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Transcript
EEE1026 Electronics II:
Experiment Instruction
Experiment EB2: IC Multivibrator Circuits
Learning Outcomes
LO4: Analyze the operation of JFET, MOSFET and BJT amplifiers and switching
circuits.
1.0 Objectives
To measure the frequency and duty cycle of an astable 555 timer
To measure the pulse width and duty cycle of a monostable 555 timer
To measure the frequency and duty cycle of a voltage-controlled oscillator
2.0 Apparatus
Equipment required
Power Supply
Oscilloscope
Multimeter
Breadboard
Function Generator
–1
–1
–1
–1
–1
Components required
Timer IC 555
Resistor 10k (1/4W)
Resistor 100k (1/4W)
Resistor 33k (1/4W)
Resistor 68k (1/4W)
Resistor 47k (1/4W)
Resistor 1k (1/4W)
Mylar Capacitor 0.01F
Potentiometer (1k)
–2
–2
–1
–1
–1
–1
–2
–4
–1
3.0 Introduction
Multivibrators are circuits that are designed to have zero, one, or two stable output states. The
555 timer is one of the most popular general purpose IC multivibrators. It can be used in a
variety of applications requiring accurate time delays, oscillation, and pulse conditioning.
Signetics Corporation first introduced it as SE555 timer, which is an 8-pin IC that can be
connected with external components for either astable or monostable operation. Figure 1
shows the simplified block diagram of a 555 timer. The circuit’s name is derived from the use
of an internal voltage divider between VCC and ground using three 5k resistors. This divider
chain is used to set a pair of reference voltages for two comparators that drive the set and
reset inputs of an R-S flip-flop.
Page: 1
EEE1026 Electronics II:
8
Experiment Instruction
+VCC
7
5k
6
THRESHOLD
CONTROL
5
DISCHARGE
+
_
S
Q
+
_
R
_
Q
5k
2
TRIGGER
5k
1
4
Q1
3
OUTPUT
RESET
GROUND
Figure 1: Block diagram of 555 Timer
Refer to Figure 1, a logic high voltage (+V0) applied to the set S input and a logic low (0V) to
the reset R input forces the output Q to high (VCC) and Q low (0V). This is referred to as the
set condition of the flip-flop. A high reset R and low set S causes the output to switch to a low
Q and a high Q . This is referred to as the reset condition of the flip-flop. The circuit latches
in either of the two states. In other words, a high S input sets Q to high; a high R input resets
Q to low. Output Q remains in a given state until triggered into the opposite state.
The comparators are simply Op-amps. Note that the upper comparator has a threshold input
(pin 6) and a control input (pin 5). In most applications, the control input is not used, so that
the control voltage equals +2VCC/3. However, applying an external voltage to this pin
provides some control over the reference voltages for both comparators. When the voltage of
pin 6 exceeds the control voltage, the high output from the Op-amp will set the flip-flop. The
high Q output from the flip-flop will turn on transistor Q1 and discharge the external timing
capacitor connected to pin 7. The complementary signal (logic low) of the flip-flop goes to
pin 3, the output.
When the external reset (pin 4) is grounded, it inhibits the device. This ON-OFF feature is
useful sometimes. In most applications, however, the external reset is not used and pin 4 is
tied directly to the supply voltage. The inverting input of the lower comparator is called the
trigger (pin 2) and its noninverting input has a fixed voltage of +VCC/3 developed by the three
5k voltage divider. When the trigger input voltage is slightly less than +VCC/3, the Op-amp
output goes high and resets the flip-flop. Lastly, pin 1 is the chip ground, while pin 8 is the
power supply pin. The 555 timer will work with any supply voltage between 4.5 and 16V.
Monostable Operation
Figure 2a shows the 555 timer connected for monostable (one-shot) operation. It produces a
single, fixed voltage, output pulse each time a trigger pulse is applied to pin 2 (Figure 2b).
Page: 2
EEE1026 Electronics II:
Experiment Instruction
+VCC
R
7
8
5k
6
C
+
_
S
Q
+
_
R
_
Q
5k
TRIGGER
2
Vout
3
5k
1
(a)
+VCC
(pin 2)
+2VCC/3
0
+VCC
0
(pin 6 & 7)
0
(pin 3)
(b)
Figure 2: (a) Monostable operation; (b) ideal waveforms
The trigger input is a narrow pulse with a quiescent value of +VCC. When the trigger input is
slightly less than +VCC/3, the lower Op-amp has a high output and resets the flip-flop. This
cuts off the transistor, allowing the capacitor to start charging up. As the capacitor charges,
the voltage at pin 6 increases. Eventually, the voltage becomes slightly greater than the
control voltage (+2VCC/3). The output of the upper Op-amp then goes high, forcing the RS
flip-flop output to be set. As soon as Q goes high, it turns on the transistor and this quickly
discharges the capacitor. As a result, we get a triangular pulse at pin 6 & 7.
