UNIVERSITY OF MASSACHUSETTS DARTMOUTH

COLLEGE OF ENGINEERING

EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I

KIRCHHOFF’S VOLTAGE LOOP LAW AND

THE VOLTAGE DIVIDER METHOD IN AC SOURCE

INTRODUCTION

The objective of this lab is to have you use MultiSim to confirm Kirchhoff’s Voltage Law for AC circuits, i.e., when the power source is time-varying.

You will use an oscilloscope to confirm that the peak-to-peak value of the voltage across a resistor is the value predicted by the voltage divider method for different source frequencies and wave shapes.

New Instruments in MultiSim:

Function Generator Source:

On the Instruments toolbar in MultiSim, you’ll find the Function Generator. When placed on your workspace, it looks like this:

If you double-click on the function generator, you’ll see 3 choices for wave forms (sinusoidal, triangle and rectangle); select the appropriate option. You’ll also need to select the signal frequency (in Hz) and the peak-to-peak amplitude. You should leave the DC Offset at 0 V and the duty cycle at 50%.

To connect the Function Generator to your circuit, connect the + and – sides as you would a battery and connect the ground (center) to the – side.

Oscilloscope:

Also on the instruments toolbar is the Oscilloscope. When placed on your workspace, it looks like this:

It can measure 2 different voltages at the same time. Connect the + and – for Channel A across resistor and the + and – for Channel B across resistor , as shown in Fig. 1.

PRELAB:

o  Before coming to lab, each team member should create a table in EXCEL that matches Table 1 and fill in their calculated values for “Predicted Peak Voltages” and “Predicted Period” in Table 1 assuming Figure 1 is built and the source waveform information provided in the table is used. The values in the “Predicted Peak Voltages” column should be calculated using the peak source voltage, the resistance values and voltage divider method. The values in the “Predicted Period” column should be 1/Frequency. Your tables need to be initialed by the instructor at the beginning of class to get full credit.

Figure 1: MultiSim circuit

IN-LAB PROCEDURE:

o  Working in pairs, use MultiSim to generate the circuit shown in Fig. 1.

o  For initial practice: Set the Function Generator voltage to 10 V peak (labeled Vpp on the Function Generator); set the waveform to sinusoidal and the frequency to 100 Hz.

o  Double click on the Oscilloscope and then start the simulation. Once the trace has crossed the entire oscilloscope screen at least once, stop the simulation.

o  Use the slide bar below the oscilloscope screen to adjust the view to start at 0 sec. Change the time base scale to 2 ms/Div and you should see 2 cycles of the waveforms. The vertical scale for both channels should be set to 5 V/Div.

o  Click and drag T1, the first vertical line marker (initially at far left of plot) to line up with the first peak of the sine wave. Drag T2, the second vertical marker (at either the far left or far right) to line up with the second peak of the sine wave. The oscilloscope transcript at the bottom should show 1.67 V for Channel A and 8.33 V for Channel B for both T1 and T2. Also T2-T1 should be 10 ms.

o  To hand in: Change the Function Generator settings to match each row of the leftmost columns in Table 1 and record the required voltage and time measurements for each setting. Be sure to adjust the time base scale and vertical scales on the oscilloscope as needed to have only 2 or 3 cycles displayed with the source voltage spanning most of the vertical range displayed.

o  In EXCEL, complete the columns as shown in Table 1.

QUESTIONS

o  How did your predicted and measured values in Table 1 compare with each other?

o  Did changing the source shape or frequency have any affect on the voltage divider results? If so, what affects were observed?

o  Is Kirchhoff’s Voltage Law satisfied?

DELIVERABLES

1.  A printout of your MultiSim circuit, with title block.

2.  Prelab EXCEL tables for each team member, initialed by instructor.

3.  1 completed EXCEL table – use Print Preview to make sure the entire table fits on one page.

4.  Answers to the questions.

Table 1 Sample spreadsheet

Name: ______Group #: ______

FUNCTION GENERATOR SETTINGS / PREDICTED
PEAK VOLTAGES
(use Voltage Division)
V1 V2 / PREDICTEDPERIOD
(T2-T1) / MEASURED PEAK VOLTAGE ON SCOPE / MEASURED TIMES ON SCOPE / Validation of Voltage Division
Wave Shape / V Peak (Amplitude) / Frequency (Hz) / V1 / V2 / T1 / T2 / T2-T1 / V1/Vs / R1/RT / V2/Vs / R2/RT
Sine / 10 / 1000
“ / 6 / 400
Triangle / 2 / 100
“ / 1 / 4000
Rectangle / 8 / 2000
“ / 4 / 600