Performance-Based Assessments for DC Circuit Competencies

I recommend choosing resistor and capacitor values that yield time constants in the range that may be accurately tracked with a stopwatch. I also recommend using resistor values significantly less than the voltmeter’s input impedance, so that voltmeter loading does not significantly contribute to the decay rate.

Good time values to use (t₁, t₂, t₃) would be in the range of 5, 10, and 15 seconds, respectively.

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.

Two very important “given” parameters are the relay coil resistance (R_coil) and the relay dropout voltage (V_dropout). These are best determined experimentally.

Many students fail to grasp the purpose of this exercise until it is explained. The idea here is to predict when the relay will “drop out” after the switch is opened. This means solving for t in the time-constant (decay) equation given the initial capacitor voltage, time constant (τ), and the capacitor voltage at time t. Because this involves the use of logarithms, students may be perplexed until given assistance.

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.

I recommend choosing resistor and capacitor values that yield time constants in the range that may be accurately tracked with a stopwatch. I also recommend using resistor values significantly less than the voltmeter’s input impedance, so that voltmeter loading does not significantly contribute to the decay rate.

Good time values to use (t₁, t₂, t₃) would be in the range of 5, 10, and 15 seconds, respectively.

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.