Design and Simulation of a Variable DC Power Supply for a Motor

This project details the design, simulation, and prototyping of a variable voltage DC power supply for controlling the speed of a permanent magnet DC motor. The project explores two design techniques: MOSFET and BJT based power supplies. A full-wave bridge rectifier with a smoothing capacitor converts low voltage AC to DC, producing an unregulated 12V output. This is then regulated using either an IC regulator or a feedback-stabilized voltage regulator to achieve a variable output ranging from 5 to 10 volts. The project includes circuit design, component selection, and simulations to verify the performance of each power supply design. The speed of the DC motor is then controlled using the variable output voltage. The report includes circuit diagrams, simulation results, component choices, and a comparison of the two design approaches, along with an evaluation of the results and a final conclusion.

Summary
The objectives of the project were to design and simulate a variable output power supply. The
variable output was then used to control the speed of a constant load permanent magnet DC
motor. The variable output power supply was accomplished using two design techniques;
MOSFET and BJT based power supply. A capacitor smoothed full bridge rectifier circuit that
converts low voltage AC to DC was designed to produce an unregulated voltage of 12 volts. The
unregulated output voltage from the rectifier was then regulated using either using an IC
regulator circuit or feedback stabilized voltage regulator. Both feedback stabilized voltage
regulators, and the IC regulator was designed to produce a variable, regulated output voltage
ranging from 5 to 10 volts. The circuit was tested to ascertain the voltage range at the output of
each power supply technique. The MOSFET based power supply produced a wide range of
output voltage, but our interest was 5-10 volts. The variable output was then used to power the
DC motor. The speed of a motor increases as the input voltages are increased.
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List of contents
1.0 Introduction…………………………………………...……………………………………….4
2.0 Circuit design…………………………………………...……………………………………..5
2.1 Rectifier design………………………………...…………………………….………..5
2.2 Smoothing capacitor design………………..………………………………………….6
2.3 Voltage regulation……………………………………………….…………………….7
2.3.1 Feedback stabilized variable regulator………………...………….……….7
2.3.2 Integrated circuit voltage regulator…………………..…………………..10
3.0 Circuit simulations…………………………………………………..……………………….12
3.1 Rectifier simulation……………………………………….………………………….12
3.2 Regulator simulation…………………………………………………………………14
3.2.1 IC regulator circuit……………………………...………………………..14
3.2.2 Screenshots of IC regulator voltage readings……..……………………..15
3.2.3 Feedback stabilized variable voltage regulator…………………..………16
3.2.4 Screenshots of feedback stabilized regulator measurements….…………17
3.3 MOSFET based power supply……………………………………………………….19
3.3.1 Screenshots of MOSFET based power supply readings…...…………….20
3.4 BJT based power supply ………………………………………………...…………..22
3.4.1 Screenshot of BJT based power supply multimeter readings……..……..23
4.0 Choice of components…………………………………………………………………..……25
4.1 Choosing the resistors……………………………………………………….……….25
4.2 Choosing the capacitors…………………………………………………….………..25
4.3 Choosing diodes and BJT’s……………………………………………...…………..25
5.0 Full system test…………………………………………………………………..…………..27
5.1 screenshot of speed measurements…………………………………………………..27
5.2 Comparison of the two designs………………………………………………………28
6.0 Evaluation of results…………………………………………………………………………29
7.0 Conclusion……………………………………………………...……………………………29
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1.0 Introduction
The project aims to design a variable voltage direct current power supply that will be used to
vary the speed of a motor. Two design approaches are used to achieve variable power supply;
BJT and MOSFET based power supply. BJT based power supply is formed by connecting a full-
wave bridge rectifier smoothen by a capacitor to feedback stabilized voltage regulator. MOSFET
based power supply is formed by connecting a rectifier with an IC regulator circuit.
Personal safety, injury and accident prevention, and compliance with environmental and health
laws and regulations are key to success in every project. It is a student’s responsibility to keep
themselves informed of the conditions affecting their safety and health. It is also their
responsibility to adhere to health and safety practices in the classroom and the laboratory. Each
individual in a working area is expected to perform all work safely to promote good health and
safety practices. Other practices that promote a safe workplace are hazard identification and
correction, the shutdown of dangerous activities, and proper plans of emergency response.
