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Heavy Load Switchboard Design and Construction for the
SuPER Project
By
Kaha Sariashvili
Senior Project
ELECTRICAL ENGINEERING DEPARTMENT
California Polytechnic State University
San Luis Obispo
Spring 2007
i
Acknowledgements
I would like to thank my family and friends for all the support they have provided
me throughout my college career. I would also like to thank my advisor, Dr. Harris as
well as the rest of the SuPER team for their academic advice and support.
ii
Abstract
This senior project focuses on the design and construction of a heavy load switch
and sensor board for use in the Cal Poly SuPER (Sustainable Power for Electrical
Resources) project. This board is made to replace the heavy load portion of the original
switchboard [1] which was unable to handle the high current load. The board consists of a
MOSFET switch able to handle a 30 Amp line and is equipped with current, voltage, and
temperature sensors to monitor the load. The board was designed, laid out, and ordered
using the free software available for download at http://www.pcb123.com. Once the
board was assembled and tested - and the proper field modifications were made - it was
integrated into the SuPER system.
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Table of Contents
Acknowledgements ............................................................................................................ i Abstract.............................................................................................................................. ii Table of Contents ............................................................................................................. iii List of Figures................................................................................................................... iv List of Tables ..................................................................................................................... v I. Introduction........................................................................................................... 1
1. The SuPER Project ................................................................................................. 1 2. Solar Power Systems............................................................................................... 2 3. System Overview.................................................................................................... 4 4. Document Overview ............................................................................................... 7
II. Requirements......................................................................................................... 9 1. Requirements from Main Switchboard................................................................... 9 2. New Requirements................................................................................................ 12
III. Design ................................................................................................................... 13 1. Changes From Main Switchboard ........................................................................ 13 2. Schematic and Layout........................................................................................... 13 3. Parts Used ............................................................................................................. 16 4. Design Notes......................................................................................................... 18
IV. Fabrication and Construction............................................................................ 23 1. PCB123 Method ................................................................................................... 23 2. On-Campus Method.............................................................................................. 23 3. Alternative Methods.............................................................................................. 25
V. Field Modifications and Testing ........................................................................ 26 1. Field Modifications............................................................................................... 26 2. Test Results........................................................................................................... 27
VI. System Integration .............................................................................................. 30 VII. Conclusion ........................................................................................................... 31 Bibliography .................................................................................................................... 32 Appendix A: Printed Circuit Board .............................................................................. 34 Appendix B: Parts List and Costs ................................................................................. 37 Appendix C: On-Campus Etching Process................................................................... 38 Appendix D: PCB123 HOWTO..................................................................................... 40 Appendix E: Analysis of Senior Project Design ........................................................... 42
iv
List of Figures
Figure 1: SuPER System Overview [11] ............................................................................ 5 Figure 2: 4 Load / 2 Day Scenario. a) Load Schedule b) Battery SOC Estimation [4] ...... 6 Figure 3: Main Switchboard Schematic [1] ........................................................................ 9 Figure 4: Current Sensors Portion of Main Switchboard [1]............................................ 10 Figure 5: Voltage Sensors Portion of Main Switchboard [1] ........................................... 10 Figure 6: Voltage Regulator and Snubbers for Main Switchboard [1]............................. 11 Figure 7: Main Switchboard Layout [1] ........................................................................... 11 Figure 8: Heavy Load Switchboard Schematic ................................................................ 14 Figure 9: Heavy Load Switchboard Layout...................................................................... 15 Figure 10: LM50 Temperature Sensor [14] ...................................................................... 16 Figure 11: Allegro 50A Current Sensor and Characteristics [15]..................................... 17 Figure 12: International Rectifier 75A MOSFET Pinout [16].......................................... 18 Figure 13: LM324 OpAmp Pinout.................................................................................... 18 Figure 14: Temperature Sensor Circuit............................................................................. 19 Figure 15: Current Sensor Circuit..................................................................................... 19 Figure 16: Voltage Sensor Circuit .................................................................................... 19 Figure 17: Voltage Regulator Circuit ............................................................................... 20 Figure 18: MOSFET Controller Circuit............................................................................ 20 Figure 19: Heavy Load Switchboard w/ Field Mods........................................................ 26 Figure 20: SuPER System Overview................................................................................ 30 Figure 21: Heavy Load Switchboard Schematic .............................................................. 34 Figure 22: Heavy Load Switchboard Layout.................................................................... 35 Figure 23: Heavy Load Switchboard Modified Layout.................................................... 36
v
List of Tables
Table 1: Phase 1 Student Contributors [4].......................................................................... 4 Table 2: SuPER System Energy Allocation [4].................................................................. 6 Table 3: Changes from Main Switchboard to Heavy Load Switchboard ......................... 13 Table 4: Parts List ............................................................................................................. 16 Table 5: LM50 Characteristics [14].................................................................................. 17 Table 6: Allegro 50A Current Sensor Characteristics [15]............................................... 17 Table 7: International Rectifier 75A MOSFET Parameters [16]...................................... 18 Table 8: Component Maximum Temperatures ................................................................. 22 Table 9: Current Sensor Test Results (Allegro Sensor).................................................... 28 Table 10: Current Sensor Test Results (ZAP50 Sensor) .................................................. 28 Table 11: Voltage Sensor Test Results (Supply Voltage) ................................................ 28 Table 12: Voltage Sensor Test Results (Load Voltage).................................................... 29 Table 13: Heavy Load Switchboard Parts List and Costs................................................. 37
1
I. Introduction
1. The SuPER Project
The Sustainable Power for Electrical Resources project was born in July of 2005 with
Dr. James Harris’ white paper describing a durable, low-cost, family owned solar power
system [2]. It is intended as a self-contained and self-monitoring off-grid DC system with
energy storage capability that will service a wide variety of loads. It was anticipated that
development time of the system would be around five years, with the first three years
dedicated to research, design, and the building of a prototype system. The project will be
composed of several sub-systems for easy upgrading and swapping of parts. Thus, when
lower cost and better performance sub-systems are made available, the SuPER system
can easily be modified to accompany any changes.