The capacitor C is charged through resistance R. For a larger RC time constant, the capacitor
will take longer time to charge to +2VCC/3. In other words, the RC time constant controls the
width of the output pulse. Solving the exponential equation for capacitor voltage gives the
formula for its pulse width as
(1)
W  1.1RC
Astable Operation
Figure 3a shows the 555 timer connected for astable or free-running operation. The output is
a square-wave signal. When Q is low, the transistor is cut off and the capacitor is charged
through ( RA  RB ). Because of this, the charging time constant is ( RA  RB )C . When the
voltage at pin 6 is slightly greater than +2VCC/3, the upper Op-amp has a high output and this
sets the flip-flop. With Q high, it turns on the transistor and grounds pin 7. Now the capacitor
discharges through RB . The discharging time constant is RBC . When the capacitor voltage
drops slightly below +VCC/3, the lower Op-amp has a high output and this resets the flip-flop.
Page: 3
EEE1026 Electronics II:
Experiment Instruction
+VCC
8
RA
7
5k
RB
6
+
_
S
Q
+
_
R
_
Q
5k
C
2
Vout
3
5k
1
(a)
+2VCC/3
(pin 2 & pin6)
+VCC/3
+VCC
(pin 3)
0
W
T
(b)
Figure 3: (a) Astable operation; (b) ideal waveforms
Figure 3b illustrates the waveforms; the timing capacitor has an exponentially rising and
falling voltage and the output of Q is a rectangular wave. Since the charging time constant is
longer than the discharging time constant, the output is not symmetrical; the high state lasts
longer than the low state. To specify how unsymmetrical the output is, we can define duty
cycle as
W
D  100%
(2)
T
Depending on the resistances RA and RB , the duty cycle is between 50 and 100 percent. The
mathematical solutions of the charging and discharging equations give the following
formulas. The output frequency is
f 
1.44
( R A  2 RB )C
(3)
and the duty cycle is
R A  RB
100%
(4)
R A  2 RB
If RA is much smaller than RB , the duty cycle approaches 50 percent.
D
Page: 4
EEE1026 Electronics II:
Experiment Instruction
Voltage-Controlled Oscillator (Pulse Position Modulator)
The free-running multivibrator can be modified to become a voltage-controlled oscillator
(VCO). Recall that pin 5 (control) is connected to the inverting input of the upper Op-amp.
Normally, the control voltage is +2VCC/3 because of the internal voltage divider. In VCO,
however, the voltage from an external potentiometer overrides the internal voltage. In other
words, by adjusting the potentiometer, we can change the control voltage level. If we increase
Vcontrol, the capacitor will take a longer time to charge and discharge; therefore, the frequency
decreases. As a result, we can change the frequency of the circuit by varying the control
voltage.
4.0 Procedures
A. Astable 555 Timer
1. Refer to the 555 timer circuit shown in Figure 4-1. The schematic diagram does not show
the op-amps, flip-flop, and other components inside the 555 timer, but only the pins and
external components.
2. Notice that pin 5 (control) is bypassed to ground through a small capacitor, typically
0.01F. This provides some noise filtering for the control voltage.
3. Based on equations (3) and (4), calculate and record the frequencies (fcal) and duty cycles
(Dcal) for the resistances listed in Table 4-1 in Appendix D2.
4. Connect the circuit of Figure 4-1 on a breadboard with RA = 10k and RB = 10k.
Measure and record the supply voltage VCC(meas) with a multimeter.
5. Using an oscilloscope (set CH1 and CH2 to DC coupling and trigger source to CH1), and
connect the probes at pin 3 (CH1) and pin 6 (CH2), measure the waveforms Vout (at CH1)
and Vpin-6 (at CH2). If the circuit is functioning properly, these waveforms will be similar
to those in Figure 3(b).
6. Align the ground levels of CH1 and CH2 as indicated on Graph 4-1 in Appendix D2.
Adjust Volts/div and Time/div to display the waveforms on the screen as big as possible
with one to two cycles. Sketch Vout and Vpin-6 waveforms on Graph 4-1.
7. Measure and record the period, T and the high portion of the pulse width, W. Determine
the frequency f and duty cycle D from the measurement results.
