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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