A goal of SuPER is to demonstrate that the system can extend component life,
especially that of the battery, and achieve very low failure rate. The goal of the SuPER
project is to achieve a mean-time-between-failure (MTBF) of 25 years and a mean-time-
to-repair (MTTR) of 1 hour (assuming the parts are available at the site). The life cycle is
designed to be 20 years, the life of most household appliances and electronic equipment.
The initial cost goal which includes scheduled battery replacement every 5 years is less
than $500, which works out to about $2-3 per month for the owner [2]. It is expected that
SuPER will be used by family units in low-income, high-insolation areas of the world.
The first prototype of the SuPER System was built in May 2006 mostly under the
guidance of grad student Eran Tal [3]. Tal led the development, testing, and assembly of
the prototype SuPER System. Grad student Tyler Sheffield [4] took over Tal’s position
after his graduation in Summer 2006. Sheffield led the further development,
2
characterization, and simulation of the SuPER System. I would recommend using both
Tal’s and Sheffield’s Thesis papers to get a good overview of the SuPER System.
2. Solar Power Systems
In section 1.3 of Sheffield’s thesis paper [4], he presents a summary argument making
a case for SuPER. The argument will be reproduced here for your convenience.
There are multitudinous opinions on whether or not rising global temperatures are
directly caused by human activity; regardless of the cause, it is nevertheless a fact that
atmospheric carbon dioxide levels, and by extension, temperatures, have been sharply on
the rise in the last 25 years [5]. Such changes will have consequences for life on this
planet as we know it. SuPER harvests energy from a renewable source, and contributes
no direct air pollution to the environment. It is a device designed with the goal of
sustainability in mind. It is also intended to be a low-cost system (which will “pay for
itself” within a short time of activation [2]) in order to provide advantages to lower
income families who have not previously had access to a power generation system. There
is no grid infrastructure required as all issues associated with long distance power
distribution are removed as costs, obstacles, and energy sinks.
Solar cell technology is becoming increasingly important as an energy source for
reasons alluded to above. As a result, it is also becoming a more ubiquitous, better
researched, more efficient and more cost-effective technology [6]. The technology is
quickly developing into a preferable option among those in SuPER’s target market,
where cooking, heating and lighting energy needs are largely still provided by fossil-fuels
[7].
3
There are a few commercially-available solar power systems similar in scope to
SuPER, such as those manufactured by SunWize (http://www.sunwize.com). SuPER is
an attempt to develop one of these types of systems at much lower cost, and the team
anticipates future advancements in technology that will make this possible. This is
especially true of solar cell and battery technology. What is unique about SuPER is how
it is put together, and perhaps more importantly, why.
In [8], Sharaf and Ul Haque present a DC motor solar power system, along with
Simulink models, but there is no storage in the system. In [9], by Chiang, Chang, and
Yen a system very similar to SuPER is proposed and prototyped, although it is designed
to be a supplement to grid power rather than a replacement. This is the case with many
commercial systems. For related reasons the authors are unconcerned with managing
individual loads and optimizing battery life. There are countless additional published
papers that address a wide variety of other issues with the components that make up
SuPER, including, but not limited to, converter topologies, battery state of charge (SOC)
measurement, and maximum power point tracking (MPPT) techniques [10]. Most
publications referenced by the SuPER team do not propose or demonstrate a system on
the scale of the SuPER project.
The SuPER project is currently at phase 1 development, with phase 0 completed in
May 2006. Phase development timelines and goals may be found in Tal’s thesis [3]. The
current phase 1 student contributors are listed in the table below.
4
Table 1: Phase 1 Student Contributors [4]
Project Student Contributors DC-DC converter development (device modification) Robert Casanova Joe Shein DC-DC converter development (computer-controlled) Thaddeus Guno Koosh Shah Kunal Shah High current PCB development, thermocouples Shane Murphy* Juan Uribe* Pyranometer integration Slavic Orzhakovsky* High current PCB development Kaha Sariashvili Simulation and software control Tyler Sheffield LED lights subsystem integration Joey Zukowski Ultracapacitor integration Joseph Witts* * denotes independent study, as opposed to senior project contributors
3. System Overview
The figure below should help in gaining a general understanding of the SuPER
System and how the sub-systems interact with each other.
5
Figure 1: SuPER System Overview [11]
The table below shows the SuPER system’s energy allocation assuming 1600 W h
of input to the system over a 2 day period. Irradiance data from Golden, CO on May 20,
2005 is used to estimate solar energy production [12].
6
Table 2: SuPER System Energy Allocation [4]
Load Watts Hours / Day W h (Over 2 Days)Television 8 2 32Refrigerator 65 0 0LED Lights 4.5 3 27Laptop* 35 14 980DC Pump Motor 237 1 474 Total Energy Use Allocation: 1513 Total Solar Energy Production: 1600*Laptop uses 35W most of the time, but will sometimes use 65W * Energy Storage: 12V Valve-Regulated Lead-Acid (VRLA) battery rated at 98 A h
The data above can be seen represented as a Simulink simulation in the figure below. For
more Simulink simulations of this type, refer to Sheffield’s thesis.
Figure 2: 4 Load / 2 Day Scenario. a) Load Schedule b) Battery SOC Estimation [4]
7
It can be noticed from the above figures and tables that the largest drain on the battery
is the DC Motor load with a steady state power draw of 237 W (12 V, 19.75 A). One
should also take note that given this high power draw, the motor can not be run for long
(about 1 hour / day) without heavily draining the battery.
