Questions
Question 1
If the only instrument you had in your possession to detect AC voltage signals was an audio speaker,
how could you use it to determine which of two AC voltage waveforms has the greatest period?
FUNCTION GENERATOR
Hz
1
10 100 1k 10k 100k 1M
Speaker
coarse
fine
DC
output
FUNCTION GENERATOR
Hz
1
coarse
fine
10 100 1k 10k 100k 1M
DC
output
file 00387
Question 2
What is the difference between DC and AC electricity? Identify some common sources of each type of
electricity.
file 00028
Question 3
All other factors being equal, which possesses a greater potential for inducing harmful electric shock,
DC electricity or AC electricity at a frequency of 60 Hertz? Be sure to back up your answer with research
data!
file 03289
Question 4
Suppose a DC power source with a voltage of 50 volts is connected to a 10 Ω load. How much power
will this load dissipate?
Now suppose the same 10 Ω load is connected to a sinusoidal AC power source with a peak voltage of 50
volts. Will the load dissipate the same amount of power, more power, or less power? Explain your answer.
file 00401
1
Question 5
Is it appropriate to assign a phasor angle to a single AC voltage, all by itself in a circuit?
100 Ω
24 VAC
What if there is more than one AC voltage source in a circuit?
100 Ω
24 VAC
15 VAC
file 00496
Question 6
If the power waveform is plotted for a resistive AC circuit, it will look like this:
e
i
p
e
i
Time
What is the significance of the power value always being positive (above the zero line) and never negative
(below the zero line)?
file 02174
2
Question 7
If a sinusoidal voltage is applied to an impedance with a phase angle of 90o, the resulting voltage and
current waveforms will look like this:
e
i
e
i
Time
Given that power is the product of voltage and current (p = ie), plot the waveform for power in this
circuit. Also, explain how the mnemonic phrase ”ELI the ICE man” applies to these waveforms.
file 00632
Question 8
If a sinusoidal voltage is applied to an impedance with a phase angle of -90o, the resulting voltage and
current waveforms will look like this:
e
i
e
i
Time
Given that power is the product of voltage and current (p = ie), plot the waveform for power in this
circuit. Also, explain how the mnemonic phrase ”ELI the ICE man” applies to these waveforms.
file 00633
3
Question 9
Suppose that a variable-voltage AC source is adjusted until it dissipates the exact same amount of power
in a standard load resistance as a DC voltage source with an output of 120 volts:
Standard load
resistances (equal number of ohms)
120 V
In this condition of equal power dissipation, how much voltage is the AC power supply outputting? Be
as specific as you can in your answer.
file 00402
Question 10
Determine the RMS amplitude of this sinusoidal waveform, as displayed by an oscilloscope with a vertical
sensitivity of 0.2 volts per division:
file 01818
4
Question 11
Solve for all voltages and currents in this series RC circuit, and also calculate the phase angle of the
total impedance:
220n
3k3
48 V peak
30 Hz
file 01849
5
Question 12
A student measures voltage drops in an AC circuit using three voltmeters and arrives at the following
measurements:
V
V
A
A
V
A
OFF
V
A
OFF
A
A
COM
COM
V
A
V
A
OFF
A
COM
Upon viewing these measurements, the student becomes very perplexed. Aren’t voltage drops supposed
to add in series, just as in DC circuits? Why, then, is the total voltage in this circuit only 10.8 volts and not
15.74 volts? How is it possible for the total voltage in an AC circuit to be substantially less than the simple
sum of the components’ voltage drops?
Another student, trying to be helpful, suggests that the answer to this question might have something
to do with RMS versus peak measurements. A third student disagrees, proposing instead that at least one
of the meters is badly out of calibration and thus not reading correctly.
When you are asked for your thoughts on this problem, you realize that neither of the answers proposed
thus far are correct. Explain the real reason for the ”discrepancy” in voltage measurements, and also explain
how you could experimentally disprove the other answers (RMS vs. peak, and bad calibration).
file 01566
6
Question 13
A parallel RC circuit has 10 S of susceptance (B ). How much conductance (G ) is necessary to give
the circuit a (total) phase angle of 22 degrees?
22o
B = 10 µS
G = ???
B = 10 µS
G = ???
file 02090
Question 14
A parallel AC circuit draws 8 amps of current through a purely resistive branch and 14 amps of current
through a purely inductive branch:
IR = 8 A
θ
IL = 14 A
8A
Itotal = ???
14 A
Calculate the total current and the angle Θ of the total current, explaining your trigonometric method(s)
of solution.
file 02089
Question 15
A parallel AC circuit draws 100 mA of current through a purely resistive branch and 85 mA of current
through a purely capacitive branch:
Itotal = ???
