Find Resistor Value From Circuit

Discussion in 'Homework Help' started by rmrps, Mar 4, 2015.

  1. rmrps

    Thread Starter New Member

    Feb 24, 2012
    Hi Friends I want to find the value of resistor Rx Value from the circuit i know the value of V1 across the 4k7 resistor please give me the equation to find the value of Resistor Rx i think i have to use kcl or kvl law please give me your guidance
  2. #12


    Nov 30, 2010
    1)This is Homework and is posted in the wrong place.
    2) This is a multi-step process. Trying to write it as all one big equation would give me a headache.
    3) Handing you that equation is against the rules of the Homework forum. You have to show us how far you got with your own abilities.
    jjw likes this.
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    you can't work it out without voltage and current.
  4. #12


    Nov 30, 2010
    You can make an equation for this if you include 4 variables for current and another variable for voltage, but that will make one big, messy, equation. Better to work it out in pieces.
  5. WBahn


    Mar 31, 2012
    You can do it several ways. The first would be to analyze the circuit to solve for V1 in terms of the circuit components, one of which happens to be Rx. At the end of that you have an equation with two "unknowns", V1 and Rx. But you know V1, so you can just plug it in and solve for Rx.

    The second way, which is easier in this case, is to walk the back starting from what you do know, which is the voltage across the 4.7kΩ resistor (even if you only "know" V1 as being the symbol V1).

    What does that knowledge tell you? What does that new knowledge tell you about the voltage and/or current elsewhere in the circuit?

    In order to find Rx you need to determine both the voltage across it and the current through it. Working your way toward those two pieces of information is what needs to guide you.
    rmrps likes this.
  6. rmrps

    Thread Starter New Member

    Feb 24, 2012
    Thanks for the reply friends
  7. MrAl

    Well-Known Member

    Jun 17, 2014

    What i would probably do here is write the equation for V1 in terms of all the resistors and 3.3v voltage source, substitute in the known value of V1, then solve that for Rx.
    Once you have the equation for V1 it's probably easy after that. You can use Norton/Thev too if you like, or any other way to get the solution for V1 first.