This is where the heavy load switchboard comes into play. The switch is only
expected to be open for about 1 hour each day with an expected current draw of about
20 A at steady state operation. The switchboard design in use is rated to handle a steady
current of 30 A, and contains sensors to report any unusual behavior to the system so that
appropriate action may be taken.
4. Document Overview
The section following the introduction discusses the requirements of this senior
project. It highlights what went wrong with the previous design [1] and the required
improvements. The requirements section also covers the possible uses of the sensor
circuitry.
The next section covers the design of the heavy load switchboard and how it differs
from the previous design. The section covers schematic and layout design in both OrCAD
and PCB123, and includes recommendations.
The fourth section of this senior project covers the fabrication and construction
methods for the switchboard. The section mentions both on-campus fabrication and
ordering through PCB123.
The next section of this senior project details field modification and testing of the
switchboard. The section will cover test methods and results, as well as any field
8
modifications that had to be made to the board in order to get it working properly. This
section contains the modified layout design, depicting all field mods.
Next, the system integration section describes the process of integrating the PCB into
the SuPER system. This section contains useful notes for anyone wanting to rewire or
reorganize the SuPER system.
Next, the conclusion highlights the errors encountered in the design, how they were
overcome, and any recommended changes to future designs. This section also states what
was learned from the project and wraps up the entire report.
Lastly there are the appendices. This portion of the senior project would be most
useful for someone attempting to reproduce or redesign this senior project. The
appendices cover many of the resources and methods used to create the switchboard, as
well as the PCB schematic, layout, parts list, and senior project analysis.
9
II. Requirements
1. Requirements from Main Switchboard
The main requirements for the main switchboard are very similar to the requirements
for the heavy load switchboard. The main switchboard requires 5 externally controlled
MOSTFET switches to switch 3 normal loads, 1 internal load (DC), and 1 heavy load.
Each load also has a current sensor and voltage sensor output associated with it. Each
load also had snubber circuitry across it for high frequency switching. The schematic and
layout for the main switchboard can be seen in the figures below.
Figure 3: Main Switchboard Schematic [1]
10
Figure 4: Current Sensors Portion of Main Switchboard [1]
Figure 5: Voltage Sensors Portion of Main Switchboard [1]
11
Figure 6: Voltage Regulator and Snubbers for Main Switchboard [1]
Figure 7: Main Switchboard Layout [1]
12
As can be seen from the layout above, the Main Switchboard trace widths are not
very large. This is fine for normal operation; however such a skinny trace would not be
able to handle a steady output of 20 A. Unfortunately the trace got too hot during
operation one day and melted the solder connections of the MOSFET switch creating a
short and destroying the heavy load switching portion of the board. It was deemed that
the current design of the board was not usable and a new heavy load switchboard was
proposed. This was favored over redesigning the Main Switchboard because it would
further modularize the system; meaning easier repair and troubleshooting.
2. New Requirements
The requirements for the Heavy Load Switchboard are to incorporate all the features
of the heavy load of the Main Switchboard including computer controlled switch, current
sensor, and voltage sensors- with the addition of a temperature sensor. The board is
expected to switch on a DC Pump Motor which will run at 20 A steady state; possibly
higher for short periods of time while adjusting torque. Joe Witts is currently working on
an Ultra-Capacitor Circuit which will prevent voltage sag in the battery when switching
on the DC Motor. The board should be able to handle a steady 30 A load line.
The sensor circuits are in place to prevent board failures. Perhaps the most useful of
the sensor circuits is the temperature sensor, which should be placed near the MOSFET
switch. If the temperature sensor reports melting point temperatures to the system, the
switch can be turned off by the software before any damage occurs.
13
III. Design
1. Changes From Main Switchboard
There are several components on the Heavy Load Switchboard which differ from
those on the Main Switchboard. Changes in components are outlined in the table below.
Table 3: Changes from Main Switchboard to Heavy Load Switchboard
Main Switchboard Part Heavy Load Switchboard Replacement Reason
5 and 10 Pin Headers Single 12-Pin Header Convenience; Less IOs required
LM340 5V Regulator LM2937-5.0 5V Regulator Convenience; One part #
LM338 3.3V Regulator LM2937-3.3 3.3V Regulator Convenience; One part #
IRFBF30 MOSFET IRF2804SPBF D2Pak MOSFET Better temperature dissipation and package
MAX622 High-Side Driver MAX1822 High-Side Driver Same Chip, pin-for-pin replacement
ZAP25 25A Current Sensor ACS750SCA-050 50A Current Sensor* Cheaper; Better output characteristics
Metal/Plastic Power Connectors 30A Terminal Blocks (all metal) Better heat dissipation *Not actually used in the final circuit, replaced during field-mod.
The Heavy Load Switchboard also differs from the Main Switchboard in that it
doesn’t use a snubber. The snubber is only necessary for high frequency switching.
Another difference is the addition of a temperature sensor. The temperature sensor used is
an LM50BIM3 10mV/Centigrade surface mount chip.
2. Schematic and Layout
The schematic and layout designs for the Main Switchboard may be seen in the
Requirements section above. The schematic design for the Heavy Load Switchboard may
be seen in the figure below. Please note that the final circuit differs from this circuit
because a few field modifications had to be made.
14
Figure 8: Heavy Load Switchboard Schematic
15
Figure 9: Heavy Load Switchboard Layout
16
3. Parts Used
Refer to the table below for a list of the parts used.
