# How to Measure input AC power through a coil using formulas If the voltage and current is known?

#### Lighttree18

Joined Feb 14, 2021
5
I am using LTSPICE to couple toghter two RLC circuits for frequency analysis. I need to measure the power flowing through the primary side and induced powered in the secondary. I am able to measure the voltage and current at different frequencies and need a formula to find out the power. This is my first dive into AC circuits and am a bit confused regarding RMS power.

#### ericgibbs

Joined Jan 29, 2010
12,901
hi 18,
Welcome to AAC.
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#### Lighttree18

Joined Feb 14, 2021
5
Here is My Schematic. Thank you for responding. Quick Question How do I get rid of the dotted graph curve that seems to follow the main curve?.

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#### ericgibbs

Joined Jan 29, 2010
12,901
hi,
For the plotted dotted phase line , right click on the rightside degrees scale, pop up menu , 'dont show phase'' button.
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#### crutschow

Joined Mar 14, 2008
27,188
The power can be calculated when doing a transient analysis.
For example, the V2 supply average Power (RMS) is the average of the instantaneous voltage across a device times its current at the same point in time.

In LTspice plot the voltage across the component.
Right click on the plot name to open up the plot expression editor and change to V(part) * I(part) (first attachment below).
Then Ctrl-Left Click that plot to get the power average value (second attachment below).

The power dissipated in a resistance can also be calculated by measuring the RMS voltage across it, squaring that voltage, and then dividing that by the resistance.

#### schmitt trigger

Joined Jul 12, 2010
438
The advantage of using LTSpice, is that it is ubiquitous and there will always be someone who knows a trick or shortcut.