voltage in RC/RL circuits - simple problem

Discussion in 'Homework Help' started by studentofcomputerscience, Oct 21, 2009.

  1. studentofcomputerscience

    Thread Starter New Member

    Oct 21, 2009

    I've got two exercises:
    1. Draw every time function (voltage across any element and current in any circuit element) which might possibly appear in first-order circuit i.e. in a circuit with one and only one capacitor or inductor and an undefined number (possibly large) of resistors and constant sources (both voltage and current) switched at t=0.
    2. Assume a simple serial RL circuit with a real inductor (i.e. with internal resistance R_L). How can you calculate R_L from the voltage U_L(t)? Give appropriate expression.

    It is my solution:
    However, I don't know how to sketch graph of Uc (first exercise) and calculate internal resistance of coil (second exercise). How to take into account many sources (especially I don't know what to do with ideal current sources) or resistors (first exercise)? Can you help me, please?

  2. studentofcomputerscience

    Thread Starter New Member

    Oct 21, 2009
    Hello! I don't understand why there are no answers. I presented my attempt to solution and I guess it is rather simple problem. Of course I know nobody is obliged to answer my question, however I don't know how to solve these two problems correctly (I only noticed one error in my solution - i.e. in first exercise for t=infinity, value of U_c is not zero). Greetings!
  3. hgmjr


    Jan 28, 2005
    Be aware that many members who are able to help you are not on the forum at this time. Usually you will get a reply in several hours but sometimes it takes the better part of a day for a member to notice your post among all of the others in the forum and take the time to develop an answer.

  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    In response to the first question:

    There are really only two basic forms for a first order response - whether it's for a voltage or a current.

    If the effective circuit time constant is T, then the two types of expression for a voltage response across any ideal element (R, L or C) driven by constant current or voltage sources would be either




    where Vm is the maximum value. Similar forms for current response would apply.

    In response to the second:

    If the only goal is to find the resistive component of the inductance then you would simply consider the circuit when it reaches steady state condition (notionally at t=∞)

    Under this condition the only voltage drops would be in the main series resistor and the inductor's resistance. So if the circuit were driven by a constant voltage source E then the value of R_L would be found from the relationship


    So if E=10V, R=100Ω and U_L(∞)=1V

    Solve the relationship 1=10*RL(100+RL) to find the unknown RL (100/9=11.11Ω)

    Hope this helps.