Figure 1.1 below shows feedback stabilized voltage regulator. The voltage regulator uses a
principle of negative feedback to keep the voltage at its output constant despite the variation in
its input.. Transistor Q1 acts as a control element, resistor R4, and Zener diode forms the
reference element. The voltage divider is comprising of resistors R1 and R2. The voltage divider
feeds back the output voltage to the base of transistor Q2. This feedback voltage controls the
flow of current in the collector keeping the voltage constant[1].
Figure 1.1: feedback stabilized voltage regulator. Source [1]
The adjustable voltage regulator can be achieved by using an integrated circuit regulator. The
integrated circuit regulator has three terminals, input and output voltage, and adjustable
terminals. Figure 1.2 shows an IC voltage regulator. The output voltage can be adjusted using
equation 1.1
Vout = Vref * (1 + R 2
R 1 )……………………………………….1.1
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The LM317H develops a 1.25V voltage reference between its output and adjustment terminal.
Placing resistor R1 between the output and adjustment terminal causes a constant current to flow
through resistor R1 and down to the ground through R2 to set the overall output voltage [2].
Figure 1.2 IC voltage regulator
2.0 Circuit design
The power supply circuit consists of four main parts: a transformer, rectifier, a smoothing
capacitor, and a series regulator. The figure 2.1 shows parts of a DC power supply
Figure 2.1 power supply circuit. Source: [2]
The transformer converts 230V AC from the mains to an alternating current of about 12V,
depending on its ratings. The rectifier circuit employs four diodes, to transform the alternating
current to direct current. Smoothing capacitor reduces the quantity of alternating current ripple
on direct current voltage[3]. The regulator can either be BJT or MOSFET based; the two
techniques will be analyzed in detail.
2.1 Rectifier design
Rectification of alternating current is achieved using a full-wave rectifier. A full-wave rectifier
accepts single phase input voltage. The bridge is using four diodes connected together in a
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closed-loop as shown in figure 2.2. The four diodes are arranged in series pairs with a pair
conducting during each cycle[4]. The diodes’ arrangement is as shown in figure 2.2
Figure 2.2 Full wave bridge rectifier circuit
Practically, a single rectifier diode develops a voltage of about 0.7V during its forward
conduction. Therefore, a voltage drop of about 2V is allowed for the full-wave bridge. So, if the
output voltage of a rectifier is to be 12 V,
Input to the rectifier = output voltage of a rectifier + voltage drop across the rectifier
= 12 + 2
= 14 Volts
RMS value of input voltage to the rectifier would be 14 V
But in the case of simulation, the rectifier is ideal, therefore no voltage across it.
2.2 Smoothing capacitor design
The output from the rectifier is a dc voltage containing an unwanted AC component. The AC
component is reduced by adding a capacitor. The effect of this capacitor is to increase the
average output voltage. The capacitor also provides current when the output voltage drops[5].
Relationship between the ripple voltage and the capacitor value is given by the equation 2.1
below
Vp-p ripple = I
Four∗f ∗C ……………..2.1
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Where I am the load current, f is the frequency of AC signal
For our design, we require a peak to peak ripple voltage of less than 1 V, for a load current of
2A.
Making C the subject of the formula in equation 2.1, we get
C = I
Four∗f ∗Vp− p
= 2
4∗60∗1
= 8.33mF
The standard capacitor value around the value is 10mF
Figure 2.3: rectifier circuit connected to a smoothing capacitor
2.3 Voltage regulation
2.3.1 Feedback stabilized variable voltage regulator
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Figure 2.4 BJT based voltage regulator.
Resistor values for the regulator are calculated below
RZ = VRZ / IZ, assuming a current of 2mA
= (12 – 5) V / 2mA
= 3.5 kΩ
The standard available resistor, is a 3.7kΩ resistor
Voltage at the input of a regulator, Vin = 12 Volts
Voltage at the base of pass transistor, Q1, Vb = Vout + Vbe1…………………2.3
= 9 + 0.6 = 9.6
Voltage drop across R3, VR3 = Vin – Vb………………………………………...2.4
= 12 – 9.6
Value of R3 = VR3 / …………………………………………………………………………………………..2.5
But IR3 = 3 / 290 + 6.8μA = 10.307 mA
Value of R3 = 2.4V / 10.307mA
= 232.8Ω
An available resistor value around this figure is 240Ω. R3 = 240Ω
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Since variable output is required, we will achieve it by inserting a potentiometer in a divider
chain.