Table 4: Parts List
Part Part # Newark Part # Description Q1 IRF2804SPBF 73K8240 75A MOSFET; D2Pak Package U1 LM2937ET-3.3 41K4552 3.3V Regulator U2 LM324 43K5293 OpAmp U3 LM2937ET-5.0 41K4553 5.0V Regulator U4 LM50BIM3 41K6016 Temp. Sensor; 10mV/°C + .5V U5 MAX1822 Sample from MAXIM High-Side Driver; Vin + 11V U6 MM74C906N 58K1928 Buffer Logic IC U7 ACS750SCA-050 81H6588 Current Sensor; Vs/2 + 40mV/A J1 1-640445-2 90F5352 12 Pin Header; 0.156" Pitch Spacing J2,J3 91B6718 30A Terminal Block C1-C6 69K7895 1uF Aluminum Electrolytic Capacitor R1 Just a wire R2 72K6178 1K Carbon Composite Resistor R3 73K0339 560 Carbon Composite Resistor R4 72K6188 2.2K Carbon Composite Resistor R5,R7 73K0272 150K Carbon Composite Resistor R6,R8 73K0336 51K Carbon Composite Resistor
All datasheets for the parts used may be found on the SuPER wiki website [13]. Some of
the parts used and their characteristics may be seen in the figures and tables below.
Figure 10: LM50 Temperature Sensor [14]
17
Table 5: LM50 Characteristics [14]
Parameter Value Temperature Accuracy (+/-) 3, 2 deg C Supply Min 4.5 Volt Quiescent Current 0.13 mA Temperature Min -40 deg C Temperature Max 125 deg C Sensor Gain 10 mV/Deg CSupply Max 10 Volt Single Supply Yes Output Impedance 2000 Ohm *Vout = (10mV x °C) + 500mV
Figure 11: Allegro 50A Current Sensor and Characteristics [15]
Table 6: Allegro 50A Current Sensor Characteristics [15]
Parameter Value Current Range -50 - 50 A Supply Voltage 4.5 - 5.5 V Sensitivity 40 mV/A Zero Current Output Voltage Vcc / 2 Total Output Error ±2 % Primary Conductor Resistance 130 uΩ Operating Temperature -40°C to +150°C*Parameters are given at Vcc = 5.0V, 25°C *Vout = 2.5V + 40mV/A
18
Figure 12: International Rectifier 75A MOSFET Pinout [16]
Table 7: International Rectifier 75A MOSFET Parameters [16]
Parameter Value Drain-to-Source Breakdown Voltage 40 V Continuous Drain Current 75 A Drain-to-Source On Resistance 1.5 mΩ Gate Threshold Voltage 3.0 V Drain-to-Source Leakage Current 20 uA Operating Temperature 175 °C
Figure 13: LM324 OpAmp Pinout
4. Design Notes
The images below depict the configurations used for the sensor and switching
circuitry.
19
Figure 14: Temperature Sensor Circuit
Figure 15: Current Sensor Circuit
Figure 16: Voltage Sensor Circuit
20
Figure 17: Voltage Regulator Circuit
Figure 18: MOSFET Controller Circuit
The temperature sensor circuit consists of an LM50 temperature sensor and an
LM324 Op Amp configured as a non-inverting Op Amp. The formula for a non-inverting
Op Amp is Vout = Vin * (1 + R2/R1), which gives a gain of 1 + (2200/560) = 4.9. The
21
usual output range (0 – 100 deg C) of the temperature circuit in this configuration is
500mV * 5 = 2.5V to 4.5V * 5 = 22.5V.
The current sensor circuit consists only of the Allegro 50A current sensor which has
an output of Vout = 2.5V + 40mV/A. The expected output of the current sensor when off
(0 A) and on (20 A) is 2.5V and 3.3V.
The voltage sensor circuit consists of a voltage divider and a buffer circuit. The
voltage divider output is 1/4th of the input voltage, and the LM324 Op Amp circuit is set
up as a voltage follower for a high input resistance. The expected output of the voltage
sensor circuit is 12 / 4 = 3V.
The MOSFET control circuit provides the gate voltage for the MOSFET. The off
voltage of the control circuit is 0V and the on voltage is about 23V. The MAX1822
provides a constant voltage of 23V (Vin of 12V + 11V) and the MM74C90N buffer acts
as a switch to pull the 23V to ground or not. An input voltage of 2V will allow the 23V to
reach the gate, and an input of 0V will pull the 23V down to ground.
The other aspects of the design to keep in consideration were ensuring the circuit
design could handle the required 30A and would not get too hot during normal operation.
These requirements apply only to the MOSFET, terminal block connections, and the
traces between them. The previous section contains a figure of the heavy load
switchboard layout. As can be seen in the layout, the traces for the MOSFET source and
drain pins are about 250 mils (.250”) wide. There are many ways to determine required
trace widths able to handle a continuous amount of a given current, but by far the
simplest method I’ve found is an online trace-width calculator [17]. The original graphs
that the calculator is based on (published in IPC-D-275) only cover up to 35 Amps, up to
22
0.4 inches of trace width, from 10 to 100 degrees C of temperature rise, and copper of 0.5
to 3 ounces per square foot. The formulas extrapolate when the values are outside of
these ranges. Another important fact to note is the thickness of the copper is 2.5 times the
usual thickness. Most boards use 1 oz / sq. ft weight copper, but this board uses 2.5 oz
copper. With these factors in mind, the trace width calculator was used to find the
required trace width using the following data: Current = 30A, Thickness = 2.5 oz, Trise =
33 deg C, Tambient = 30 deg C. The resulting external trace width was 250 mils. Using
the combined temperatures of Tambient and Trise, we would expect the trace to stay
below 63 deg C. Even with this knowledge, extra precautions were taken in the design.
The PCB footprint for the terminal blocks is just one large pad (top and bottom of the
PCB) with 6 holes for mounting. The terminal block itself is made entirely out of metal.
The MOSFET used has a D2PAK package which is able to handle high temperatures and
dissipate heat really well. Below is a table of maximum temperatures for the components
used in the switching circuitry. As can be seen in the table, the limiting factor is the PCB
which will begin to melt after 130 deg C (sustained temp). Fortunately, the maximum
sustained temperature of the board should be no higher than 65 deg C.
