Download Lab 8 Capacitors and Inductors

Villanova University
ECE 2053 Fundamentals of Electrical Engineering I Lab
8 – Capacitors and Inductors
During this lab you will examine capacitors and inductors, measure the parameters of
capacitance and inductance, and build circuits to examine their dc and ac characteristics.
Parts List
Resistors: 100 Ω, 2.2 kΩ all 5%, ¼ W
Capacitor: 0.15 F
Inductor: 40 mH
Exercise 1: DC Characteristics of Capacitors
1. Select a capacitor of 0.15 microfarad (F) and measure its capacitance by using either
of these meters:
The B&K Precision Universal LCR meter, or
The B&K Precision 830 Capacitance meter.
See if you can measure the resistance of the capacitor. Since the resistance is very
large you may not see a readable display. Try using an analog ohmmeter — see
instructor.
2. Question: How much current would pass through this capacitor if it were connected in
series with a 5-volt dc source and a 2200 ohm () resistor?
3. Connect a 5-volt dc source, the capacitor, and a 2200  resistor in series on your
breadboard. Connect the capacitor to the negative side of the source.
4. Use the digital multimeter (DVM) set for dc voltage measurements to measure the
voltage across all 3 elements.
5. Compute the current using the measured resistor voltage. How does this computed
compare with your predicted value?
6. What is your conclusion about dc current through a capacitor?
Exercise 2: AC Characteristics of Capacitors
1. Exchange the 5-volt dc source with a function generator (FG).
2. Select a sine wave at a frequency of 500 hertz (Hz).
3. Set the DVM for ac voltage measurements. Note: The DVM set for ac measurements
is calibrated to display the effective (rms) value for sine wave signals.
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4. Use the DVM to set the voltage across the source (with the circuit connected) to 5.0
volts rms.
5. Measure the resistor voltage and the capacitor voltage.
6. Use Ohm's law to compute the rms value of the ac current.
7. Next measure the circuit current as follows. Obtain a Tektronix DMM and set it for
ac current measurements. Next make a break in your circuit between the resistor and
capacitor; connect the meter in series with the break.
8. Record the current. Note: A fuse protects the meter and it is possible that the fuse is
blown from a previous user. Note: We are using the Tektronix DMM because the
Agilent DMM set for AC current measurements does not give the correct results.
9. To check your results compare these three values of current:
- the computed dc current
- the computed ac current
- the measured ac current.
10. What is your conclusion about the different types of current through a capacitor?
Exercise 3: DC Characteristics of Inductors
1. Select an inductor in the range of 3 mH to 4 mH (millihenry).
2. Use the B&K LCR meter to measure the resistance and the inductance of the inductor.
3. Question: How much current would pass through this inductor if it were connected in
series with a 5-volt dc source and a 100  resistor?
4. On your breadboard, build a series circuit consisting of a 5-volt dc source, the
inductor and a 100  resistor. Connect the inductor to the negative side of the dc
source. Use the DVM to measure the dc voltage across each of the elements.
5. Compute the current from the resistor voltage measurement. How does the computed
value compare with your predicted value? Compute the percentage difference.
Exercise 4: AC Characteristics of Inductors
1. Exchange the 5 volt dc source with the FG.
2. Select a sine wave with a frequency between 400 Hz and 500 Hz (assuming your
inductance is between 3 and 4 mH).
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3. Adjust the voltage across the source to 5.0 volts rms with the DVM. Measure the
remaining two voltages.
4. Use Ohm's law to compute the rms value of the ac current.
5. Now break the circuit between the resistor and inductor and insert the Tektronix
DMM (set for ac current measurements) in series with the break.
6. Record the rms value of current.
7. To check your results compare these three values of current:
- the computed dc current
- the computed ac current
- the measured ac current.
8. What is your conclusion about the different types of current through an inductor?
Exercise 5: RC Exponential Decay
An exponential decay is found in many every day situations. For example, the
temperature of a pot of hot water will decay when it is removed from the burner of a stove
– the decay is exponential. Here are some other examples. The sound level of a guitar
string decays exponentially after being struck. A charged capacitor discharges
exponentially through a shunt resistor. And finally the temperature in a house drops
exponentially as the thermostat is turned down.
1. Select a 2.2 k resistor and a 0.15 F capacitor, measure their values and then
determine the time constant in milliseconds (ms).
2. Wire the FG in series with these components. Connect the resistor to the negative
side of the FG.
3. Select a square wave and set the FG voltage to 2.5 volts peak to peak as read on the
FG display. Pick the frequency of the FG such that the period of the square wave is at
least ten times the time constant of the circuit.
4. Connect your scoope as follows. Connect probe 1 to display the waveform of the FG.
Connect probe 2 to display the waveform of the resistor voltage (the voltage
waveform has the same waveshape as the current waveform). Adjust the offset
control on the FG so that the FG signal, as seen on the scope, has a minimum value of
zero volts. You can save these signal settings in the FG by pressing the Shift Store
keys and selecting memory location 1, 2 or 3. To retrieve a stored signal setting, press
the Recall key. The FG square wave, as seen on the scope, should be 5 volts peakto-peak.
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5. Display the positive decaying waveform – make the display large. Using the cursors
(press the Cursors key) measure a value of voltage (in volts) for a corresponding
value of time (in ms) of the waveform. Record the pair of values. Do this for 6 or 7
data points.
6. Next display the negative decaying waveform. Repeat the above.
7. Plot the data for the combined waveform – mark off the zero-volt (base) line on your
graph. Plot your FG waveform below this graph.
8. Indicate, on each portion of your graph, where the capacitor is charging and where it
is discharging.
9. Readjust the scope to display the positive decay part of the waveform. Press the
Quick Meas button on the scope and measure the fall time of the waveform. See
Appendix for a discussion of fall time.
10. From the value of Tf determine the value of the time constant τ. Compare this
measured value with the predicted value in step 1.
Appendix: Fall Time
An exponentially decaying signal can be described by a parameter called the time
constant. When making measurements, however, the signal's fall time is easily obtained.
The fall time is based upon spreading the signal amplitude over a 100 % vertical scale.
Then the fall time, Tf, is the amount of time for the signal to decay from the 90 % point
down to the 10 % point. The equation for the fall time is
Tf = τ ln 9 = 2.197 τ
where τ is the time constant.
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