Configuration 1
The diagram shows potentiometer R3 at its minimum level, we start the computation by
assuming R1 = 1kΩ
Total resistance across the divider is
Rtotal = R1 + R2 + R3
Current through resistors R1 is equal to current through resistors R2 and R3
Voltage at the potentiometer = 5V
IR1 = (5-0)/(R1=1kΩ)
= 5 mA
If the regulator is designed to produce a maximum voltage of 15V,
Rtotal = V / I = 15 V / 5mA
= 3 kΩ
Configuration 2
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In this configuration potentiometer is the maximum level. At this setting, the regulator will
produce the minimum level of an output voltage.
Current through the divider, I = Vout / Rtotal
= 5.1V / 3kΩ
= 1.7 mA
R1 + R3 = 5 V / 1.7mA
= 2.94kΩ
But R1 = 1kΩ, so R3 = 1.94kΩ
The standard resistor value around 1.94kΩ is 2.2kΩ, therefore R3 = 2.2kΩ
R2 = (5.1-5)V/1.7mA
= 58 Ω
The standard resistor value around this figure is 68Ω. R2 is thus 68Ω
2.3.2 Integrated circuit voltage regulator
MOSFET based power supply uses a regulator integrated circuit for its voltage regulation. The
IC has three terminals, and the output voltage can be easily varied through one of the
terminals[6]. A fixed resistor and a potentiometer are connected between the output and
adjustment terminals, as shown in figure 2.5.
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Figure 2.5: IC regulator circuit
The output voltage relation of the circuit above is given by the equation below
Vout = Vref * (1 + R 1
R 2 )…………………..2.6
LM317H integrated circuit develops a voltage reference of 1.25 volts between the output and the
adjustable terminal.
Assuming a resistor value of R2 = 100Ω, range of resistor values of R1 is computed using
equation 2.2
For the maximum desired output voltage of 20V, R1 is
15 = 1.25 * (1 + R 1
100)
15/1.25 = 1 + R 1
100
R1 = 1.1kΩ
For a minimum desired output voltage of 5 volts, R1 is
5 = 1.25 * (1 + R 1
100)
5/1.25 = 1 + R 1
100
R1 = 400Ω
2kΩ potentiometer is chosen to cover all ranges
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A decoupling capacitor of 10mF is used to reduce the effects of noise. The capacitor also
prevents the coupling of one part of the circuit from another[7].
3.0 Circuit simulations
3.1 Rectifier simulation
The circuit to investigate the operation of a rectifier is shown below. The input to the rectifier is
a 14rms AC signal.
Figure 3.1: rectifier circuit without a capacitor
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Figure 3.2: Input and output waveform of a rectifier
The waveform in blue is the output of a rectifier. The negative part of the signal has been
removed, giving a signal with a positive part only. The waveform in red is the input signal and is
sinusoidal in shape, cantered at 0 Volts. The peak voltage of the input is 17 volts (14 * √ 2).
Figure 3.3 below shows a rectifier circuit with a 10mF capacitor connected at the output of a full-
wave bridge rectifier. The output of the circuit of figure 3.3 as displayed by the oscilloscope is
shown in figure 3.4
Figure 3.3: rectifier circuit with a smoothing capacitor
Figure 3.4: output voltage of the rectifier
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Figure 3.4 shows the effect of adding a capacitor to the rectifier circuit. The signal is free of the
alternating current component. The capacitor provides currents at the output when the voltage
from the rectifier drops.
3.2 Regulator simulation
3.2.1 IC regulator circuit
IC regulator circuit was tested using the 12 DC power source. The output voltage of the regulator
was varied using potentiometer R1. The measurements were taken using the multimeter, as
shown in figure 3.5 below.
fig
ure 3.5: IC regulator circuit
Output voltages measurements with their respective potentiometer settings are as shown in table
3.1 below
Table 3.1: output voltages with R1 settings
Potentiometer (R1)
settings
Voltage readings (V)
85% 5.022
81% 6.026
77% 7.03
73% 8.033
69% 9.035
65% 10.038
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3.2.2 Screenshots of the IC regulator voltage readings
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3.2.3 Feedback stabilized variable voltage regulator
Feedback stabilized variable voltage regulator was tested using a 12V direct current power
source, and the measurements were taken using the multimeter. The output of the regulator was
varied using a potentiometer R5, as shown in figure 3.6.
