Exercise 1: Capacitors

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1 Capacitance AC 1 Fundamentals Exercise 1: Capacitors EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the effect a capacitor has on dc and ac circuits by using measured values. You will verify your results with a multimeter and an oscilloscope. DISCUSSION zero (assuming the capacitor did not have an initial charge). is stopped (except for the small amount of leakage current through the dielectric). The charge on the capacitor remains after the applied dc is removed. A charged capacitor can be discharged when a resistor or a direct short provides a discharge path across the capacitor. In this circuit, R2 provides a discharge path for the capacitor current (I DISC ). is interrupted. 152 FACET by Lab-Volt

2 AC 1 Fundamentals Capacitance Although ac passes through a capacitor, the capacitor creates opposition in the form of impedance. result, total circuit current increases. The circuit current decreases when the value of capacitance is reduced. If the value of C1 were increased to 0.9 F, the circuit current (I C1 ) would a. increase. b. decrease. c. remain the same. Increasing the frequency of the applied signal decreases the impedance of the capacitor, resulting in a higher circuit current. Circuit current will decrease and the capacitor impedance will increase if the frequency of the applied signal is decreased. If the frequency of the signal source were changed to 5 khz, circuit current (I C1 ) would a. increase. b. decrease. c. remain the same. Capacitance also affects the phase relationship between the applied voltage and current. The voltage across a capacitor lags the current by 90º. Put another way, the current through the capacitor leads the voltage by 90º. PROCEDURE If necessary, clear the AC 1 FUNDAMENTALS circuit board of all two-post connectors and any other connections. FACET by Lab-Volt 153

3 Capacitance AC 1 Fundamentals Locate the RC TIME CONSTANTS circuit block, and connect the circuit shown. Switches S1 and S2 are normally open. They must be pushed and held in order to be closed. S2 provides a means of discharging C1 through R3. In order to start with C1 fully discharged, press and hold S2 for several seconds, then release it. Connect channel 1 of the oscilloscope (use a X10 probe) across C1 and set the vertical coupling control to DC. S1 applies the 15 Vdc to the circuit. While monitoring the oscilloscope for voltage across C1, close S1. Does the capacitor charge up when the dc voltage is applied? Close S1 for about 15 seconds. While observing the voltage across C1 with the oscilloscope, release (open) S1 to remove the dc source from the circuit. 154 FACET by Lab-Volt

4 AC 1 Fundamentals Capacitance Does the charge on C1 remain even after the dc source is removed? Discharge the capacitor by pressing S2 until the voltage across C1 is zero. Remove the oscilloscope probe from the circuit. Set up a multimeter to read dc milliamps. Replace the two-post connector, between S1 and R2, with the multimeter probes. While monitoring the multimeter display, hold S1 closed for about 15 seconds. Repeat several times (discharge C1 each time by using S2). Based on the reaction of the multimeter FACET by Lab-Volt 155

5 Capacitance AC 1 Fundamentals became fully charged? Unlike dc, a capacitor will not block ac because the voltage level and polarity are constantly changing. In the following steps, you will change the value of capacitance and frequency of the applied signal to determine the effect on circuit current in a resistor-capacitor (RC) series circuit. shown. Use the oscilloscope to adjust V GEN for a 10 V pk-pk, 1 khz sine wave. Determine circuit current (I) by using current-sensing resistor R2. NOTE: To determine the ac circuit current, remove the two-post connector and use the oscilloscope to measure the peak-to-peak voltage drop across sensing resistor R2. Take the measurement and divide the value by the resistance of R2 (10 ). Replace the two-post connector before moving on to the next step. I = V R2 I = ma pk-pk (Recall Value 1) 156 FACET by Lab-Volt

6 AC 1 Fundamentals Capacitance Monitor the circuit current on the oscilloscope by observing the amplitude of the voltage across current-sensing resistor R2. Place CM switch 10 in the ON position to increase the capacitance of C3 from 0.1 F to 0.2 F. While observing the oscilloscope, toggle the CM switch off and on. Does an increase in capacitance increase or decrease circuit current? a. increase b. decrease Monitor the circuit current on the oscilloscope. Increase the generator frequency. Does increasing the frequency of the applied signal increase or decrease circuit current? a. decrease b. increase Readjust the generator frequency to 1 khz. Using V C3 as the reference, connect the channel 1 input of the oscilloscope to measure V C3, and connect the channel 2 input to measure V R2 (circuit current and V R2 have identical phase). Observe the phase angle ( ) between the circuit current (V R2 ) and V C3. Does the circuit current lead or lag the capacitor voltage? a. lead b. lag Do not turn off the equipment. The FACET setup is needed to answer a review question. Make sure all CMs are cleared (turned off) before proceeding to the next section. FACET by Lab-Volt 157

7 Capacitance AC 1 Fundamentals CONCLUSION A charge on a capacitor remains after the voltage source is removed. A capacitor passes ac current. REVIEW QUESTIONS 1. A capacitor a. blocks ac and passes dc. b. blocks dc and passes ac. c. passes ac and dc. d. blocks ac and dc. 2. Adjust V GEN for a 10 V pk-pk, 1 khz sine wave. Monitor the circuit current on the oscilloscope by observing the amplitude of the voltage across current-sensing resistor R2. Place the CM switch 9 in the ON position to alter the value of C3. While observing the oscilloscope, toggle CM switch 9 off and on. What can you conclude based on the reaction of the circuit current? a. The capacitance of C3 increased in value. b. Changing the capacitance of C3 had no effect on circuit current. c. The capacitance of C3 decreased in value. d. Changing the capacitance of C3 increased circuit current. 3. Decreasing the frequency of the signal applied to a capacitor a. b. c. d. decreases impedance. 158 FACET by Lab-Volt

8 AC 1 Fundamentals Capacitance 4. In a capacitor, a. current lags voltage by 90º. b. current leads voltage by 90º. c. voltage leads current by 90º. d. voltage and current remain in phase. 5. a. only while it is charging. b. when it is fully charged. c. only while it is discharging. d. while it is charging or discharging. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. FACET by Lab-Volt 159