# Solar Power Source for Sensors

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```							Solar Power Source for
Sensors
Group 10
Steven Portscheller
Pavlina Akritas &
Gunjan Tejani

ECE 445 Senior Design
April 28, 2006
Introduction
   Solar panel and battery system provide
independent power source for a wireless
sensor node
   Utilizes peak power tracking to extract the
maximum power from the solar panel under
variable conditions (sunlight, load)
   Provides a regulated 3.3 Volts independent of
a power grid
Objectives
   Provide approximately 30-50 mW at 3.3 V
   Small size
   Implement maximum peak power tracking on
the solar panel
   Monitor battery charge status
Overall Design
Initial Idea
   Constant Voltage Fraction
   Voltage at which peak power occurs is a constant
fraction of open circuit voltage under given light
conditions
   Not necessary to measure current
   Model based design (non adaptable)
   Dependant on accuracy of preproduction testing
Challenges
   Hard to search for PV’s
that are small.
   Power Film 3.6 V, 50 mA
Flexible Solar Panel
MPT3.6-75 by Sundance
Solar with size 2.9" X 3“.
Testing
   Solar panel tests to determine peak power voltage
constant k
Determining k
2.5

y = 0.74*x - 0.094
2

1.5
Solar Panels        1
Vmax (V)

V                                   1
Rv ar
2                      0.5

0
data
linear fit

-0.5
0       0.5        1     1.5       2     2.5   3            3.5
Voc (V)
Testing cont.
           BUT, the current is too low, therefore the resulting
power was low.
-3                                                                                 -3
x 10                  V versus P with a Lamp                                       x 10                  V versus I with a Lamp
3                                                                                 1.8

2.8
1.6
2.6
1.4
2.4
1.2
2.2

Current (A)
Power (W)

2                                                                                  1

1.8
0.8
1.6
0.6
1.4
0.4
1.2

1                                                                                 0.2
1.6              1.8   2    2.2       2.4     2.6   2.8   3   3.2                   1.6             1.8   2   2.2       2.4     2.6    2.8   3   3.2
Voltage (V)                                                                       Voltage (V)
Challenges Overcame
   Final PV used, Edmund Scientific item
#3039811 with specifications of 0.45 V, 800 mA
and size of 33/4" x 29/16" x 1/4“.
   Given the low VOC, used 6 PV’s in series.
   I SC is never reached so model would work.
Temperature Test Results
   When cooled VOCincreases (from 2.870 V to 3.035 V)
   When heated VOCdecreases (from 2.837 V to 2.685 V)
   Possible reasons include the solar insolation, different
ambient temperatures, electrons and holes
   Another possible reason could be the internal cell diode

Id  I0 e   qVd / kT

1
Typical Test Results
Temperature results:
• When cooled
V       increases (from 2.870 V to 3.035 V)
OC

• When heated
V        decreases (from 2.837 V to 2.685 V)
OC

• V versus I, V variation throughout a day
OC

V versus I at Voc = 3.301V                                     P versus V at Voc = 3.301V
0.7                                                                1.6

0.6                                                                1.4

0.5                                                                1.2
Voltage (V)

0.4                                                                 1

0.3                                                    Power (W)
0.8

0.2
0.6

0.1
0.4

0
0.5       1    1.5         2          2.5   3   3.5               0.2
Current (A)                                   0.5   1   1.5         2          2.5   3   3.5
Voltage (V)
Voc throughout a day

Voc throughout a day
3.8

3.6

3.4

3.2
Voc (V)

3

2.8

2.6

2.4

2.2
6   8   10      12          14      16   18   20
Time
Changing ideas
   Maximum power point tracking (MPPT) based on
measuring voltage and current and calculating
power
   Operating points kept at knee of the PV I-V curves.
Power Tracking Hardware
   Dc/dc converter with large (1 F) capacitor
   Current sense amplifier and sense resistor
   PIC to measure voltage and current and
enable/disable converter accordingly
Dc/dc converter
   Acts as voltage control for solar panel
   Dc/dc converter is enabled to discharge
capacitor and lower operating voltage of solar
panel
   Converter is disabled to allow capacitor to
charge and raise operating voltage of solar
panel
   Boosts solar panel voltage (0 to 3.3 V) to a
voltage sufficient to charge the battery
Dc/dc converter
   MAX1675 chosen for small size and high
efficiency (up to 94 % claimed)
   Cooper Bussmann PowerStor® carbon
aerogel supercapacitor chosen for high
energy density and low equivalent series
resistance (ESR)
Testing
   Testing indicated an efficiency of approx.
85%
MAX1675
0.9

0.85

0.8
Efficiency

0.75

R=   100
0.7                                                R=   90
R=   68
R=   48

0.65
1.4   1.6   1.8   2   2.2    2.4     2.6   2.8   3    3.2   3.4
Input Voltage (V)
Microcontroller
   MSP430 used in target circuit
   PIC16F876A was chosen for availability and
support provided in the class (examples, etc.)
   Software written in C so algorithms could be
Current Sense Amplifier
   MAX4173 chosen for small size, low power
consumption
   Testing showed 20 V/V amp with 0.2 ohm
sense resistor allowed measurable current
range of 0.004 to 0.75 amps (approx.)
   Care must be taken in choosing resistor/amp
combination to prevent saturation of amplifier
Testing
MAX4173 w/ Rsense = 0.2 ohms
3