Table 8: Component Maximum Temperatures
Item Max Temp (°C)IRF2804SPBF D2Pak MOSFET [16] 175SN63 Common Solder Alloy [18] 183PCB123 Printed Circuit Board [19] 130
23
IV. Fabrication and Construction
1. PCB123 Method
The heavy load switchboard was ordered through the PCB123 software and
fabricated by PCBExpress. PCB123 is free software provided by PCBExpress and it may
be downloaded at http://www.pcb123.com. Some important resources to keep in mind
while using PCB123 is the PCB123 Knowledgebase [19] and the SuPER Wiki website
[13]. You should also refer to the appendices for a PCB123 how-to.
In my overall experience with the software, I would rate the schematic software a bit
low due to various bugs, and the layout software on par with OrCad Layout. Overall, the
program was useful for its convenience of ordering straight from the layout program and
knowing how much it would cost right away.
2. On-Campus Method
Alternatively, the PCB could be manufactured on campus. In fact, one of the goals of
the SuPER project is to manufacture everything on campus except the battery and the
solar panel. Refer to the appendices for an on campus manufacturing howto. It should be
noted that at the time of writing this, the IME department is in the process of moving to a
new building as well as updating their fabrication methods. The instructions provided are
still applicable as of the time of this report, but they will most likely change in the near
future.
The following on-campus fabrication method is summarized from Alexander Gee’s
senior project [1]. It should be noted that PCB manufacturing is a privilege granted
mainly to IME 156/157 TAs, and it would be difficult to get a PCB manufactured on
campus without one. A few useful resources for using OrCAD and the on-campus
24
manufacturing method are the connexions website (http://www.cnx.org) [20] and the
IME156/157 Lab Manual which is available in the IME lab. The IME professors are very
helpful and would be pleased to answer any questions related to on-campus fabrication.
The first step in creating the PCB's starts with the creation of the photo-tools and the
drill-hole files. These two tasks are created virtually simultaneously using the OrCAD
Layout program. Use the OrCAD Layout program to print out a copy of a black and
white version of the layout for the front and back layers of the PCB's to be made. When
doing this, the program also creates a drill-hole file. Copy the drill-hole file onto a floppy
disk so that it can be used in conjunction with the drill machine to drill the holes onto the
copper wafer before the etching process. Once the machine is finished drilling the holes
on the copper wafer chosen for the PCB, let it sit heating with a UV heat lamp to prep the
board for lamination. In order to create the photo-tools, take the two sheets for the front
and back layers to a photo room. First the photo-tool is aligned with the sheet with the
layout design on a UV light. This exposes the photo-tool and imprints the design onto it.
Then the photo-tool is dipped in developer. This causes all of the lines that have not been
exposed to the UV light to become clear. Then the photo-tool is dipped in water and then
fixer to hold the design. When the copper wafer is finished in the UV heat lamp, both
sides of the wafer are laminated. A similar step to that of the photo-tool is used to burn
the image of the photo-tool onto the copper wafer. Then the copper wafer is put through a
series of chemical baths in order to etch away the unneeded copper in order to produce
the traces designed using the OrCAD Layout program. The copper is first sprayed with
the developer, then rinsed off, and finally sprayed with the etching solution. Once the
25
unneeded copper is completely etched away the PCB is dipped in a solution to get rid of
the photo-resist used in the lamination process.
3. Alternative Methods
There are many fabrication methods available to create your PCB, and I have found
that PCB123 is one of the more expensive and buggy methods, and using the resources
on campus is both tedious and error-prone. In the future, Dr. Harris is considering
moving to a program named DipTrace (http://www.diptrace.com) which provides a
freeware version and also has a 500 pin, 2 layer limit version for $145. The software
would be available for installation and use by all the SuPER team-members (unlike
OrCAD) and would also generate the files necessary to have the PCB made by any
manufacturer (unlike PCB123).
26
V. Field Modifications and Testing
1. Field Modifications
The testing and field modification of the PCB is the most important step in the
process of integrating the board with the SuPER System. Without a properly working
PCB, the system is unable to function as desired. The field modified heavy load
switchboard layout design is given in the figure below.
Figure 19: Heavy Load Switchboard w/ Field Mods
As can be seen in the figure above, 7 modifications had to be made to the board design
to get it working properly. The first modification made was reversing the polarity of C4
to properly set up the MAX1822 chip. This improper setup also caused the MM74C09N
chip to break and it had to be replaced. Next, R2 had to be changed to a 10k resistor.
27
When R2 was 1k, the MOSFET gate voltage would not drop below 12V, and thus never
turn off. The third modification made was putting a 10uF capacitor across the output and
ground of the 3.3V regulator. This capacitor is required for stability and the regulator
won’t function properly without it. The fourth modification made was to switch the 2nd
and 3rd legs of the 5.0V regulator chip. The design originally called for a different voltage
regulator with a different pinout; when the regulator was changed to a different chip, the
design was never updated. The next modification made was to supply power and ground
to the LM324 Op Amp chip. The power pins are hidden in PCB123 and never showed up
when creating the layout design. To ensure that hidden pins are properly connected, there
must be a power net with a text label that matches the hidden pin name (e.g. A net with a
text label attached to it named “Vcc” or “GND”). One other field mod that had to be
made is filing down the pins for the 12 pin header. The holes were not large enough for
the header and the pins had to be individually filed down to fit. The last field mod
depicted in the image above is the replacement of the original current sensor with a
ZAP50 current sensor. Although both sensors use hall effect to sense the current the
ZAP50 senses current from a wire passing though a loop, while the Allegro sensor senses
current passing though a shunt. The ZAP50 sensor has the same pinout as the Allegro
sensor, so I was easily able to replace the Allegro sensor. The ZAP50 outputs 23mV/A
instead of 40mV/A. The test results for the ZAP50 sensor may be found in one of the
tables below.
