Performance-Based Assessments for Semiconductor Circuit Competencies

Use a Variac at the test bench to provide variable-voltage AC power for the students’ power supply circuits.

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 variable-voltage, regulated power supply to supply any amount of DC voltage below 30 volts. Specify standard load resistor values, all between 1 kΩ and 100 kΩ (1k5, 2k2, 2k7, 3k3, 4k7, 5k1, 6k8, 10k, 22k, 33k, 39k 47k, 68k, etc.), and let the students determine the proper resistance values for their series dropping resistors.

I recommend specifying a series resistor value (R_series) high enough that there will little danger in damaging the zener diode due to excessive supply voltage, but also low enough so that the normal operating current of the zener diode is great enough for it to drop its rated voltage. If R_series is too large, the zener diode’s current will be too small, resulting in lower than expected voltage drop and poorer regulation (operating near the flatter end of the characteristic curve).

Values I have used with success are as follows:

R_series = 1 kΩ

R_load = 10 kΩ

V_zener = 5.1 volts (diode part number 1N4733)

V_supply = 12 volts

Measuring the minimum supply voltage is a difficult thing to do, because students must look for a point where the output voltage begins to directly follow the input voltage (going down) instead of holding relatively stable. One interesting way to measure the rate of output voltage change is to set a DMM on the AC voltage setting, then use that to measure V_load as V_supply is decreased. While turning the voltage adjustment knob on V_supply at a steady rate, students will look for an increase in AC voltage (a greater rate of change) at V_load. Essentially, what students are looking for is the point where \(\frac{dV_{load}}{dV_{supply}}\) begins to increase.

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’ve used 47 μF electrolytic capacitors and 1N4001 diodes with good success on a 10 volt AC (RMS) power supply. I recommend that students measure their own diodes to determine typical forward voltage (V_F).

Don’t forget to mention the polarity sensitivity of these capacitors! Electrolytic capacitors can explode violently if reverse-connected!