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Complex Power in AC Circuit Analysis Experiment 27

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Changes to Circuit

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Analysis Section Step 6: – Use MatLAB to plot a graph of the instantaneous power, p(t), for each value of R 2.

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Tables 1-4 and Step 16 of Measurements Change to – You must calculate the value of the capacitor that is required to balance the inductive portion of the impedance of the resistor and inductor in series.

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Measurements To measure the instantaneous power, you will measure the voltage dropped across the components and the current flowing through them, as measured by the voltage dropped across the shunt resistor. These measurements can be multiplied together to obtain the instantaneous power. The latest version of the oscilloscope software program has a CH1*CH2 option. However, it is difficult to read data off of the plot.

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Export Data to MatLAB Select File/Save DSO Data from upper toolbar.

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Data Format The number of data points in the first column (N) are equivalent to a period of time (50 s in this case). To change the data listed under N to time, you must multiply each value by the fraction 50 s/125.

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Data Format 0V is equal to the value of 167 for the data displayed under CH1 and to the value of 87 for the data displayed under CH2. This offset must be removed before converting the data listed under CH1 and CH2 can be converted to voltage.

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Data Format The conversion factor needed to change the data listed under CH1 and CH2 into voltage. To change the data to voltage after you have substract off the offset for ground, you must multiply each value by the fraction 1V/32 for CH1 and 0.1V/32 for CH2 in this example. To convert the voltage across the shunt resistor to current, divide by the value of the shunt resistor. Only then can you can multiple the data listed in CH1 by the data listed in CH2 to calculate the power.

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Importing Data File to MatLAB You may do the preprocessing of the data saved from the oscilloscope in either Excel or in MatLAB. However, the graphs to be included in Step 5 of the report template must be generated using MatLAB. – There is an Import Wizard in MatLAB. For instructions, see: http://www.mathworks.com/help/techdoc/import_export/br5wz4t.html#f5-91134 http://www.mathworks.com/help/techdoc/import_export/f5-35378.html These instructions are also available in the MatLAB program in the Help function

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Measurements The m-file referred to in the lab manual (complexpower.m) has been modified. – Use complexpower_changed.m is posted under Resources/Technical Support: MatLAB

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Conclusions You should comment on the accuracy of your analysis and measurements due to: – Deviations from nominal values of your components including the parasitic resistance of the inductor – The effect of the shunt resistor – Digitizing errors associated with the data outputted by the oscilloscope

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