2. Test Results
After all the proper field modifications were made to the PCB, I was able to begin
testing and taking data for the circuitry. The board was tested in the Power Lab on
28
campus. The power lab has a power supply able to supply up to 6A @ 12V and an
electronic load able to pull from the supply. The tables below show test results for all the
sensor circuits.
Table 9: Current Sensor Test Results (Allegro Sensor)
Sensed Current (A) Output Voltage (V) Difference (mV)0.02 2.496 01.03 2.536 402.02 2.575 393.03 2.615 404.03 2.654 39
5 2.694 406.03 2.734 40
Table 10: Current Sensor Test Results (ZAP50 Sensor)
Sensed Current (A) Output Voltage (V) Difference (mV)0.02 2.480 01.03 2.503 232.02 2.526 233.03 2.549 234.03 2.573 24
5 2.596 236.03 2.619 23
Table 11: Voltage Sensor Test Results (Supply Voltage)
Input Voltage (V) Output Voltage (V) Ratio 4 1.01 3.960396048 2.03 3.9408867
12 3.03 3.9603960416 4.02 3.98009950220 5.02 3.98406374524 6.01 3.99334442628 7.01 3.994293866
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Table 12: Voltage Sensor Test Results (Load Voltage)
Input Voltage (V) Output Voltage (V) Ratio 4 1.01 3.960396048 2.03 3.9408867
12 3.03 3.9603960416 4.02 3.98009950220 5.02 3.98406374524 6.01 3.99334442628 7.01 3.994293866
As can be seen from the tables above, the sensor circuits behave as expected. Data
is missing for the temperature sensor because it burned out before it could be tested, and
there was no replacement sensor. The board is set up to have a temperature sensor added
to it, however it currently has none.
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VI. System Integration
An overview of the SuPER system wiring may be seen in the figure below.
Figure 20: SuPER System Overview
The Heavy Load Switchboard is placed right over the main switchboard. The lines
that go through the Main Switchboard to the Pump now go through the Heavy Load
Switchboard. In terms of communicating with the system’s data acquisition units, the
switch control, load voltage sensor, and load current sensor lines were taken off the Main
Switchboard and attached to the Heavy Load Switchboard. There were no existing lines
for the supply voltage sensor and the temperature sensor so those still need to be attached
to a DAQ and programmed in. It should be noted that the temperature sensor IC was
burnt out. There are wires there ready for an IC to be soldered to them, but there is no IC.
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VII. Conclusion
Although the board design feels fairly simple in retrospect, the complications
presented during the process of design and integration made this project a worthwhile and
satisfying educational experience. The most valuable knowledge gained during this
senior project is learning to never underestimate a task. Many of the road blocks
encountered were small unanticipated problems which turned into large time sinks. A
simple avoidable problem such as ordering more parts than you need for a few more
cents could have saved several days of delays.
A few of the challenges I faced during this project was getting to know a new
program- PCB123- and learning to work around its many little kinks and bugs. Although
I would not recommend the PCB123 Schematic program, it is quite tedious to create the
schematic in OrCAD and port it over to PCB123 Layout. After several hours of tinkering
with the program, however, I was able to become more intimately associated with it and
soon became the person to ask questions about using the program.
A recommendation I would make to someone attempting to recreate this circuit board
would be to either do the entire thing in OrCAD or some other design program, then send
the gerber files to PCB123 for manufacturing. I would not recommend manufacturing on
campus until the final prototype of the SuPER system is made due to severe restrictions
to on campus manufacturing. I would also recommend the next design of this board is
based off the field modified version of the board which can be found in the “Field
Modifications and Testing” section.
32
Bibliography
1. Gee, Alexander J., Printed Circuit Board Design and Construction for the SuPER Project, Senior Project, California Polytechnic State University: San Luis Obispo, in EE Dept., J.G. Harris, 2006
2. Harris, James G., White Paper for Sustainable Power for Electrical Resources - SuPER. 2005.
3. Tal, Eran, SuPER System Prototype Design and Implementation, Masters Thesis, California Polytechnic State University: San Luis Obispo, in EE Dept., J.G. Harris, July 2006
4. Sheffield, Tyler, Cal Poly SuPER System Simulink Model and Status and Control System, Masters Thesis, California Polytechnic State University: San Luis Obispo, in EE Dept., J.G. Harris, June 2006
5. Administration, National Oceanic and Atmospheric, Trends in Atmospheric Carbon Dioxide., http://www.esrl.noaa.gov/gmd/ccgg/trends/, Editor. 2007.
6. MSNBC. Venture Capitalists Embrace Solar Energy. 28 December 2005 [cited; Available from: http://www.msnbc.msn.com/id/10625903.
7. Mills, E., ENVIRONMENT: The Specter of Fuel-Based Lighting. 2005. 8. Sharaf, A. M. and A. Reaz, A low cost stand alone photovoltaic scheme for
motorized hybrid loads. System Theory, 2004. Proceedings of the Thirty-Sixth Southeastern Symposium on, 2004: p. 175-179.
9. Chiang, S. J., K. T. Chang, and C. Y. Yen, Residential photovoltaic energy storage system. Industrial Electronics, IEEE Transactions on, 1998. 45(3): p. 385-394.
