Performance-Based Assessments for AC Circuit Competencies

Use a sine-wave function generator for the AC voltage source. Specify a resonant frequency within the audio range.

This exercise assumes access to a variety of capacitor and/or inductor sizes, so the student may make series-parallel networks to achieve the necessary values of L and/or C. It would be wise for you (the instructor) to check your students’ component kits for L and C values prior to choosing resonant frequencies for them to try to achieve.

I also recommend having students use an oscilloscope to measure AC voltage in a circuit such as this, because some digital multimeters have difficulty accurately measuring AC voltage much beyond line frequency range. I find it particularly helpful to set the oscilloscope to the “X-Y” mode so that it draws a thin line on the screen rather than sweeps across the screen to show an actual waveform. This makes it easier to measure peak-to-peak voltage.

Here, students must calculate values for C₁ and R₁ that will produce the V_out waveshape specified in the “Given conditions” oscilloscope plot. The input signal, of course, is a square wave. Students must calculate the time constant of the circuit (τ) such that the pulse fully decays within the pulse width (half-period) of the square wave. With 5 τ being the accepted standard for full charge/discharge of a time-constant circuit, this is an easy calculation.

There are many different combinations of values for C₁ and R₁ possible for any given square-wave signal frequencies. The purpose of this exercise is for students to be able to predict and select practical component values from their parts kits.

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.

Here, students must calculate values for C₁ and R₁ that will produce the V_out waveshape specified in the “Given conditions” oscilloscope plot. The input signal, of course, is a square wave. Students should be able to show mathematically why the time constant of the integrator (τ) must be 69.3% of the waveform’s half-period. Instructors, note: the calculations for this circuit, with V_out = ¹/₃V_in, are exactly the same as for a 555 timer circuit, because 555 timers also cycle their capacitors’ voltages at peak-to-peak values equal to one-third of the supply voltage.

There are many different combinations of values for C₁ and R₁ possible for any given square-wave signal frequencies. The purpose of this exercise is for students to be able to predict and select practical component values from their parts kits.

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.