Performance-Based Assessments for AC Circuit Competencies

You will need an AC signal source of variable voltage, so that the reference voltage at test point A (V_A) may be precisely adjusted to 1 volt RMS. The frequency of the signal source must be within the range of the voltmeter to measure, of course. You may use a sine-wave signal generator as the source, but a step-down transformer and series potentiometer works just as well! The purpose of this exercise is not how to calculate voltage divider outputs, but rather how to utilize a dB-reading meter to measure signal strength in decibels rather than volts AC.

Most high-quality digital multimeters will measure dBV in addition to AC volts RMS. Some have a “zero” or “relative” button which acts like the “tare” button on an electronic weigh scale. With such a meter, one may set 0 dB to reference at any measurable level of AC voltage. Of course, this means the meter will be measuring dB instead of dBV, since the reference point is made arbitrary with the press of the “relative” button. However, this is a very useful tool for electronic signal measurements, where source strength may be set to 0 dB and then losses measured directly in -dB (relative to the source) just as easily as normal voltage measurements.

Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard resistor and capacitor values.

If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range.

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.

Use a sine-wave function generator for the AC voltage source. I recommend against using line-power AC because of strong harmonic frequencies which may be present (due to nonlinear loads operating on the same power circuit). Specify standard resistor and capacitor values.

If students are to use a multimeter to make their current and voltage measurements, be sure it is capable of accurate measurement at the circuit frequency! Inexpensive digital multimeters often experience difficulty measuring AC voltage and current toward the high end of the audio-frequency range.

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.