source transformations

Discussion in 'General Electronics Chat' started by ericleerice, Oct 31, 2013.

  1. ericleerice

    Thread Starter New Member

    Oct 31, 2013
    2
    0
    I am trying to analyze a circuit using source transformations (converting from a voltage source to a current source). I am trying to understand how they work based on a book, but I keep getting confused based on a simple example. If I have a circuit with a voltage source and two resistors in series, according to the book I'm looking at, I can convert the voltage source and resistor into a current source and a resistor in parallel. The current value of the current source will be the voltage source divided by the resistor, and the new resistor in parallel will have the same value for resistance.

    The problem is, if we have two resistors in series, if we choose to perform the transformation about one resistor, we get one answer for the value of the new current source, and if we choose the other resistor, we get a different answer. The resistors will be the same, but the value of current coming out of the current source will be a different magnitude and a different direction. Am I just looking at this wrong?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    Yup.

    Current sums for parallel resistors, but is identical in series connected resistors. If you draw it out and calculate the current in every resistor, you should arrive at that conclusion.
     
  3. ericleerice

    Thread Starter New Member

    Oct 31, 2013
    2
    0
    I understand that. Maybe it would be helpful to be more concrete. Suppose I have a 10V source, and a 1000 ohm and 2000 ohm resistor in series. If I do the transformation about the 1000 ohm resistor, then I have a current source of 0.01A and two resistors in parallel (1000 and 2000 ohm resistors). If I do the transformation about the 2000 ohm resistor, then I have a current source of 0.005A and two resistors in parallel (1000 and 2000 ohm resistors).
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    You use the "equivalent" resistance of the network for the conversions. So you can't use the resistors separately for that. Since resistors in series add you would use a value of 3000 ohms for both the voltage and current sources.
     
    anhnha likes this.
Loading...