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Build this
SOLAR CHARGER
FOR 12V BATTERIES
Keep that 12V battery topped up with this
solar cell charger & matching voltage
regulator. It's ideal for use on camping trips &
in boats, tractors & electric fence installations.
By BRANCO JUSTIC
Do you have a 12V battery that's
seldom used and often goes flat due
to self-discharge? Or how about a battery that's used continuously but
which should be trickle charged to
keep it operating correctly (eg, in an
electric fence or on camping trips)?
If you answered "yes" to either of
these questions, this project will solve
20
SILICON CHIP
your problems. It uses a low-cost amorphous solar-cell array to provide a
charging current for the battery, plus
a simple voltage regulator to ensure
that the battery cannot be overcharged.
The solar cells specified in the parts
list are 150 x 150mm units rated at 6V
1W. Depending on the application,
they can be made up into either 2-cell
or 4-cell panels. The 2-cell version
has an open circuit output voltage
(Voc) of about 20V and a short circuit
current (Isc) of about 160mA in bright
sunlight.
By contrast, the larger 4-cell panel
has an Isc of about 32DmA (Voc =
Z0V). Note, however, that the actual
power. that a panel can deliver to a
load is just under 2/3(Voc x Isc) (not
simply Voc x Isc). In practice, this
means that a 2-cell panel can deliver
just over 120mA of load current at
15V in bright sunlight, while a 4-cell
panel can deliver around 240mA.
The voltage regulator contains all
the circuitry necessary to interface
the solar panel to the battery. Its main
features include an indicator LED to
indicate that the solar panel is working, a shunt regulator circuit to prevent the battery from overcharging, a
Z-LED charging current indicator, and
an isolating diode to ensure that the
battery cannot discharge back into the
panel when there is no sunlight.
The regulator circuit is suitable for
use with solar panels rated from 0.5W
to 10W. However, if a solar panel with
an output above about 3W is used, a
bigger heatsink than the one originally specified should be fitted to the
shunt regulator transistor (Ql).
D1
1N4004
D2
1N4004
VR1
1000
02
BC558
C
FROM
SOLAR
PANEL
TO
BATTERY
K
LED2
R8
4-?k
LED3
R7
1k
How it works
Fig.1 shows the circuit details. LED
1 functions as a voltage indicator and
is connected in series with its current
limiting resistor directly across the
output of the solar panel. When LED
1 lights, the solar panel is delivering
at least ZV.
Darlington power transistor Ql
serves as the shunt regulator. This
transistor remains off when .the output from the solar panel is less than
about 15.3Vand so the battery charges
via diodes Dl & DZ (provided the
battery voltage is 1.ZV less than the
panel voltage).
However, if the output from the
solar panel exceeds the voltage across
ZDl, ZDZ and Ql's base-emitter junction (about 15.3V), Ql begins to turn
on. Ql thus loads the solar panel by
shunting part of the charging current
to ground and this prevents the panel's output from rising above 15.3V.
Because Ql 's collector is isolated from
the battery by diodes Dl & DZ, this
corresponds to a battery charging voltage of about 13.8V.
Essentially, the shunt regulator stage
prevents the battery from being overcharged by high output voltages from
the solar panel during bright sunlight
conditions.
Current indicator
QZ, LED Z and LED 3 serve as a
charging current indicator. LED Z
comes on when the charging current
reaches about Z0% of maximum,
while LED 3 comes on when it reaches
80%.
Essentially, the charging current
indicator monitors the voltage across
Dl. This voltage varies from approximately 0.5V at a few milliamperes to
about 1V at 1A (ie, the greater the
current, the greater the voltage across
Dl). A voltage proportional to this
PLASTIC
SIDE
A
ITT
B
EQc
VIEWED
FROM BELOW
ECB
SOLAR PANEL VOLTAGE REGULATOR
Fig.1: the circuit uses shunt regulator transistor Ql to limit the voltage from the
solar panel to about 15.3V. It does this by shunting part of the charging current
to ground when the voltage rises above this level so that the battery is not
overcharged. Q2, LED 2 & LED 3 provide charge current indication.
,
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Fig.2: install the parts on the PC board
exactly as shown in this diagram.
Note that Ql is installed with its
metal face towards ZD1 & ZD2.
current appears at the wiper of VR1
and is applied via a 22Q resistor to
the base of QZ.
QZ serves as a current amplifier.
When its collector current reaches
0.5mA, LED Z begins to light. If the
charging current now increases, QZ's
collector current also increases and
the brightness of the LED increases
accordingly. This continues until QZ's
collector current reaches about ZmA,
at which point LED 3 also begins to
light.
Both LEDs then further increase
their brightness as the collector current increases beyond ZmA.
When VR1 is properly adjusted, the
maximum current through QZ is about
5mA. Although this is negligible for
the solar panels specified, it may be a
problem if very small panels are substituted. This "waste" current can be
eliminated by connecting a normally
closed switch across Dl to hold QZ
off. The charging current could then
be checked by pressing the switch.
Alternatively, a link could be substituted for D1 and QZ and its associated parts left off the board.
Construction
Fig.Z shows the parts layout on the
PC board (code OESOLARSC). Install
the parts as shown, taking care to
ensure that all polarised parts are correctly oriented. These include the diodes, zener diodes, transistors and
LEDs. It's easy to identify the LED
terminals, as the anode lead is always
the longer of the two.
Transistor Ql is installed with its
metal face towards the two adjacent
zener diodes. A small finned heatsink
is then bolted to the transistor to provide cooling.
