Performance-Based Assessments for AC Circuit Competencies

The real challenge in this assessment is for students to determine their transformers’ “polarities” before connecting them to the AC voltage source! For this, they should have access to a small battery and a DC voltmeter (at their desks).

You may use a Variac at the test bench to provide variable-voltage AC power for the students’ transformer circuits. I recommend specifying load resistance values low enough that the load current completely “swamps” the transformer’s magnetization current. This may mean using wire-wound power resistors instead of [1/4] watt carbon composition resistors.

Note that there may very well be a shock hazard associated with this circuit, as the output voltage is not far below the line voltage! Be sure to take this into consideration when specifying load resistor values. You may also want to use low supply voltage levels (turn the Variac way down).

In lieu of using a line-powered transformer, you may also do this exercise with a signal generator and an audio-frequency transformer. The principles are the same, and the safety hazard will be vastly reduced. If using a 1000:8 ohm transformer, the winding turns ratio is approximately 11.18 to 1.

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.

The real challenge in this assessment is for students to determine their transformers’ “polarities” before connecting them to the AC voltage source! For this, they should have access to a small battery and a DC voltmeter (at their desks).

You may use a Variac at the test bench to provide variable-voltage AC power for the students’ transformer circuits. I recommend specifying load resistance values low enough that the load current completely “swamps” the transformer’s magnetization current. This may mean using wire-wound power resistors instead of [1/4] watt carbon composition resistors.

Note that there may very well be a shock hazard associated with this circuit, as you are “boosting” the supply voltage! Be sure to take this into consideration when specifying load resistor values. You may also want to use low supply voltage levels (turn the Variac way down).

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.

The nature of the AC signal source is not crucial, so long as an accurate peak measurement may be obtained. I recommend specifying equal-value resistors to make the voltage drop calculation as easy as possible. The purpose of this exercise is not how to calculate voltage divider outputs, but rather how to utilize both inputs of a dual-trace oscilloscope to perform differential voltage measurements.