8. Repeat steps 5 through 7 for the other resistances of Table 4-1.
9. For RA = 100 k and RB = 10 k case, measure and record the maximum and minimum
voltage levels of Vout and Vpin 6 waveforms.
10. Ask the instructor to check all of your results. You must show the last oscilloscope
waveforms to the instructor.
VCC=+5V
RA
RB
C1=0.01F
4
7
6
8
3
555
Timer
2
Vout
5
1
C2=0.01F
1
8
2
7
3
6
4
5
SE/NE555
Figure 4-1: Astable 555 Timer Circuit
(TOP VIEW)
Page: 5
EEE1026 Electronics II:
Experiment Instruction
B. Monostable 555 Timer
*Do not remove the circuit from the previous part. The connection of IC U1 is
similar with Part A.
1. IC U2 of Figure 4-2 is a 555 timer connected for monostable operation. Calculate the
pulse width for each resistance value R listed in Table 4-2. Record the results under Wcal.
2. IC U1 of Figure 4-2 is the astable multivibrator circuit of Part A. It is used here to provide
the trigger input to the monostable circuit (U2).
3. Connect the circuit of Figure 4-2 with resistance R = 33k. Measure and record VCC(meas).
4. Using an oscilloscope, measure the waveforms at pin 2, Vpin2 (at CH1) and pin 3, Vout (at
CH2) of the monostable circuit (U2). Set CH1 and CH2 to DC input coupling and trigger
source to CH1. Align the ground levels of CH1 and CH2 as indicated on Graph 4-4. Set
Time/div to display the waveforms with one to two cycles on the screen. Set Volt/div to
display the waveforms as big as possible but not overlapping. Sketch the waveforms.
5. Measure and record the pulse width, W, at the output of U2.
6. Repeat steps 4 to 5 for the other resistances R in Table 4-2.
7. For R = 68 k case, connect CH2 to pin 6, measure and record the maximum and
minimum voltage levels of pin 6 waveform.
8. Ask the instructor to check all of your results. You must show the last oscilloscope
waveforms to the instructor.
VCC=+5V
4
100k
7
10k
6
4
R
3
U1
555
Timer
2
0.01F
8
7
6
5
1
0.01F
0.01F
8
3
U2
555
Timer
2
Vout
5
0.01F
1
Figure 4-2: Monostable 555 Timer Circuit
Page: 6
EEE1026 Electronics II:
Experiment Instruction
C. Voltage-Controlled Oscillator
1. Connect the voltage-controlled oscillator (VCO) of Figure 4-3 (refer to Appendix A for
the potentiometer legs). Measure and record VCC(meas).
2. Measure the output, Vout (CH1) and pin 6, Vpin 6 (CH2) waveforms with an oscilloscope.
Set CH1 and CH2 to DC input coupling and trigger source to CH1. Align the channel
ground levels as indicated on Graph 4-5. Adjust Volts/div to display the waveforms on
the screen as big as possible.
3. Vary the 1-k potentiometer and notice the changes in the waveforms. Adjust Time/div if
necessary.
4. Turn the potentiometer to get the minimum frequency. Adjust Time/div to display the
waveforms with one to two cycles on the screen. Sketch Vout and Vpin 6 waveforms.
5. Measure and record T, W, Vpin 6 (max) and Vpin 6 (min). Measure and record the DC voltage at
pin 5, Vpin 5 with a multimeter. Calculate the frequency and duty cycle.
6. Turn the potentiometer to get the maximum frequency. Repeat steps 4 and 5.
7. Ask the instructor to check all of your results. You must show the last oscilloscope
waveforms to the instructor.
VCC=+5V
10k
100k
C1=0.01F
4
7
6
8
3
555
Timer
2
5
Vout
Var
1
1k
A
1k
B
1k
Figure 4-3: Voltage-Controlled Oscillator Circuit
Report Submission
You must obtain the signature of the Instructor after completing each section of the experiment.
Submit your report together with the Rubric assessment form to your Lab Instructor, immediately
after your experiment session.
Page: 7
EEE1026 Electronics II:
Experiment Instruction
APPENDIX A
The Resistor color code chart
Capacitance
.abc
ABC
AB x 10C pF
0.abc F
Potentiometer
A Var B
Log Scale
The distance in a decade of the log scale in the figure below is x mm. Since log101 = 0, it is
used as a refernce point (0 mm) in the linear scale. Then, the reading 10 is located at x mm
and the reading 0.1 is located at –x mm. For a reading F, it is located at [1og10(F)]*x mm.