IC = 85 mA
IR = 100 mA
θ
IC = 85 mA
IR = 100 mA
Calculate the total current and the angle Θ of the total current, explaining your trigonometric method(s)
of solution.
file 02091
7
Question 16
A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage
is supposed to be 15 volts DC:
Fuse
TP2
TP3
TP4
TP1
TP5
TP8
TP6
TP7
-
DC voltage output
TP10
TP9
The technician begins making voltage measurements between some of the test points (TP) on the circuit
board. What follows is a sequential record of his measurements:
1.
2.
3.
4.
5.
6.
7.
VT P 9
VT P 8
VT P 8
VT P 6
VT P 4
VT P 1
VT P 1
= 0 volts DC
= 0 volts DC
= 0 volts DC
= 0 volts DC
= 0 volts AC
= 0 volts AC
= 116 volts AC
T P 10
TP7
TP5
TP7
TP5
TP3
TP2
Based on these measurements, what do you suspect has failed in this supply circuit? Explain your
answer. Also, critique this technician’s troubleshooting technique and make your own suggestions for a more
efficient pattern of steps.
file 00795
8
Question 17
A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage
is supposed to be 15 volts DC:
Fuse
TP2
TP3
TP4
TP1
TP5
TP8
TP6
TP7
-
DC voltage output
TP10
TP9
The technician begins making voltage measurements between some of the test points (TP) on the circuit
board. What follows is a sequential record of her measurements:
1.
2.
3.
4.
5.
6.
VT P 1
VT P 3
VT P 1
VT P 4
VT P 7
VT P 9
= 118 volts AC
T P 2 = 0 volts AC
T P 3 = 118 volts AC
T P 5 = 0.5 volts AC
T P 8 = 1.1 volts DC
T P 10 = 1.1 volts DC
TP2
Based on these measurements, what do you suspect has failed in this supply circuit? Explain your
answer. Also, critique this technician’s troubleshooting technique and make your own suggestions for a more
efficient pattern of steps.
file 00796
9
Question 18
A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage
is supposed to be 15 volts DC:
Fuse
TP2
TP3
TP4
TP1
TP5
TP8
TP6
TP7
-
DC voltage output
TP10
TP9
The technician begins making voltage measurements between some of the test points (TP) on the circuit
board. What follows is a sequential record of his measurements:
1.
2.
3.
4.
5.
6.
7.
VT P 9
VT P 1
VT P 1
VT P 5
VT P 7
VT P 5
VT P 7
= 0 volts DC
= 117 volts AC
= 117 volts AC
= 0 volts AC
= 0.1 volts DC
= 12 volts AC
= 0 volts DC
T P 10
TP2
TP3
TP6
TP8
TP4
TP6
Based on these measurements, what do you suspect has failed in this supply circuit? Explain your
answer. Also, critique this technician’s troubleshooting technique and make your own suggestions for a more
efficient pattern of steps.
file 00797
10
Question 19
A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage
is supposed to be 15 volts DC, but instead it is actually outputting nothing at all (zero volts):
Fuse
TP2
TP3
T1
TP4
TP1
TP5
TP8
D1
D2
TP6
D3
TP7
D4
-
DC voltage output
TP10
TP9
C1
The technician measures 120 volts AC between test points TP1 and TP3. Based on this voltage
measurement and the knowledge that there is zero DC output voltage, identify two possible faults that
could account for the problem and all measured values in this circuit, and also identify two circuit elements
that could not possibly be to blame (i.e. two things that you know must be functioning properly, no
matter what else may be faulted). The circuit elements you identify as either possibly faulted or properly
functioning can be wires, traces, and connections as well as components. Be as specific as you can in your
answers, identifying both the circuit element and the type of fault.
Circuit elements that are possibly faulted
1.
2.
Circuit elements that must be functioning properly
1.
2.
file 02306
11
Question 20
Suppose this power supply circuit was working fine for several years, then one day failed to output any
DC voltage at all:
Fuse
T1
D1
D2
D3
D4
115 VAC
RMS
C1
When you open the case of this power supply, you immediately notice the strong odor of burnt
components. From this information, determine some likely component faults and explain your reasoning.
file 03708
12
Answers
Answer 1
Connecting the speaker to each AC voltage source, one at a time, will result in two different audio tones
output by the speaker. Whichever tone is lower in pitch is the waveform with the greatest period.
Answer 2
DC is an acronym meaning Direct Current: that is, electrical current that moves in one direction only.
