Thursday, 1 October 2015

What is the phase of Voltage and Current in AC signal

Circuit to demo phase between Voltage and Current

I always perceived that the Voltage and Current are in same phase in a AC voltage source, and using LTSpice probing for Voltage at V1 source and and another probe measuring current through V1 shows a plot as shown below that indicates indeed there is a 180 degree phase difference.

Wave of 180 degree difference between V and I by LTSpice simulation

But actually in real world there is no phase difference between Voltage and Current in the Voltage Source. The above plot is because of the way LTSpice probes the Current through the Voltage Source (V1).

If you notice the probe at the (V1)  is pointed downwards and the probe at the R1 load resistor is also pointed downwards. That means the current is measured in the reverse direction in the voltage source (V1) so the actual plot shown is -I (V1). This is the reason we get 180 degree phase shift in LTSpice Simulator for Current measured through Voltage Source.

The below is the plot of current measured across the R1 load resistor.

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