E.g.:
Reading 0.25 is located at [1og10(0.25)]*x mm = -0.602x mm
Reading 2.5 is loacted at [1og10(2.5)]*x mm = 0.398x mm
Reading 25 is located at [1og10(25)]*x mm = 1.398x mm (not shown in the figure)
Reading 250 is located at [1og10(250)]*x mm = 2.398x mm (not shown)
Conversely, a point at z mm location is read as 10 z / x .
E.g.:
-0.3x mm is read as 10(-0.3x/x) = 0.501
0.6x mm is read as 10(0.6x/x) = 3.98
1.5x mm is read as 10(1.5x/x) = 31.6 (not shown)
2.7x mm is read as 10(2.7x/x) = 501 (not shown)
-x
-0.602x
-0.3x
0
0.398x0.6x
x
Linear scale
(mm)
0.1
2 3 4 5 6 7 8910
0.2 0.3 0.5
1
0.4 0.6
0.7 0.9
0.25
2.5 3.98
0.501 0.8
Log scale
(unit)
Page: 8
EEE1026 Electronics II
Appendix B: Breadboard Internal Connections
Internal
connections
Horizontally connected
Horizontally connected
+VCC
0.1 F
Vertically
connected
8 7 6 5
555
Vertically
connected
1 2 3 4
0V
GND
Internal
connections
General mistakes:
The legs of the resistors and the transistor are shorted
by the breadboard internal connections.
EEE1026 Electronics II
Appendix D2
Experiment EB2: IC Multivibrator Circuits
Lab Report
(Together with Rubric form, Submit your report on the same day immediately after the
experiment)
Name: ________________________ Student I.D.: _______________ Date: __________
Majoring: ____________________
Group: ____________
Table No.: ____________
4. Astable 555 Timer
VCC(meas) = _________V
Table 4-1: Astable Operation for various RA and RB
RA (k) RB (k)
f cal
Dcal
T
W
f
D
10
10
10
100
100
10
For RA = 100 k, RB = 10 k case (Step 9):
Vout (max) = ______ V
VCC(meas) – Vout (max) = ______ V
Vout (min) = ______ V
Vpin 6 (max) = ______ V
Vpin 6 (max) / VCC(meas) = ______
Vpin 6 (min) = ______ V
Vpin 6 (min) / VCC(meas) = ______
Graph 4-1: Astable Operation for RA = 10 k, RB = 10 k
Time base : ______ s/div, CH1 (Vout) : ______ V/div, CH2 (Vpin 6) : ______ V/div
CH1 & CH2
ground
EEN1026 Electronics II
Experiment EB2
Graph 4-2: Astable Operation for RA = 10 k, RB = 100 k
Time base : ______ s/div, CH1 (Vout) : ______ V/div, CH2 (Vpin 6) : ______ V/div
CH1 & CH2
ground
Graph 4-3: Astable Operation for RA = 100 k, RB = 10 k
Time base : ______ s/div, CH1 (Vout) : ______ V/div, CH2 (Vpin 6) : ______ V/div
CH1 & CH2
ground
* Note: Ask your instructor to verify your results before you proceed to Part B.
Signature: ______________ Time: ___________ Remarks _________________
Page: 2
EEN1026 Electronics II
Experiment EB2
B. Monostable 555 Timer
VCC(meas) = _________V
Table 4-2: Monostable Operation for various R
R (k)
33
47
68
Wcal
W
For R = 68 k case (Step 7):
Vpin 6 (max) = ______ V
Vpin 6 (max) / VCC(meas) = ______
Vpin 6 (min) = ______ V
Graph 4-4: Monostable Operation for R = 33 k
Time base : ______ s/div, CH1 (Vpin 2) : ______ V/div, CH2 (Vout) : ______ V/div
CH2 ground
CH1 ground
* Note: Ask your instructor to verify your results before you proceed to Part C.
Signature: ______________ Time: ___________ Remarks _________________
Page: 3
EEN1026 Electronics II
Experiment EB2
C. Voltage-Controlled Oscillator
VCC(meas) = _________V
Graph 4-5: Voltage-Controlled Oscillator at minimum frequency
Time base : ______ s/div, CH1 (Vout) : ______ V/div, CH2 (Vpin 6) : ______ V/div
T
= ________s
W
= ________s
Vpin 6 (max) = ________V
Vpin 6 (min) = ________V
Vpin 5
= ________V
f
= ________Hz
D
= ________%
CH1 & CH2
ground
Voltage-Controlled Oscillator at maximum frequency
T
= ________s
W
= ________s
Vpin 6 (max) = ________V
Vpin 6 (min) = ________V
Vpin 5
= ________V
f
= ________Hz
D
= ________%
* Note: Ask your instructor to verify your results.
