ELECTRICAL ENGINEERING DEPARTMENT
California Polytechnic State University
EE 361 – 06
Digital Pulse Width Modulator Controlled DC Motor
Lab 8
Objective
Design, build, and test a Digital Pulse Width Modulator Controlled DC Motor circuit.
Procedure 1: Digital PWM DC Motor Controller pt 1
Built: JZ Verified: CB 1st Time: Y
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Use the circuit shown in Figure 1.1.
Input clock as a square wave with 1KHz, 5V p-p and an offset of + 5V
Before enabling 10 V power supply, capture Vin and Vo on oscilloscope at DIP settings,
1, 4, A, E
Enable the DC power supply last and record Vin and Vo output and a picture of LEDs for
each DIP setting
Figure 1.1 Circuit
Figure 1.2 Vo and Vin Output on Oscilloscope Without 10V Power Supply.
DIP order: 1, 4, A, E (left to right, up to down)
Figure 1.4 LED and Breadboard Pictures. DIP order: 1, 4, A, E (left to right, up to down)
Figure 1.3 Vo and Vin Output on Oscilloscope Complete Circuit.
DIP order: 1, 4, A, E (left to right, up to down)
Takeaways
Like before, a clock input is used to create a 16-bit counter. However, unlike another lab, the
outputs are fed into a digital comparator rather than an op-amp comparator. The comparator uses
boolean logic and the four switches to output signals of varying strength tied to the 0-15 numbers
the asynchronous counter generates. LED are also used for a visual display of the count. As the
strength of the output voltage creates a square wave with a duty cycle ranging from 0% at count
0 and 100% at count 15. This square wave drives the motor by increasing or decreasing current
by use of a transistor. The benefit of this system is that a specific duty cycle/motor speed can be
easily repeated from an earlier experiment. However, because the system is digital, only 16
speeds are available rather than an infinite amount in an analog system.
Procedure 2:
Digital PWM DC Motor Controller pt 2
Built: CB Verified: JZ 1st Time: Y
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Take your previous circuit and insert a MOSFET in place of the existing BJT
Test the output power limit by increasing the 10V supply to 20V in one volt increments,
record fan speed and relative temperature of MOSFET
At max voltage capture Vin and Vo on oscilloscope
Figure 2.1 MOSFET Implementation
Figure 2.2 Vo and Vin Output on Oscilloscope MOSFET
Takeaways
The addition of the MOSFET made little difference to the circuit as a whole. Even when the 10volt DC power supply was increased, the only change was the motor speed. Under higher
voltages and currents than the BJT experienced, the MOSFET had no temperature changes and
continued to operate normally showing one of its advantages over the other transistor.
Discussion
1. Did motor turn(spin) at setting 1 hex? Why or why not? What is the lowest hex
value that motor began spinning? How did motor behave at setting “E” hex?
Our motor only began spinning at 12.5% duty cycle. Because the duty cycle of the square wave
increased the current flowing through the motor, at low duty cycles the motor didn't spin. The
lowest hex we could get the motor to spin was 0. At E the motor was at its second fastest setting
only slower than F.
2. Do you see a benefit of a digital motor control over the analog (POT & comparator)
motor control used in previous lab 5 experiment? Be specific, show examples.
Analog systems benefit from being able to access an infinite amount of motor speeds. Because
the analog systems relied on a variable resistor, potentiometer, the variety of motor speed is only
capped by the sensitivity of the resistor. For example, if a 50kΩ resistor can be increased by a
nob by 1Ω each, it gives the motor thousands of possible speeds. This precision comes at the cost
of decreased repeatability as it would be extremely difficult to return to the exact same speed
consistently. A digital system like today's lab counters many of these shortcomings. The digital
system divides the 100% mad duty cycle into 16 equal parts because of the 16-bit counter giving
each count a 6.25% duty cycle increase. This allows the system to easily select 50%, 18.75% or
87.5% duty cycle consistently and repeatedly. However, unlike the analog system, this trade off
includes the digital system losing precision as under a 16-bit counter duty cycles like 3%, 95% or
25.98% are impossible to generate.
3. Did DC motor load effect observed results? Any anomalies? Why or why not?
The motor through off the duty cycle of the whole circuit because of the noise it created. The
Vout vs Vin scope with the motor running was inconsistent and you couldn’t determine a specific
duty cycle because the Vout was all over the place. This is because the circulating current
produces a magnetic force which in return made a lot of noise. The noise increased as the speed
of the motor increased.
4. On Procedure 2 what was the effect of replacing the BJT (2N2222) with the
MOSFET? What are the advantages or limitations? What is the effect of having a
low Rds on? How does FET consumed power compare to power through the BJT?
In replacing the mosfet with the BJT, the motor was off when the dip switch was 0. Each step up,
or binary addition, the motor would be more responsive than the bjt. Limitations of using the
mosfet would be it would turn off when the dip switch is off, if you needed an application that it
needed to be running the whole time. Also because the mosfet is so sensitive, it was harder to get
intermediate speeds that the bjt provided because the bjt increased the speed ever so slightly. The
mosfet is much more efficient than the bjt, when using the mosfet, the motor was able to run at
much higher top speeds. In have a low Rds on, the mosfet had very little power loss across it,
given that it didn’t heat up as we increased the voltage in the motor. The BJT got really hot when
we increased the voltage of the motor, proving that the bjt consumes a lot more power than the
mosfet.
Conclusion:
In this lab we experimented with Pulse Width Modulators (PWM’s) by using a sequence of a
counter and a comparator. We learned the comparator compares two binary inputs from two
different voltage sources. There are binary inputs A and they are compared to binary inputs B. If
the binary inut B is larger than A, there is an output from the comparator. Through this, we were
able to control the voltage of the output circuit. In our case, our comparator swung from 5 volts
to zero volts by the controlling of a dip switch, which controlled out B input. By changing the
dip switch into certain binary and hex values, we were able to change the duty cycle. Since we
were using a 16-bit counter, the duty cycle changed by 6.25 for each addition to our binary
number. We analyzed our circuits at 1, 4, A, E in binary. A and E are hex from 10 and 14
respectively. After Analyzing that our duty cycle was changing correctly by each binary value
change, we added the motor portion of the circuit with a BJT to amplify the current. When 10V
was applied to this end of the circuit, the motor initially turned on, even when the dip switch was
all zeros. As we increased the value of the dip switch, the duty cycle increased and the motor
spun faster, but not by a huge margin. After analyzing the motor at 1, 4, A and, E again, we
switched the BJT out for a Mosfet. With the mosfet in, we didn’t see the motor spin as the dip
switch read 0. The motor then turned on when we switched the dip switch to 1. With the mosfet,
the motor gained speed more quickly through each step up on the switch. This told us that the
mosfet was more sensitive to the voltage than the bjt. The mosfet also never heated up, even
when we increased the voltage to 20V. It is an effective transistor, especially compared to the
BJT, who heated up quickly. We went slow to go fast in this lab, ensuring that our circuit was
correct in the first go, we were able to complete everything in an hour and a half which has been
unlike us the past few labs. Our main takeaways from this lab included how the dip switch
worked like a potentiometer paired with a comparator and a counter. We could also use code to
operate our circuit instead of the dip switch which was intriguing to us.
Accreditation:
Jack Zilligen: DQ: 3,4, Conclusion
Colin Brace: Procedure 1,2 DQ 1,2