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How to draw phasor diagrams?

Discussion in 'Electronics Homework Help' started by MrAlex, Aug 14, 2013.

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  1. MrAlex


    Aug 14, 2013
    Hi everyone!

    So I'm doing an assignment and it requires me to draw the phasor diagrams for the circuit below. I need to draw the phases for each of the components, with the voltage source being the reference phase.

    So I understand what phasor diagrams look like, but have no idea how to actually draw them. I've got all the values for the components, so what calculations do I need to perform? How is this translated onto a diagram? I've tried looking online but nothing that says "Ok, using your values do X then Y etc".

    Any help would greatly be appreciated!

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Apart from the component values, you'll need the frequency. I assume you have that, too.

    Next step is to name the components. THis will make the following calculations easier. For example name the top left resistor R1, the 2.2k resistor R2, the 4.7k resistor R3 and the capacitor C1.
    You should also clearly label the currents in and the volatges across the components. E.g. name the source V0, the voltage across R1=V1, the current through R1=I1 etc. Take care and note where the same current flows through two components (e.g. I2 through 2.2k and 22nF) or where the same voltage is across two components (e.g. the voltage across (2.2k+22nF) and 4.7k). Same currents have the same name as do same voltages.

    Now you set up the network equations (e.g. using kirchhoff's laws) and solve the system of equations. for the different currents and voltages.
    The result of this bit of math will be a set of equations in the form
    I1= f(R1, R2, R3, C1, V0, frq) etc.
    - where R1...C1 are the components, V0 is the input voltage, frq is the frequency
    - f() is a complex expression which will usually contain fractions.

    Rearrange the complex expression such that it has the form of either
    I1= freal() + fimag() which expresses the phasor in Cartesian coordinates
    I1= A() /_() which expresses the phasor in polar coordinates (/_ stands for the phase angle)

    Per definition V0 = 1 /_0

    Note that the phasor's amplitude and phase will depend on the frequency. In your example, if frq=0, no current at all will flow through the capacitor and the circuit acts as avoltage divider with 0 phase. With increasing frequency. more and more current will flow through the capacitor which will lead to a shift in amplitude and phase of all currents and voltages.

    Using either of above representations you can now draw the phasors into a diagram. Some more info is here.
    Last edited: Aug 14, 2013
  3. MrAlex


    Aug 14, 2013
    That's great! Thanks for your help :)
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