ELEC 242 Lab

Experiment 5.1

RC Circuits

Equipment

Components

Part 1: Zero Input Response



Step 1:

 Set the DMM to DC Volts. Remove the probes from the DMM and replace them with your BNC-banana adapter. Make sure the ground bump goes to the COMMON terminal.


Step 2:

 Wire the following circuit. Use the BNC clip leads to connect the DMM to the circuit.


Warning
Electrolytic capacitors are polarized, i.e. they have a + terminal and a - terminal. Be sure to connect the + terminal to the positive supply and the - terminal to ground. If connected backwards, an electrolytic capacitor stops being a capacitor and conducts a large DC current. This current can heat up the capacitor to the point where it may EXPLODE.

Unfortunately it's not immediately obvious by looking at most electrolytic capacitors which terminal is positive and which is negative. Most are marked only no the negative terminal, and not with a simple minus sign, but a weird "minus inside an oval" symbol ( ).



Step 3:

 Turn on the power supply and adjust to 10 V.

Step 4:

 Without otherwise disturbing the circuit, disconnect the power supply, i.e. the circuit should now look like this:

Using a stopwatch, the second hand of your watch, or the "Clock" program from the "Accessories" menu, read and record the voltage at 5 second intervals for 90 sec.

Question 1:

 Why can't we just turn off the power supply to get the zero input response?

Step 5:

 Plot vs. time.

Step 6:

 Determine the time constant. How does it compare with the expected time constant (RC) of the circuit?

Question 2:

 Take the ratio of successive measured values of . What pattern emerges in the successive ratios? Why? Can you determine the time constant from this information? If so, how does it compare with your other determinations?

Part 2: Step Response

Although straightforward to understand, the technique used in Part 1 doesn't have much else to recommend it. It's tedious, prone to errors, and impractical for time constants smaller than a few seconds. This time we'll use the function generator and scope, and measure the step response of the circuit.


Step 1:

 Select a 2.2 k resistor and a .33  F capacitor from your parts kit.

Note
Ceramic capacitors use the same labeling codes as the potentiometers except that the units are picofarads (pF) instead of ohms. So a .33  F capacitor would be a 330,000 pF capacitor which would have the code 334 ( ).


Step 2:

 Wire the following circuit.


Step 3:

 Connect CH1 of the scope to and CH2 to .

Step 4:

 Set the CH1 VOLTS/DIV to 2 and the CH2 VOLTS/DIV to 1. Set the trigger source to CH2 and slope to +.

Step 5:

 Set the TIME/DIV switch to 2 mSEC.

Step 6:

 Set the function generator to produce an 80 Hz square wave. Adjust the amplitude so that has a peak-to-peak amplitude of 8 V, i.e. so that it completely fills the scope screen.


Step 7:

 Sketch and .

Step 8:

 Set the TIME/DIV switch to .1 mSEC. Measure the length of time taken by the wavefrom to rise from its initial value (the bottom of the screen) to away from its final value ( divisions from the top of the screen).


Step 9:

 Increase the function generator frequency to 1kHz. Sketch and .

Step 10:

 Set the function generator waveshape to triangle. Sketch and .

Step 11:

 Set the function generator waveshape to sine. Sketch and .

Step 12:

 Exchange the resistor and capacitor.


Step 13:

 Reset the function generator waveshape to square and the frequency to 80 Hz. Reset the scope to 2ms/div. Sketch and .

Question 3:

 Using the techniques we have developed in class, determine analytically what the waveforms should be in Steps 6 and 12. Do your measured waveforms agree?