10. Oi, Akihiro, Design and Simulation of Photovoltaic Water Pumping System, Masters Thesis, California Polytechnic State University: San Luis Obispo, in EE Dept., Taufik, September 2005
11. Cao, Jennifer, SuPER Project Wiring and System Protection, Senior Project, California Polytechnic State University: San Luis Obispo, in EE Dept., J.G. Harris, 2006
12. DenHerder, Tyson, Design and Simulation of Photovoltaic SuPER System Using Simulink, Senior Project, California Polytechnic State University: San Luis Obispo, in EE Dept., J.G. Harris, 2006
13. SuPER. SuPER Wiki. [cited; Available from: http://cla.calpoly.edu:16080/course/engl500/doku.php?id=super:home.
14. Semiconductor, National. LM50 Datasheet. 2006 [cited; Available from: http://www.national.com/pf/LM/LM50.html.
15. Allegro. ACS750xCA-050 Data Sheet. [cited; Available from: http://www.allegromicro.com/en/Products/Part_Numbers/0750/index.asp.
16. Rectifier, International. IRF2804SPBF Data Sheet. 2005 [cited; Available from: http://www.irf.com/product-info/datasheets/data/irf2804pbf.pdf.
17. Brad. Trace Width Calculator. 2006 [cited; Available from: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/.
18. Wikipedia. Solder. 2007 [cited; Available from: http://en.wikipedia.org/wiki/Solder.
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19. PCB123. PCB123 Knowledgebase. 2007 [cited; Available from: http://www.pcb123.com/community/content/modules.php?name=knowledgebase.
20. Connexions. OrCAD Tutorials. 2004 [cited; Available from: http://cnx.org/content/search?words=orcad.
34
Appendix A: Printed Circuit Board
Figure 21: Heavy Load Switchboard Schematic
35
Figure 22: Heavy Load Switchboard Layout
36
Figure 23: Heavy Load Switchboard Modified Layout
37
Appendix B: Parts List and Costs
Table 13: Heavy Load Switchboard Parts List and Costs
Part Footprint Part # Newark Part # Description Price Quantity Total Q1 D2PAK IRF2804SPBF 73K8240 30A MOSFET; D2Pak Package $3.55 1 $3.55U1 TO220U LM2937ET-3.3 41K4552 3.3V Regulator $2.49 1 $2.49U2 14 Pin DIP LM324 43K5293 OpAmp $0.50 1 $0.50U3 TO220U LM2937ET-5.0 41K4553 5.0V Regulator $2.62 1 $2.62U4 LM50 LM50BIM3 41K6016 Temp. Sensor; 10mV/°C + .5V $1.70 1 $1.70U5 8 Pin DIP MAX1822 Sample from MAXIM High-Side Driver; Vin + 11V $0.00 1 $0.00U6 14 Pin DIP MM74C906N 58K1928 Buffer Logic IC $2.58 1 $2.58U7 TO220 ACS750SCA-050 81H6588 Current Sensor; Vcc/2 + 40mV/A $5.97 1 $5.97J1 Custom 1-640445-2 90F5352 12 Pin Header; 0.156" Pitch Spacing $0.47 1 $0.47 640250-4 90F4297 Connector Housing 4 terminals, 0.156" Pitch $0.23 1 $0.23 640250-8 90F4301 Connector Housing 8 terminals, 0.156" Pitch $0.39 1 $0.39J2,J3 Custom 91B6718 30A Terminal Block $0.35 2 $0.70C1-C6 Cap 69K7895 1uF Aluminum Electrolytic Capacitor $0.04 6 $0.24R1 Res Just a wire $0.00 1 $0.00R2 Res 72K6178 1K Carbon Composite Resistor $0.13 1 $0.13R3 Res 73K0339 560 Carbon Composite Resistor $0.13 1 $0.13R4 Res 72K6188 2.2K Carbon Composite Resistor $0.13 1 $0.13R5,R7 Res 73K0272 150K Carbon Composite Resistor $0.13 2 $0.26R6,R8 Res 73K0336 51K Carbon Composite Resistor $0.13 2 $0.26 PCB Heavy Load Switchboard Printed Circuit Board $197.15 1 $197.15 Total: $219.50
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Appendix C: On-Campus Etching Process
Please note that the IME Lab is moving the quarter after this HOWTO is written and
this process will most likely not be the same in the future. This guide is therefore only
provided as a reference to help establish an educational foundation. One should also note
that access to the lab and machines is limited to TAs or former TAs for IME156 or
IME157. To use the lab, you will either need to be a TA or work with a TA. This might
not be such a bad thing as the TAs will be able to guide you in most of the fabrication
process. Some useful resources to help understand the etching process are the IME156
Lab Manual, or for a quick and dirty explanation of the process, go to
http://styley.net/?p=52#more-52. Much of this appendix is summarized from Alexander
Gee’s On-Campus Etching HOWTO [1].
1) Follow instructions in the lab to print out paper used for the photo-tool and to create
the drill-hole file.
2) Cut the wafer down to size and use the drill-hole file to drill the through-holes needed
for the layout design. Use the CNC Drill instructions to drill your board.
3) Create the photo-tool used for the etching process. Use the instructions in the photo-
tool room to mix chemicals and expose and develop your printout.
4) Expose the wafer to the UV heat for about 15 minutes while letting the lamination
machine heat up to get the wafer ready to be laminated.
5) Laminate the top and bottom portions of the board one at a time (depending on if your
board is single-sided or double sided).
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6) Peel the top layer off. Line up the photo-tool(s) with the drill-holes on the wafer and
tape the photo-tool(s) onto the wafer. Be careful to make sure that the top and bottom are
aligned properly.
7) Expose the side(s) to the UV light for 45 seconds at a time with 15 minute intervals.
8) Hang the wafer in the developer machine and rinse with the developer for 50 seconds.
9) Rinse off with water and rinse in the etching solution machine for about 4 minutes or
until the unneeded copper is completely rinsed off.
10) Dip the new board in the next solution until the remaining lamination screen is
completely rinsed off. This can take about 15 minutes, but you may leave it in as long as
you wish.
