Power, Voltage, and Current Meter

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Power, Voltage, and Current Meter

• Pim Shih
• Eric Wild
• Professor Gary Swenson
• T.A. Julio Urbina
• University of Illinois at Urbana Champaign
• Department of Electrical and Computer Engineering

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Power Voltage and Current Meter
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Project Goals

• Build a power supply able to display voltage,
  current, and power.
• The device will be used as an educational tool
  for students in early stages of studies.
• Device is upgradeable to be used in various
  applications.
• Device is inexpensive and marketable.

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Block Diagram
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Board Layout
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Schematics Main Power Supply
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Schematics Magnetic Field Sensor
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Schematics Micro-Controller to Liquid Crystal
Display
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Description

• Power Supply
• 0-9V DC
• Voltage is varied using potentiometer
• AA004-02 Magnetic Field Sensor
• Measures the magnetic field
• MC68HC705P6 Microprocessor
• Converts the analog signal to a digital signal
• Sends signal to an LCD after programming

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Hardware

• Power Supply
• 0 to 9 volt DC power supply
• LCD display
• DMC Series LCD
• Displays voltage, current, and power generated by
  the power supply

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Hardware (cont.)

• Voltage Regulator
• MC7800 series
• Constant 5 volts is supplied to the Magnetic
  field sensor, Op-amp, Micro-Controller, and LCD
  display
• Op-amp
• Amplifies signal from the field sensor to
  readable levels for the micro-controller

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Hardware (cont.)

• The MC68HC705P6 Microcontroller
• Most important part of the project
• Converts the voltage and current to ASCII
  characters
• Outputs ASCII characters to an LCD display

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Power Supply

• Transformer
• Transforms 120 Volts AC down to 9 volts AC
• Rectification
• Converts AC to DC using diodes and capacitors
• Filtering
• Smoothes out pulsating DC to just DC using
  capacitors
• Voltage Regulation
• Used Voltage Regulator ICs to keep voltages
  constant and steady across loads

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Steps in Designing the Power Supply
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Results
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Power Supply (cont.)

• Variable DC Voltages
• Between 0-9 volts
• Switched between using potentiometer
• Implemented using a resistor/transistor array
• Output Isolation and Stability
• Diodes used for isolation of outputs
• Capacitors ensure low-noise voltage outputs

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GMR Magnetic Field Sensor

• General Comments
• Designed to measure or sense magnetic filed
  strength over a wide range of fields
• Designed to directly detect magnetic field rather
  than rate of change in magnetic field
• Very sensitive to small changes in magnetic field
• Magnetic fields produced by current carrying
  devices makes it usable as current sensors

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GMR Magnetic Field Sensor (cont.)

• Current measurement concepts
• Below illustrates the sensor package orientation
  for detecting the field from a current carrying
  wire.
• This application allows for current measurement
  without breaking or interfering with the circuit
  of interest.
• Note wire can be located above or below chip as
  long as it is oriented perpendicular to the
  sensitive axis

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M68HC05 Microcontroller

• Definition
• A microcontroller is a very small product that
  contains many of the functions found in a
  computer system
• A microcontroller is packaged as a single chip
  that can be programmed by the user with a series
  of instructions loaded into its memory

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M68HC05 Microcontroller (cont.)

• Hardware features
• HCMOS technology
• 8-bit architecture
• Internal 16 bit timer
• 2.1 MHz internal operating frequency, 5 volts
  1.0 MHz, 3 volts
• Serial Communications Interface System
• Serial Peripheral Interface System

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M68HC05 Microcontroller (cont.)

