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I am trying to figure out how to correctly measure the phase difference of a circuit's different currents using multisim. After trying several examples, I noticed that spawpping the position of two in series elements (a restistor and a capacitor), it completely changes the measurement, as you can see below. enter image description here

enter image description here

As you can see, just by swapping R2 and C1, I get a different time difference between two concecutive peaks (don't mind the negative sign). This confuses me, because I can't explain why it happens.

To make things worse, this difference becomes even greater with a similar circuit that uses different elements, as seen below.

enter image description here

enter image description here

To sum up, I'd appreciate it if someone could explain to me:

  1. Why is this difference? How do I understand it?
  2. How do I decide which one to actually measure if I want to calculate the phase of the current? Does it purely depend on the given circuit?
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    \$\begingroup\$ If you swapped the two resistors in a resistive voltage divider (assuming they're not equal) the output voltage would change. Why do you think it would stay the same in the case of an RC voltage divider? \$\endgroup\$
    – The Photon
    Commented Feb 25, 2020 at 16:26

1 Answer 1

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The resistor and capacitor are in series. This means (assuming the probe impedance is negligible, as it should be in the sim) that the same current is passing through both the resistor and the capacitor. That is not changing when you switch the two.

You are measuring voltage relative to ground (i.e., single-endedly). The reason is that you're measuring the voltage and phase as different is that in one configuration, you're measuring the voltage across the resistor, which is in phase with the current. In another configuration, you're measuring the voltage across the capacitor, which is 90 degrees out of phase with the current. In fact, in both cases shown above, you can see that the phase moves 90 degrees, or 1/4 of a cycle (2.5ms) when you switch which element you're measuring the voltage across.

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  • \$\begingroup\$ Thanks for your quick answer. Just an insight for anyone trying to understand the same thing as me:Don't forget that the oscilloscope measures voltage, not current. So, basically you want to measure the voltage after the element that changes its phase (i.e. the capacitor), but also across an element that doesn't change its phase (i.e. the resistor). This way you make sure that the voltage phase you measure is the same for the current, too (I=V/R). Hope it helps someone in the future. \$\endgroup\$ Commented Feb 25, 2020 at 18:09

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