 2017 #1
    TLbgLkr.png

    Could I combine the two resistors together in parallel to make an equivalent resistor, and then use that equivalent resistor in series with the capacitor?

    It would be great if there was some rule of thumb that I can use to know what I can make into an equivalent resistor/capacitor down the line once I come across different circuit combinations.

    Thanks so much!
     
    Last edited: Mar 16, 2017
  2. jcsd
  3. Mar 16, 2017 #2
    If I understand your questions correctly, the answer is yes to all of them.
    Here is a link about calculating the resistance of a resistors wired in parallel:http://www.electronics-tutorials.ws/resistor/res_4.html
     
  4. Mar 16, 2017 #3
    Ah, I see. So to confirm, I can make an equivalent resistor even though the capacitor is there?

    And thanks for the link!
     
  5. Mar 16, 2017 #4
    Yes, as long as the resistors are connected to each other directly.
     
  6. Mar 16, 2017 #5
    Hmm.. Can you please clarify what you mean by "directly?"
     
  7. Mar 16, 2017 #6
    In order for the resistors to be in parallel, the resistors need to be connected at both ends to the opposite ends of the other resistor(s).

    For example, if I have:
    * a 10-ohm resistor tied to nodes A and B;
    * another 10-ohm resistor tied to nodes A and B; and
    * a capacitor tied across nodes A and C (or node A and B).
    then those two 10-ohm resistors are equivalent to a single 5-ohm resistor. The same as this:
    * a 5-ohm resistor tied to nodes A and B; and
    * a capacitor tied across nodes A and C (or node A and B).

    On the other hand, if you have this:
    * a 10-ohm resistor tied to nodes A and B;
    * a 10-ohm resistor tied to nodes A and C; and
    * a capacitor tied across nodes B and C
    then these resistors are not in parallel and, unless nodes B and C are connected, are not equivalent to 5-ohm (or any other value) resistor.
     
  8. Mar 16, 2017 #7

    davenn

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    upload_2017-3-17_9-46-33.png
     
  9. Mar 16, 2017 #8
    Ah, I see. Thank you for the detailss!
     
  10. Mar 16, 2017 #9
    Are you saying that that is what resistors that are connected directly look like? Because then looking at my circuit, the resistors are not connected directly, since the capacitor is in between the upper node that they share. Is my intuition correct?
     
  11. Mar 16, 2017 #10

    davenn

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    yes, I am saying my pic is what you should have .... anything else is incorrect for what you described in your OP as what you wanted

    your image shows that the resistors are NOT in parallel so isn't in line with what you wanted



    Dave
     
  12. Mar 16, 2017 #11
    Oohh I see... If I wanted to find the resistance of the entire circuit, how would I since I can't combine them into an equivalent resistor?
     
  13. Mar 16, 2017 #12

    davenn

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    way back in post #2, Scott gave you a link to show you how to calculate parallel resistors

    http://www.electronics-tutorials.ws/resistor/res_4.html

    you can only measure the combined parallel resistance across the ends of the resistors not from the far side of the capacitor
    as it will make the resistance look like an open circuit
     
  14. Mar 17, 2017 #13

    sophiecentaur

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    The Equivalent single resistance in the first diagram will depend upon the value of the C and the operating frequency. Taking two extremes: for DC or very low frequencies, the equivalent resistance is the value of the right hand resistor (the C isolates the left hand R completely) and for very high frequencies (where the C behaves like a short circuit), the equivalent resistor is the same as for the two resistors connected in parallel.
    For frequencies in between (i.e. one a large range) you need to consider the Impedance of all three circuit elements.
     
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