11) Rinse the board off with water, give it a quick scrub, and it is ready to be populated.
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Appendix D: PCB123 HOWTO
The following are useful tips when using the PCB123 program. There are many bugs
and tricks with the program; these are just a few of the basics. For help getting started
with the program and doing some basics, refer to the online PCB123 tutorial located at
http://www.pcb123.com/pcb123tutorial.php. Another very extensive tutorial is available
at http://www.pcb123.com/tutorials/PDF%20Documents/PCBDesignTutorialRevA.pdf.
The PCB123 site is a very useful resource as it has both a forum and a knowledgebase. I
would recommend using all of the resources they provide as they are very helpful. If
anything, they are very easy to contact and very friendly, so you could communicate over
the phone or through email.
Going from OrCAD Schematic to PCB123 Layout
1) Select your design in OrCAD.
i. Click Tools > Create Netlist.
ii. Click the “Other” tab. Under “Formatters,” select either Tango or
Protel format.
iii. Save your netlist.
2) Open PCB123 Layout.
i. Click File > New.
ii. For the netlist file, select the netlist you saved earlier.
iii. The program will ask for footprints for each of your parts. I would
recommend using the SuPER PCB123 Library downloadable from the
Wiki Website.
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Creating Custom Footprints
Before creating your own footprints, make sure the part you’re looking for isn’t
already either in the SuPER PCB123 library (downloadable from the Wiki Website) or
the downloadable PCB123 Layout library (under PCB123 > Download Library Parts).
A very nice visual tutorial is available on the PCB123 website for creating simple
footprints at http://www.pcb123.com/pcb123visualtutorial.php. The visual tutorial site
also has guides for routing and editing components.
Fixing Common Layout DRC Errors
A couple errors encountered frequently in Layout are easily fixable by increasing pad
and track sizes. Track widths are changeable by right clicking a net and selecting “Net
Width.” To change pad sizes, right click a component and select properties. The pin and
pad sizes can be changed by selecting the “Pins” tab. Other DRC errors can be found and
fixed by searching the PCB123 website.
More Tips
PCB123 usually offers a few promotions and discounts. First time customers might
get 10% off their first order. Other discounts can be found at
http://www.pcb123.com/pcb123current_promo.php. Sunstone Circuits, a partner of PCB
Express, also offers each campus a $350 sponsorship once a year. Contact Sunstone for
more details.
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Appendix E: Analysis of Senior Project Design
Summary of Functional Requirements
This senior project focused on the design and construction of a heavy load switch
and sensor board for use in the Cal Poly SuPER (Sustainable Power for Economic
Resources) project. The board was made to replace the heavy load portion of the original
Main Switchboard which was unable to handle the high current load. The board consists
of a switch able to handle a high current line and is equipped with current, voltage, and
temperature sensors to monitor the load. The board was designed, laid out, and ordered
using the free software available for download at http://www.pcb123.com. Once the
board was assembled and tested - and the proper field modifications were made - it was
integrated into the SuPER system.
Primary Constraints
The biggest constraint I faced were communication of information delays and
shipping delays. My first quarter of working on the SuPER project, I was unable to make
the weekly meetings and had to receive most of my information second hand. This added
a lot of delays in my design because every modification I made to the design had to work
well with the overall system, and I didn’t know much about the overall system due to
limited communication. The shipping delays were not to be unexpected, but were not
fully accounted for until after the fact. One of the parts the design called for was no
longer being manufactured, and I also had to make sure all the parts were ROHS
compliant for future production and use. Some parts also took longer than expected to
arrive because they were not in stock.
Economic
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The cost of the board can be found in an earlier appendix of this report. As can be
seen in the table, the bulk of the cost went towards the board itself. I did not spend much
money on the project as it is sponsored by SuPER.
Environmental
My senior project is based on solar power energy thus making it safe for the
environment. The solar power will supply voltage to a refrigerator, a battery charger,
pump, LED lights and a TV. The goal is to keep everything DC for low power
consumption.
Manufacturability
My senior project is based on a five-year plan to design a prototype system that
will be able to only supply DC voltage to various loads. My part in the project focused on
the design and construction of PCB's for use within the SuPER system. The
manufacturing process of the PCB's is fairly simple, especially when using an off-campus
vendor such as PCB Express. The only draw-back from going off-campus is the slight
increase in cost for the fabrication of the PCB. Using an on-campus method for
manufacturing would be more efficient and cost-effective after the system is complete
and ready for production.
Sustainability
The main focus of the project is low cost sustainable power. The goal of the
SuPER project is to achieve a mean-time-between-failure (MTBF) of 25 years and a
mean-time-to-repair (MTTR) of 1 hour (assuming the parts are available at the site). The
life cycle is designed to be 20 years, the life of most household appliances and electronic
44
equipment. The system runs in a stand alone environment and supplies all its power from
a solar panel.
Ethical
Some ethical issues related to the project is ensuring the board is ROHS
compliant, meaning it is lead-free and thus more environmentally friendly to produce.
Health and Safety
Since the board switches a high current line, it is important to ensure all high
current pins are in a secure location when the cart is in operation. The board is located
behind a Plexiglas pane which protects someone from accidentally touching dangerous
circuitry.
Social and Political
The social impact that this project will have on people that don't access to power
will be tremendous. This system will give them a way to refrigerate food, have lighting,
pump water, and overall just have access to power. Politically, I can see certain
governments or perhaps even entrepreneurs purchasing the system to help those
individuals in their country that can't afford the system and have some type of payback
plan. The system should cost under $500 and have a lifetime of 20 years. The cost turns
out to be only $2-3 a month.
Development
This project has helped teach me much about working with a team and the
importance of communication. I have also learned a lot about the fabrication process,
some useful chips and how to use them in different ways in a system, and the delays to
expect when applying my expertise in a career environment.
