Chapter 8
AC Power

m8.1 Periodic Waveforms

Plot the half-wave rectifier output and full-wave rectifier output for each of the three standard waveforms shown in Figure m8.1. Then:

1. Determine the general expressions for the (1) average value and (2) rms value for each of the six rectified waveforms.
2. Evaluate your expressions for average and rms values for V _m = 10 V and T = 10 ms.

NI Multisim Measurements

1. Create the half-wave rectified and full-wave rectified versions of the three standard waveforms of Figure m8.1 using the following approach:
   • Create the unrectified original waveform with the Function Generator,
   • Connect a wire to the + terminal of the function generator and double-click to terminate the wire in “empty space” without connecting to any other terminal. Display the net names to determine the number of this net. Connect the Common terminal to ground.
   • Place the ABM_VOLTAGE source, ground its negative terminal, and enter the function positive(v(n)) (where n is the net number of the wire connected to the function generator output) to make the ABM source create the half-wave rectified version of the function generator waveform,
   • Place another ABM source (also with its negative terminal grounded) with the absolute value function abs(v(n)) to create the full-wave rectified waveform, and
   • Place an SPDT switch (single-pole double-throw) to conveniently connect one ABM source or the other to the oscilloscope to display the rectified waveform.
2. Place a Measurement Probe to display the average value and rms value of the rectified waveform. Determine the average and rms values of each of the rectified waveforms.

Additional tips:

• The “Analog Behavior Modeling” (ABM) voltage source “senses” the voltage of net n with the function v(n). The connection between the function generator and the ABM source does not appear visually.
• The Multisim User Manual describes a wide variety of functions that can be used by the ABM source. Visit ni.com/manuals and enter “Multisim User Manual” into the search box. See the “Mathematical Expressions” section of Chapter 7.
• Set up the Simulate → Instruments → Measurement Probe to display V(dc) for average value and V(rms) for rms value.

NI myDAQ Measurements

1. Create the half-wave rectified and full-wave rectified versions of the three standard waveforms of Figure m8.1 with the arbitrary waveform generator on AO0; use 200 kS/s for the sampling rate. Display this signal on the oscilloscope using AI0. Extract the average value of the waveform using the RC circuit shown in Figure 8.1a and display the output of this circuit on AI1.
2. Read the average value of the rectified waveform using either the cursor value or the “RMS” indicator below the waveform display for AI1. Read the “RMS” indicator for AI0 to measure the rms value of the rectified waveform. Display at least two periods of the waveform to ensure accuracy of the “RMS” indicator.

Additional helpful tips:

• Set both channels of the oscilloscope to the same volts per division. In this way the RC circuit output (average value) overlays the rectified waveform and you can visually see how the average value tends toward the effective center of the waveform.
• IMPORTANT: Be sure to observe proper polarity for the electrolytic capacitor. The capacitor may mark the negative lead rather than the positive lead.