• Software Features
• Upward Software Compatible
• Efficient Instruction set
• Memory Mapped I/O
• On Chip Memory
• 176-304 bytes of random access memory
• 240 bytes of read only memory
• 7600 to 7744 bytes of programmable memory

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Microcontroller Procedure

• Voltage measurement
• Done by sending the output of the power supply to
  a pin on the microcontroller
• Current measurement
• Output of power supply is sent through a
  transducer which transformed current into voltage
• Power measurement
• Used multiply command in the microcontroller

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Coding Procedure

• Algorithm to display voltage
• 1)Set Port C, Bit 2 to output
• 2)Loop forever
• 3)Set RC oscillator
• 4)Turn on Analog to Digital converter
• 5)Check ready flag
• 6)Toggle Enable

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Flow Chart
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Original Design

• Differences on original design
• Used resistors instead of potentiometers to
  calibrate the magnetic field sensor
• Used VIR instead of magnetic field sensor to
  measure current
• Used less number of capacitors to reduce voltage
  ripple

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Performance Requirements

• Requirements
• The accuracy of the current, voltage, and power
  meter will be accurate to within five percent of
  the actual value.
• This is due to the fact that the LCD display is
  2 X 16. Which means there are two lines
  each holding 16 characters.
• Values will be in milliamps to make values as
  accurate as possible

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Verification of Load Voltage
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Tests Performed

• Tested load voltage, and current using different
  values of load resistances.
• Tested the effects of temperature, and air flow
  on circuit.
• Tested outside interference.

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Problems and Challenges

• Outside Interference
• Outside magnetic field interfered with magnetic
  field sensor reading on breadboard
• Noise due to wires inductance/capacitance EM
  noise
• Capacitors
• Original number of capacitors did not reduce
  voltage ripple to desirable levels
• Resistors
• Load resistor too small
• Clock
• Clock signal must be placed very close to the
  micro-controller, otherwise circuit does not work

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Successes

• Successfully implemented a power supply, magnetic
  field sensor, and a microprocessor to LCD
• Improved project over time while overcoming
  original design problems and challenges
• Attained an efficient and accurate working device
  as proposed
• Attained a very high quality device with
  practical purposes while keeping costs low
• Learned a great deal through this project

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Competitive Analysis

• Competitors
• ? Agilent Technologies, Fluke, Philips,
  Tektronix, many others
• Strengths
• Affordable, portable, easy to use, easily
  upgradable
• Weaknesses
• Limited but ample functionality

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Parts Cost

• For Main Power Supply Quantity Cost
• 1)117-9 Volt (100 mA) transformer 1 3.00
• 2)Switch, SPST 1 1.12
• 3)AC Line Cord 1 2.00
• 4)Diodes 1N914 Diode 4 0.22
• 5)LED 1 0.10
• 6)Resistors 100 100 K ohms 5 0.05
• 7)10 Kohm Potentiometer 1 0.49
• 8)Capacitors 1 microfarad 1 nanofarad/16V
  5 0.50
• 9)Transistors 3 0.26
• 10)Three Terminal Positive Fixed Voltage Reg.
  1 1.60
• Total for main power supply 10.96

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Parts Cost (cont.)

• For Current Measurement Quantity Cost
• 1)AA004-02 Magnetic Field Sensor 1
  11.00
• 2)U1 LM324 Low Power Quad op-amp 1 3.00
• 3)Resistors 100 ohm 1 Mohm 8 0.05
• 4)R8, R12 1 Mohm 2 0.05
• 5)Capacitors 2 0.10
• Total Cost for Current Measurement 14.70

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Parts Cost (cont.)

• For Micro-chip to LCD Quantity Cost
• 1)MC68HC705P6 Micro-controller 1
  8.50
• 2)16 pin Display Connector/to display
  1 5.05
• 3)Resistors 15 ohm-4.7 Mohm 1 0.05
• 4)Capacitors 0.1 microfarad, 27 Pico farad
  2 0.10
• 5)Oscillator 4 MHz 1 1.59
• Total Cost for Micro-Chip to LCD 15.39

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Total Cost

• Total cost of entire project (total of three plus
  breadboard) 46.05 (estimate)
• Total cost Labor Parts
• 50/hour2.5120246.05 30,046.05

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Potential Improvements

• Ways to gain stability
• Placing shields around chips and magnetic sensor
  to decrease outside magnetic field
• Design a different setup of amplifier
  configuration using feedback loops to give up
  gain for stability
• Use a different op-amp configuration to decrease
  temperature dependence

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Potential Markets

• Educators
• Labs
• Software could be reconfigured to be used in many
  applications.

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Conclusions

• Great tool for engineering college students
• Flexible design

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Any Questions