ELEC 226, Spring 2003

Prof. Rich Kozick

Prof. Rich Kozick

RC Circuits, Time Constants, and an Oscillator

Figure 1Please answer the following questions in your lab notebooks. These answers should not take very long, since we have discussed this in class recently.

- If the switch has been closed for a long time so that the capacitor is fully charged, what is the voltage v(t) across the capacitor?
- Suppose that the switch opens at time t = 0 seconds. Analyze the circuit and find an equation describing the voltage v(t).
- What is the time constant for this circuit, in terms of
R
_{s}, R, and C? Make a sketch of v(t), indicating the value of v(t) after 1, 2, 3, 4, and 5 time constants. - You should be able to see from your plot where the following two
facts and "rules of thumb" about time constants come from:
- The response decays to 36.8% of its original value after one time constant.
- The response has decayed to "zero" after 5 time constants, since the amplitude is less than 1% of the original value.

- What value of R should be used to obtain a time constant of 1 msec?

- Adjust the horizontal (time) axis scaling and the vertical
(voltage) scaling to values that are appropriate for the value of
V
_{s}and the time constant. - Open and close the switch a few times. Make sure the v(t) you observe on the scope matches the sketch you made earlier.
- Use the MODE key on the scope to set it to record a single trace when you open the switch. Also set the scope to trigger at a level just below v(0), and set the scope to trigger on a negative slope.
- Use the STOP, RUN, and ERASE keys to record a trace of v(t) after you open the switch.
- Use the cursors on the oscilloscope screen to measure and compute the time constant. If you use the "%" option in a clever way, then you can get the scope to do all the computations for you in checking the time constant.

Below are some specific activities and measurements to make.

- Measure the time constant of the circuit using the oscilloscope. Compare the measured value with the expected value based on the R and C component values.
- Consider the case in which the switch is initially opened and then closed to charge the capacitor. Use the oscilloscope to capture one trace of the charging capacitor. Note that the procedure needs to be modified slightly to capture this trace.
- Modify the circuit to achieve a time constant on the order of one second. Use the oscilloscope to verify that the time constant is indeed about one second.

Figure 2Perform the following activities.

- Please sketch on a single plot the capacitor voltage
v
_{c}(t) and the op amp output voltage v_{out}(t) versus time for the oscillator circuit. How is the period of the wave related to the values of R and C? -
What value should you choose for the resistor R
_{a}? Why? If R is a potentiometer that varies from 0 ohms to 100 k ohms, what value of C should you use to produce a clock frequency as low as 1000 hertz? How can you modify the circuit to produce clock frequencies in the range from 100 to 1000 hertz? -
Set up and test your circuit. Demonstrate how the frequency of your
clock circuit varies as you change the potentiometer. Observe both
v
_{c}(t) and v_{out}(t) on the oscilloscope. -
What if the pair of resistors with value R
_{a}are replaced by resistors with values that are not identical? Can you make a triangle wave generation circuit? Try it!!