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Figure 3.6: Feedback stabilized variable regulator.
The output voltages of a regulator for the potentiometer settings R5 were taken using the
multimeter. The measurements were recorded in table 3.2.
Table 3.2: Output voltage measurements of feedback stabilized regulator
Potentiometer setting Output voltages (V)
98% 6.024
78% 7.022
62% 8.035
49% 9.078
39% 10.07
3.2.4 Screenshots of feedback stabilized measurements
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3.3 MOSFET based power supply
The rectifier circuit was connected to the IC regulator circuit to achieve a MOSFET based power
supply. Figure 3.7 represents the MOSFET based power supply. The circuit is tested by taking
readings using the multimeter. Table 3.3 shows the results obtained from testing the MOSFET
based power supply
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Figure 3.7: MOSFET based power supply.
Table 3.3: Output voltages of a MOSFET based supply.
Potentiometer settings Output voltage in Volts
85% 5.029
81% 6.034
77% 7.039
73% 8.044
69% 9.048
65% 10.051
3.3.1: screenshots of MOSFET based supply readings
A screenshot of every multimeter reading was taken. The screenshots are shown below
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3.4 BJT based power supply
Feedback stabilized voltage regulator is connected to the rectifier circuit to achieve BJT based
power supply. Figure 3.8 represents BJT based power supply. The circuit is tested by varying its
output using the potentiometer while taking readings using the multimeter.
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Figure 3.8: BJT based power supply
Table 3.4: output voltage measurements of BJT based power supply
Potentiometer setting Output voltages (V)
99% 6.036
80% 7.034
64% 8.052
51% 9.087
41% 10.076
3.4.1 screenshot of BJT based power supply multimeter readings
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4.0 Choice of components
4.1 Choosing the resistors
The first thing to consider while choosing a resistor for your prototype is the resistance value.
The resistance values were calculated, as shown in the design section, to meet the design
specification. The next thing to consider is the power the resistor will dissipate, and we chose a
1/4W resistor[8]. The power supply design is not sensitive to the type of resistor. Therefore, we
preferred a carbon film.
4.2 choosing the capacitors
The capacitor bought for the design had a sufficient working voltage. Capacitors with a working
voltage of 50 Volts works best for the design. The normal voltages should not exceed 50% of the
stated working voltage of a capacitor[9]. This will help improve the reliability of a capacitor as
well as reducing stress to the capacitor. Another factor considered when choosing the capacitor is
the capacitance value.
We gave much attention to the ripple current when selecting the capacitor to be used for
smoothing. As a guide, we took a capacitor ripple current as twice the load current. For a good
margin, a capacitor with a ripple current of 10A was chosen.
4.3 choosing diodes and BJT’s
1N4007 diodes are used for the rectifier circuit due to its features. The diode has a high current
capability; it is reliable, has low power loss, has a high maximum RMS voltage of 700V, has a
high maximum DC blocking the voltage of 1000V. The maximum collector voltage, current
gain, absolute maximum collector current, leakage current, and power are key parameters used to
chose a BJT[10].
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5.0 Full system test
The full system comprises of MOSFET based power supply connected to the DC permanent
magnet. The system is as shown in figure 5.1. A MOSFET based supply has a variable output
ranging from 5-10V. To control the speed of the DC permanent magnet motor, we varied the
voltage.
The output of the variable power supply is connected to the positive and negative terminals of an
armature[11]. The speed measurements are taken from the motor shaft. The voltage on the pin
represents the mechanical speed in rad per second. The shaft is attached to a motion controller
that converts speed from rad/s to revolution per minute. The voltage measurements are taken
using the multimeter. The measurements are equivalent to the speed in revolution per minute.
Figure 5.1: full system circuit
The motor speed and voltage relationship are tabulated in Table 5.1 below
Table 5.1. Motor speed versus input voltage
The input voltage to the
motor in Volts
Motor speed in RPM
5 15683
6 18781
7 21907
8 25031
9 27544
10 28137
Given the no-load condition, the speed of a motor is affected by the input voltage. It is evident
from the measurements that, as the input voltage is increased, the speed also increases.
Therefore, the speed of a motor is directly affected by the input voltages[12].
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5.1 Screenshots of speed measurements
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5.2 Comparison of the two designs
The other design had a BJT based power supply connected to the permanent magnet DC
machine. The speed of the motor was again varied using the variable output voltage of a motor.
