Design of Experiment - Measuring Inductance & Capacitance

The primary goal of this lab procedure was to find the values of several capacitors and inductors of unknown values using only the oscilloscope and function generator. My lab partner and I went about completing this task by utilizing the time constants of an RC circuit to find the capacitance and of an RL circuit to find the inductance. The time constant, tao, is the amount of time required for a circuit to transition from its resting place to 63.2% of its final value in response to a change in the circuit. Using a square wave as the input, we measured tao. The inductance and capacitance was deduced by using the known relationships of tao to inductive and capacitive loads.

For this experiment, we used the TDS2022-Oscilloscope to measure the output voltage from the function generator as well as the desired output signal from our circuit. The BK Precision 4040 FGEN provided the input signal to the circuit in question.

The initial step was to breadboard the circuits shown in Figure 1. The L and C componenets are replaced by the unknown inductors and capacitors.

The function generator was then configured to produce a 10Hz square wave with an amplitude of 1V at a 50% duty cycle. After having issues with using a 10Hz signal, we eventually had to adjust the function generator to produce higher frequency to get a proper reading. After inserting our first unknown capacitor into the RC circuit as shown in Figure 2, we used CH1 of the oscilloscope to measure the input signal and CH2 to measure the voltage across the capacitor as shown in Figure 3. Next, using the cursor tool, we measured the RC time constant from the edge of each pulse to the time when the voltage across the capacitor changed by 1.264V. Using the mathematical relationship tao = RC, we solved for the capacitance value and recorded results in Table 1. This procedure was repeated for 6 different capacitors.

Figure 1: Breadboarded RC circuit

Figure 2: Oscilloscope display of input and output waveforms

The time constant method was effective in finding the values of our selected unknown capacitors. However, we did not get the same results with the inductors. Theoretically, we should have been able to obtain values for our inductors using the same method as we did with the capacitors and the mathematical relationship tao=L/R for the time constant. However, this proved to be a difficult task as we could not figure out why we were getting incorrect values using this method. By utilizing the concepts of transient circuit analysis from ELEC 2110, we should have easily deduced the values for inductors through the known relationships of charging/discharging time constants to inductive and capacitive loads.