AC is an acronym meaning Alternating Current: that is, electrical current that periodically reverses direction
(”alternates”).
Electrochemical batteries generate DC, as do solar cells. Microphones generate AC when sensing sound
waves (vibrations of air molecules). There are many, many other sources of DC and AC electricity than
what I have mentioned here!
Answer 3
From a perspective of inducing electric shock, AC has been experimentally proven to possess greater
hazard than DC (all other factors being equal). See the research of Charles Dalziel for supporting data.
Answer 4
50 volts DC applied to a 10 Ω load will dissipate 250 watts of power. 50 volts (peak, sinusoidal) AC
will deliver less than 250 watts to the same load.
Answer 5
Phasor angles are relative, not absolute. They have meaning only where there is another phasor to
compare against.
Angles may be associated with multiple AC voltage sources in the same circuit, but only if those voltages
are all at the same frequency.
Answer 6
Positive power represents energy flowing from the source to the (resistive) load, in this case meaning
that energy never returns from the load back to the source.
Answer 7
e
i
e
p
i
Time
The mnemonic phrase, ”ELI the ICE man” indicates that this phase shift is due to an inductance rather
than a capacitance.
13
Answer 8
e
i
e
p
i
Time
The mnemonic phrase, ”ELI the ICE man” indicates that this phase shift is due to a capacitance rather
than an inductance.
Answer 9
120 volts AC RMS, by definition.
Answer 10
The RMS amplitude of this waveform is approximately 0.32 volts.
Answer 11
VC = 47:56 volts peak
VR = 6:508 volts peak
I = 1:972 milliamps peak
ΘZ = 82:21 o
Follow-up question: what would we have to do to get these answers in units RMS instead of units
”peak”?
Answer 12
AC voltages still add in series, but phase must also be accounted for when doing so. Unfortunately,
multimeters provide no indication of phase whatsoever, and thus do not provide us with all the information
we need. (Note: just by looking at this circuit’s components, though, you should still be able to calculate
the correct result for total voltage and validate the measurements.)
I’ll let you determine how to disprove the two incorrect explanations offered by the other students!
Challenge question: calculate a set of possible values for the capacitor and resistor that would generate
these same voltage drops in a real circuit. Hint: you must also decide on a value of frequency for the power
source.
Answer 13
G = 24.75 S
Follow-up question: how much resistance is this, in ohms?
14
Answer 14
Itotal = 16.12 amps
Θ = 60.26o (negative, if you wish to represent the angle according to the standard coordinate system
for phasors).
Follow-up question: in calculating Θ, it is recommended to use the arctangent function instead of either
the arcsine or arc-cosine functions. The reason for doing this is accuracy: less possibility of compounded error,
due to either rounding and/or calculator-related (keystroke) errors. Explain why the use of the arctangent
function to calculate Θ incurs less chance of error than either of the other two arcfunctions.
Answer 15
Itotal = 131.2 mA
Θ = 40.36o
Follow-up question: in calculating Θ, it is recommended to use the arctangent function instead of either
the arcsine or arc-cosine functions. The reason for doing this is accuracy: less possibility of compounded error,
due to either rounding and/or calculator-related (keystroke) errors. Explain why the use of the arctangent
function to calculate Θ incurs less chance of error than either of the other two arcfunctions.
Answer 16
The fuse is blown open.
Follow-up question: with regard to the troubleshooting technique, this technician seems to have started
from one end of the circuit and moved incrementally toward the other, checking voltage at almost every
point in between. Can you think of a more efficient strategy than to start at one end and work slowly toward
the other?
Answer 17
The transformer has an open winding.
Follow-up question #1: with regard to the troubleshooting technique, this technician seems to have
started from one end of the circuit and moved incrementally toward the other, checking voltage at almost
every point in between. Can you think of a more efficient strategy than to start at one end and work slowly
toward the other?
Challenge question: based on the voltage measurements taken, which do you think is the more likely
failure, an open primary winding or an open secondary winding?
Follow-up question #2: how could you test the two windings of the transformer for a possible open
fault? In other words, is there another type of measurement that could verify our hypothesis of a failed
winding?
Answer 18
There is an ”open” fault between TP4 and TP6.
Follow-up question: with regard to the troubleshooting technique, this technician seems to have started
from one end of the circuit and moved incrementally toward the other, checking voltage at almost every
point in between. Can you think of a more efficient strategy than to start at one end and work slowly toward
the other?
Answer 19
I’ll let you and your classmates figure out some possibilities here!
15
Answer 20
Shorted capacitor, open transformer winding (as a result of overloading), shorted diode(s) resulting in
blown fuse.
16