Comparing the performance of the power supplies, MOSFET based power supply performs
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better compared to BJT. From the test performed on the BJT based supply, the minimum
achievable output voltage was 6Volts. As for MOSFET based power supply, the minimum
output voltage of 1 volt is achievable. Therefore, it can be concluded that the MOSFET based
power supply performs better as it provides a wide range of output voltages.
6.0 Evaluation of results
A full-wave bridge rectifier circuit was designed to be used by both power supplies. The rectifier
circuit was then tested using an AC power source. From the measurements taken by the use of an
oscilloscope, the rectifier performs as expected as the negative part of the alternating current was
chopped off, leaving the positive circle. The rectifier was connected to a smoothing capacitor to
eliminate the unwanted AC components of a power supply.
We designed two types of voltage regulators; feedback stabilized variable voltage regulator and
integrated circuit variable voltage regulator. IC variable voltage regulator uses the LM117H
integrated circuit with three terminals. The output and adjustable terminals are connected to a
100Ω fixed resistor and 2kΩ variable resistor. A variable output voltage was achieved, ranging
from 1V to 15V. The feedback stabilized variable voltage regulator achieves similar results with
voltages ranging from as low as 6 volts to 12 volts.
The rectifier circuit was connected to feedback stabilized and IC voltage regulator to form BJT
and MOSFET based power supply, respectively. The full system was then tested with an AC
power source supplied to the input terminals. A variable output is achieved for both power
supplies, which was then used to control the speed of a constant load DC motor. The voltage was
varied between 5 volts and 10 volts in steps of 1. The lowest voltage of 5 volts produced a speed
of 15653 rpm, and a voltage of 10 V produced a speed of 28137 rpm. The speed increased with
an increase in the input voltage.
7.0 Conclusion
A variable voltage power supply was successfully designed and simulated. Two approaches were
used to achieve the power supply; BJT based and MOSFET based power supply. The power
supply was designed to produce an output ranging from 5 – 10 volts. The variable output was
then used to vary the speed of a constant load permanent magnet DC motor. The speed of a
motor was measured using the multimeter. The speed is directly proportional to the voltage. The
objectives of the project were achieved.
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REFERENCE LIST
[1] "Controlled Transistor Series Regulator", Electronic Circuits and Diagrams-Electronic
Projects and Design. [Online]. Available: http://www.circuitstoday.com/controlled-transistor-
series-regulator. [Accessed: 19- Apr- 2020].
[2] "Linear DC Power Supply design", Skillbank.co.uk. [Online]. Available:
http://www.skillbank.co.uk/psu/index.htm. [Accessed: 19- Apr- 2020].
[3] "Regulated Power Supplies", Learnabout-electronics.org. [Online]. Available:
https://learnabout-electronics.org/PSU/psu22.php. [Accessed: 19- Apr- 2020].
[4] R. ERICKSON, FUNDAMENTALS OF POWER ELECTRONICS. [S.l.]: SPRINGER
NATURE, 2020.
[5] B. DOKIC, POWER ELECTRONICS. [Place of publication not identified]: SPRINGER
INTERNATIONAL PU, 2016.
[6] T. Skvarenina, The power electronics handbook. Boca Raton, Fla.: CRC Press, 2002.
[7] "An IC medium-power voltage regulator", Microelectronics Reliability, vol. 8, no. 4, p. 297,
2015. Available: 10.1016/0026-2714(69)90355-2.
[8] Z. Shao, "Research on 5V Internal Power Supply Circuit of Switching Power
Supply", Applied Mechanics and Materials, vol. 571-572, pp. 950-954, 2014. Available:
10.4028/www.scientific.net/amm.571-572.950.
[9] "Power-supply stabilising circuit", Electronics and Power, vol. 15, no. 4, p. 135, 2016.
Available: 10.1049/ep.1969.0136.
[10] W. Williams and J. Parker, "An IC medium power voltage regulator", IEEE Spectrum, vol.
6, no. 2, pp. 72-78, 2017. Available: 10.1109/mspec.1969.5213967.
[11] M. Akram Ahmad, "Speed Control of a DC motor using Controllers", Automation, Control
and Intelligent Systems, vol. 2, no. 6, p. 1, 2014. Available: 10.11648/j.acis.s.2014020601.11.
[12] N. Thornhill, An introduction to analog electronics. London: McGraw-Hill, 2015.
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