Signature: ______________ Time: ___________ Remarks _________________
Page: 4
EEN1026 Electronics II
Experiment EB2
Discussion
A. Astable 555 Timer
1. Explain the difference between the calculated fcal and the measured f.
________________________________________________________________________
________________________________________________________________________
2. Compare the calculated Dcal to the measured D, and justify their difference.
________________________________________________________________________
________________________________________________________________________
3. Identify how the voltages Vout and Vpin 6 are related in the three graphs.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
4. Compare between voltages VCC(meas) and Vout (max) and explain their difference.
________________________________________________________________________
5. Evaluate how WL, f and D changes when RA and/or RB are varied. Propose the expected
minimum and maximum duty cycle values.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
B. Monostable 555 Timer
1. Identify how the voltages Vout and Vpin 3 are related in the three graphs.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
2. Describe how W changes when R is varied.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Page: 5
EEN1026 Electronics II
Experiment EB2
C. Voltage-Controlled Oscillator
1. With the help of Figure 1 and Figure 3, compare and evaluate the relationships between
voltages Vpin 6 (max) and Vpin 5, as well as Vpin 6 (min) and Vpin 5. Include numerical calculations
in your answer.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
2. Describe how W, f and D changes when the value of Vpin 5 is varied.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
3. What is the voltage at pin 6 before the output of a monostable 555 timer changes from
ON state to OFF state? Why?
________________________________________________________________________
________________________________________________________________________
4. Why does the trigger pin 2 is pulled low only for a short period of time? How long can
pin 2 be maintained at low? What will happen if pin 2 is kept low indefinitely?
________________________________________________________________________
________________________________________________________________________
Conclusion
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
Page: 6
EEE1026 Electronics II
Experiment EB2
STUDENT'S NAME:
ID NO:
SUBJECT CODE AND TITLE: EEE1026 ELECTRONICS 2
EXPERIMENT TITLE: EB2 – IC Multivibrator Circuits
EXPERIMENT DATE:
Criteria
1
2
3
Ability in constructing the IC
Multivibrator Circuits:
Monostable, Astable and
Voltage-Controlled
Oscillator
Ability in performing data
collection using lab
equipment such as DC power
supply, digital multimeter,
oscilloscope and function
generator
Ability to determine and
draw the time domain
waveform for input and
output voltages of the
amplifier circuits
TIME:
1 (Need Improvement)
2 (Satisfactory)
3 (Good)
4 (Excellent)
Constructing the circuits and performing data collection using lab equipment
Unable to demonstrate
Able to demonstrate basic steps
Able to demonstrate good
Able to demonstrate full
proper steps to construct
required to construct the IC
knowledge on the
knowledge on the
the IC Multivibrator
Multivibrator Circuits with some
construction of the IC
construction of the IC
Circuits and not asking
help
Multivibrator Circuits with
Multivibrator Circuits
for help
minimum help
without help
Unable to record data,
Able to provide adequate
Able to record most of the
Recorded all data neatly
and no effort is shown
data, and show some efforts in
data correctly
and correctly
getting the data
Not able to determine,
and draw the time
domain waveform for
input and output voltages
(no effort was also
shown)
4
The ability to present results
and summarise final
outcomes which answers the
objectives of the lab
Unable to present results
clearly and no attempt
was made to summarise
final outcomes
5
Ability to answer the
questions in by Oral
Assessment
Not able to answer the
question, no attempt was
made to answer
Able to at least determine and draw
the time domain waveform for
input and output voltages and show
some efforts in the calculation
Conclusions
Able to present results and
summarise adequate final
outcomes and reasonably relating
them to the objectives
Able to answer questions with
some basics answers and
demonstrate some attempts to
refer to the text books, notes, lab
sheet
Able to determine and draw
the time domain waveform
for input and output
voltagesand solve the
calculations partiall
Completed all the drawing
and calculations correctly
Able to present results and
summarise mostly to the
final outcomes and answered
most of the objectives of the
lab
Able to answer most part of
the questions, with some
explanations and
elaborations and demonstrate
some attempts to refer to text
books, notes or lab sheet
Able to present results
very clearly and excellent
summary of final
outcomes which answer
the objectives of the lab
Answered all correctly
with proper explanations
and elaborations, without a
need to refer to any
references.
Rating Awarded by
Assessor
Page: 7
EEE1026 Electronics II
Experiment EB2
Page: 8