ENGR225 Circuit Analysis                           Name_______________________________

Lab #6 - First Order RC and RL Circuits                Date________________________________

 

Objectives:

 

To see the actual voltage and current responses in first-order RC and RL circuits.

To see how our calculations agree with the observed responses.

 

Procedures:

 

1.                  Use the LCR meter on the Cs & Rs range to measure your capacitor for the circuit in Figure 1.   The time constant will be the sum of the three series resistors times the capacitance.  Build the RC circuit shown in Figure 1.  Set up the Wavetek for a 0-to-8V square wave before you connect it to the circuit.  I will show you how you can effectively observe this open circuit square wave even after you connect the circuit.  Adjust the frequency of the square wave so that the transient waveforms are allowed to go to their final values after each square wave transition (frequency = 500Hz).  Record the current waveform by measuring the voltage across the 51Ω resistor R and then dividing the vertical scale by 51Ω to convert it to a current waveform.  Then interchange C and R and record the voltage across C.  Obtain scope pictures for both the current and the capacitor voltage waveforms when the input is going from 0V to 8V and then when the input is going from 8V to 0V (4 pictures).

   

2.                  Calculate the expected capacitor voltage and current waveforms for part 1 right after the input voltage rises from 0V to 8V and then again when the input voltage drops from 8V to 0V.  Compare your calculations with the measured waveforms by plotting the calculations in Mathcad on a similar time and amplitude scale or by plotting some calculated points right on the measured waveform displays.  Either way compare and discuss the agreement between the calculated and measured waveforms.

 

3.                  Use the LCR meter on the Ls & Rs range to measure your values for the L and RL (this is the Ls and Rs for the inductor) required for the circuit in Figure 2.   Build the RL circuit of Figure 2.  Set up the Wavetek as in part one again choosing a frequency to allow the waveform to reach its final value after each transition.  Measure the current waveform by measuring the voltage across the 51Ω resistor R and then dividing the vertical scale by 51Ω to convert it to a current waveform.  Obtain scope pictures for both the current and the inductor voltage waveforms when the input is going from 0V to 8V and then when the input is going from 8V to 0V (4 pictures).

 

4.                  Perform the same calculations for the voltage and current waveforms asked for in part 2 right after the input voltage changes from 0V to 8V and then again when the input voltage drops from 8V to 0V.  Compare your results with the measured waveforms as before.

 

Note: the key points to compare between the calculated and measured waveforms are the initial values, the shape of the response curves, and the time it takes to reach the final values.