7/28/2019 As the Sources of Conventional Energy Deplete Day by
Day
1/7
As the sources of conventional energy deplete day by day,
resorting to alternative sources of energylike solar and wind
energy has become need of the hour.
Solar-powered lighting systems are already available in rural as
well as urban areas. These includesolar lanterns, solar home
lighting systems, solar streetlights, solar garden lights and solar
powerpacks. All of them consist of four components: solar
photovoltaic module, rechargeable battery, solarcharge controller
and load.
In the solar-powered lighting system, the solar charge
controller plays an important role as thesystemsoverall success
depends mainly on it. It is considered as an indispensable link
between thesolar panel, battery and load.
The microcontroller-based solar charge controller described here
has the following features:
1. Automatic dusk-to-dawn operation of the load.2. Built-in
digital voltmeter (0V-20V range)3. Parallel- or shunt-type
regulation4. Overcharge protection5. System status display on LCD6.
Deep-discharge protection7. Low battery lock8. Charging current
changes to pulsed at full charge9. Low current consumption10.
Highly efficient design based on microcontroller11. Suitable for
10-40W solar panels for 10A load
7/28/2019 As the Sources of Conventional Energy Deplete Day by
Day
4/7
below.
Microcontroller. Microcontroller AT89C2051 is the heart of the
circuit. It is a low-voltage, high-
performance, 8-bit microcontroller that features 2 kB of Flash,
128 bytes of RAM, 15 input/ output(I/O) lines, two 16-bit timers/
counters, a five-vector two-level interrupt architecture, a
full-duplexserial port, a precision analogue comparator, on-chip
oscillator and clock circuitry. A 12MHz crystal isused for
providing the basic clock frequency. All I/O pins are reset to 1 as
soon as RST pin goes high.
Holding RST pin high for two machine cycles, while the
oscillator is running, resets the device. Power-on reset is derived
from resistor R1 and capacitor C4. Switch S2 is used for manual
reset.
Serial ADC.The microcontroller monitors the battery voltage with
the help of an analogue-to-digitalconverter. The ADC0831 is an
8-bit successive approximation analogueto- digital converter with
aserial I/O and very low conversion time of typically 32 s. The
differential analogue voltage inputallows increase of the
common-mode rejection and offsetting of the analogue zero input
voltage. Inaddition, the voltage reference input can be adjusted to
allow encoding of any smaller analoguevoltage span to the full
eight bits of resolution. It is available in an 8-pin PDIP package
and can beinterfaced to the microcontroller with only three
wires.
LCD module. The system status and battery voltage are displayed
on an LCD based on HD44780controller. The backlight feature of the
LCD makes it readable even in low light conditions. The LCD isused
here in 4-bit mode to save the microcontrollers port pins. Usually
the 8-bit mode of interfacing
with a microcontroller requires eleven pins, but in 4-bit mode
the LCD can be interfaced to themicrocontroller using only seven
pins.
Solar panel.The solar panel used here is meant to charge a 12V
battery and the wattage can rangefrom 10 to 40 watts. The peak
unloaded voltage output of the solar panel will be around 19
volts.Higher-wattage panels can be used with some modifications to
the controller unit.
Rechargeable battery.The solar energy is converted into
electrical energy and stored in a 12V lead-acid battery. The
ampere-hour capacity ranges from 5 Ah to 100 Ah.
Dusk-to-dawn sensor.Normally, in a solar-photovoltaic-based
installation for example, solar homelighting system, solar lantern
or solar streetlight the load (the light) is switched on at dusk
(evening)and switched off at dawn (morning). During daytime, the
load is disconnected from the battery andthe battery is recharged
withn current from the solar panel. The microcontroller needs to
know the
presence of the solar panel voltage to decide whether the load
is to be connected to or disconnectedfrom the battery, or whether
the battery should be in charging mode or discharging mode. A
simplesensor circuit is built using a potential divider formed
around resistors R8 and R9, zener diode ZD1and transistor T1 for
the presence of panel voltage.
Charge control. Relay RL1 connects the solar panel to the
battery through diode D1. Under normal
conditions, it allows the charging current from the panel to
flow into the battery. When the battery isat full charge (14.0V),
the charging current becomes pulsed. To keep the overall current
consumptionof the solar controller low, normally closed (N/C)
contacts of the relay are used and the relay isnormally in
deenergised state.
Load control. One terminal of the load is connected to the
battery through fuse F1 and another
terminal of the load to an n-channel power MOSFET T3. MOFETs are
voltage driven devices thatrequire virtually no drive current. The
load current should be limited to 10A. One additional MOSFET is
connected in parallel for more than 10A load current.
Circuit description
Basically, there are two methods of controlling the charging
current: series regulation and parallel(shunt) regulation. A series
regulator is inserted between the solar panel and the battery. The
seriestype of regulation wastes a lot of energy while charging the
battery as the control circuitry is alwaysactive and series
regulator requires the input voltage to be 3-4 volts higher than
the output voltage.The current and voltage output of a solar panel
is governed by the angle of incidence of light, whichkeeps
varying.
7/28/2019 As the Sources of Conventional Energy Deplete Day by
Day
6/7
switching on the controller unit, connect the leads of the
battery, load and solar panel at appropriateplaces on the
board.
Switch on the unit and the message Solar Charge ControllerEFY is
displayed on the LCD for two
seconds. The system status messages are displayed on line 1 of
the LCD and the battery voltage isdisplayed on line 2.
A small graphic representing the battery status is also
displayed on line 2 of the LCD .
EFY note:If the unit is switched on without the solar panel
connected, the Battery LowLoad Offmessage is displayed irrespective
of the battery voltage. The display changes to charging as soon
as
the panel is connected.
2. There will be slight variation in the voltage displayed
because of the tolerance levels of potential-divider resistors in
the ADC section and Vref of the ADC being directly connected to VCC
(the output of7805 has an accuracy of 2-5 per cent) instead of
dedicated temperature-compensated voltagereference.
Software
The source program for the project is written in Assembly
language and assembled using MetalinksASM51 assembler, which is
freely available on the Internet for download. It is well commented
foreasy understanding and works as per the flow-chart shown in
Fig.5. The hex file solar.hex is to beburnt into the
microcontroller.