2.5

2
Vout (V)

1.5

1

0.5

0
0   0.5     1          1.5        2      2.5   3
Current (A)
Algorithm
   Takes voltage and current readings each
cycle and multiplies for power
   Compares current power to previous two
   If three progressively lower power readings
have been sensed, compares three voltage
readings to determine if converter needs to
be enabled or disabled
Algorithm

tempVOLT = VOLT_2;
VOLT_2 = VOLT_1;
VOLT_3 = tempVOLT;
VOLT_1 = ADC_1; //most recent voltage reading

tempPOWER = POWER_2;
POWER_2 = POWER_1;
POWER_3 = tempPOWER;

mult(); //POWER_1 is updated
Algorithm
if(POWER_2 > POWER_1)
{
if(POWER_3 > POWER_2 // P3 > P2 > P1
{
if(VOLT_1 >= VOLT_2)
{
if(VOLT_2 >= VOLT_3) // V3 < V2 < V1 V rising
{
output_high(PIN_C0); //enable converter
}
}
else // V2 > V1
{
if(VOLT_3 > VOLT_2) //V3 > V2 > V1 V dropping
{
output_low(PIN_C0); //disable converter
}
}
}
Circuit
Testing
   Tested indoors with a lamp
   Determine voltage of maximum power for the
solar panel under controlled conditions
   Monitor voltage on solar panel with tracking
circuit implemented
   Tracker would maintain an average voltage
within approx. 10% of peak power voltage
Testing   Current Signal

V*I

Voltage
Rechargeable Battery
   Uses 3-cell 3.6V NiMH rechargeable battery
with 700-mAH capacity

   Features
   Light weight
   No memory-effect – trickle charging
   Environmentally friendly – no toxic chemicals
   Constant charging and discharging rates
   70% efficiency
Discharge Characteristic
Battery Voltage Discharge Vs. Time

   Discharge across various                 1.4

   Smaller the resistance                    1

Voltage (V)
faster the discharge                     0.8                                                              V1 w / 2.3Ohms
V2 w / 5 ohms
0.6
   At a maximum applicable                                                                                   V3 w / 3.4 ohms

0.4
load of 50 mW plus the
0.2
circuit consumption, the
0
battery can supply up to

0

7

0

0

0
15

25

45

60

75
0

7

10

10

12

13

15
21 hours without                                                  Time (mins)

charging up
Bq2012 - Gas Gauge IC
   Maintain accurate record of available battery
charge
   Monitor voltage across a sense resistor to
determine charge or discharge activity
   The bq2012 also estimates self-discharge,
monitors the battery for low-battery voltage
thresholds, and compensates for temperature
and charge/discharge rates
Bq2012 Features
        Three ways to communicate with the gas gauge IC
1.    EMPTY output
2.    LED display
3.    DQ serial I/O
communication function
Serial Communication
   Transmit bits 03hex to read NACH register
and receive bits for currently stored charge
capacity in the battery
   Using two available pins on the controller
   Shut-off
- 20% of the charge capacity remains
Serial Communication
Switching Regulator
   Buck-boost dc/dc converter
TPS61131PW
operation
   Takes the input from the
positive battery terminal,
which is connected to the
dc/dc converter MAX1675
   Higher efficiency than the
linear regulator
   Fixed 3.3 V output at a
maximum of 1300 mA
output current capacity
Load and Input Variation

• Output voltage (Ch 1) sweep    • Output voltage (Ch 1) for variation
across resistance 200 ohms and   in input voltage from 2.0 V to 4.5V
1000 ohms at a constant input    (Ch 2)
voltage of 3.6V
Regulator Efficiency
Efficiency versus Input Voltage for the Switching Regulator TPS61131PW
86
56 ohms
47 ohms
39 ohms
85.5
Efficiency (%)

85

84.5

84

83.5
3.6       3.65     3.7      3.75      3.8      3.85    3.9      3.95         4
Input Voltage (V)

• Efficiency increases as the input voltage increases as well as the load decreases
• The average efficiency is 85%
PCB
Overall Design
Conclusions
   Need to maintain voltage greater than
nominal battery voltage in order to prevent
back current and associated losses
   Probably necessary to design a dc/dc
converter instead of using off the shelf design
   Circuit still provides necessary power, but
peak power tracking did not appear to offer
significant gains
Experience Gained
   Solar panel characteristics
   MPPT algorithms
   PIC programming
   Battery monitoring and charge tracking
   Serial communication
   Low voltage power systems
   PCB design and fabrication
Cost Analysis
Component                Price (\$)              Description

MAX1675                   5.06               dc/dc converter

MAX4173                   1.88            current sense amplifier

PIC16F876A                 4.71                  controller

Sumida CR54-220               1.17                   inductor

Cooper Bussmann PB-5R0H474          5.13                  capacitor

MPF102                   0.37                      fet

HD01DICT-ND                 0.7                     diode

13FR200E                  1.55                sense resistor

Edmund Scientifics #3039811(x6)     53.7                 solar panels

Bq2012                   3.81        gas gauge IC to monitor battery

TPS61131PW                  4.61             switching Regulator

DR74                    2.19                   inductor

SNN5542 Battery Pack           15.96        3-cell 3.6 V NiMH battery pack

TOTAL                                          100.84          Single Unit Cost
Credits
   Dwayne Hagerman
   Prof. Scott Carney
   Joel Jordan
   Jonathan Kimball
   Sriram Narayanan
   Machine and Part Shop Staff

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