The regulator board can now be
placed to one side while the solar
panel is constructed. Warning: do not
try to test the regulator by connecting
it directly to a variable power supply.
The current output capability of the
MARCH
1993
21
SOLAR
CELL
+
SOLAR
CELL
+-----~- +
SOLAR
CELL
+
SOLAR
CELL
+
Fig.3: this diagram
shows the wiring
details for a 4-cell
solar panel. If you
only want a 2-cell
panel, just leave
off the bottom two
cells.
supply will be sufficient to blow the
shunt transistor (Ql) if you do.
The way around this is to connect a
22Q 5W resistor is series with one of
the supply leads. This will limit the
output current and protect the regulator transistor and the supply as well.
To test the regulator, wind the supply
up to about 20V and check the voltage
output from the regulator atDZ 's cathode. You should get a reading of about
13.8V.
350 x 350 x 3mm ALUMINIUM PANEL
MOUNT SOLAR CELLS ON MATCHSTICKS AND SEAL
EDGES WITH NEUTRAL CURE SILICONE SEALANT
This 4-cell array was made by mounting the cells on a 3mm-thick aluminium
panel. The cells are mounted on matchsticks so that they don't short on the
panel & the edges sealed using neutral-cure silicon sealant.
Building the solar panel
If you are buying the bare solar cells,
they will have to be wired together,
attached to a panel and waterproofed
at the rear (note: the panels specified
in the parts list come with a glass
cover at the front).
Fig.3 shows the wiring details for a
for a 4-way panel. If you only want a
2-cell panel, just leave off the bottom
two cells. Note that the cells in a 2way panel are simply wired in series,
whereas those in a 4-way panel are
wired in series-parallel combination.
Before wiring the cells, you first
have to identify their positive and
negative terminals using a multimeter
(this is best done in bright light). Mark
the cell terminals with a felt pen as
they are identified, then fit the spring
clips supplied to the terminals. A
small strip of copper foil goes under
each clip on the back of the cell. The
cells can then be interconnected by
soldering leads to the copper strips.
Don't try to solder the leads directly to the solar panels, as the solder won't "take" to the aluminium
backing.
The best way to mount the cells is
to attach them to a sheet of 3mm-thick
aluminium. A 4-cell panel will require a sheet of aluminium measuring 350 x 350mm. Eight matchsticks
can be used under each cell (two at
each corner) to space it off the aluminium sheet to prevent shorts.
The procedure is to first mark out
RESISTOR COLOUR CODE
0
0
0
0
0
0
22
No.
Value
4-Band Code (1%)
5-Band Code (1%)
2
1
2
4.7kQ
1.5kQ
1kQ
100Q
22Q
yellow violet red brown
brown green red brown
brown black red brown
brown black brown brown
red red black brown
yellow violet black brown brown
brown green black brown brown
brown black black brown brown
brown black black black brown
red red black gold brown
2
SILICON CHIP
A small finned heatsink is fitted to the BD679 shunt regulator transistor (Qt) to
provide cooling. Trimpot VR1 , to the right of Qt, is adjusted so that both LEDs
shine brightly when the solar panel is in full sunlight & the battery is connected.
the positions of the solar cells on the
panel - they can be carefull y aligned
on the pan el using several 20mm
spacers between them to achieve even
spacings . This done, the panels can
PARTS LIST
1 PC board, code OESOLARSC,
70 x 35mm
1 small heatsink
2 2-way PC-mount screw
terminal blocks
. 1 1oon horizontal trimpot
Semiconductors
1 B0679 NPN transistor (01)
1 BC558 PNP transistor (02)
1 6.8V 400mW zener diode (ZD1)
1 7.5V 400mW zener diode (ZD2)
2 1N4001 silicon diodes (01 ,02)
3 red LEDs (LED1 -LED3)
be lifted up and the matchsticks attached to the panel at the cell corner
positions using neutral-cure silicone
sealant. The cells can then be dropped
into position and secured by running
a fillet of n eutral-cure silicon e sealant
right around their outside edges.
Make sure that th e edges of the
cells are all properly sealed. The edges
of the terminating clips sh ould also
be sealed but try not to get too much
sealant on the front surface of the
cells. Any excess sealant that you do
get around the edges can later be
rubbed off when the sealant dries.
Alternatively, the cells can be held
together by fitting them into plastic
edging strips an d then edge-sealing
them front an d back with silicone sealant as before. The aluminium backing
of each cell must then be waterproofed
by spraying it will a clear hard-setting
lacquer.
This technique was used for the 2cell array pictured with this article,
although mounting the cells on an
alu min ium panel is the method we
recommend.
Once the panel has been completed,
place it in direct sunlight and check
th at its open-circuit output voltage is
about Z0V. After that, it's simply a
matter of installing the panel in a
convenient location and connecting
it to the battery via the shunt regulator circuit. Adjust VRl so that LEDs 2
& 3 shin e brightly when the solar panel
is in full sunlight & the battery is
connected.
Performance
The full voltage and current capabilities of a solar panel can only be
checked in direct sunlight in the middle of the day, during the summer
months. The output from the panel
will be reduced under any other conditions.
Obviously, weather factors play an
important role in determining the output from any solar panel installation.
If we assume that the amount of full
sunlight averages about five hours per
day, it follows that the 2-cell panel
can provide about 0.6Ah/day while
the 4-cell panel can provide about
l .2Ah/day.
In practical terms, this means that
the battery can be continuously discharged at 24mA if a 2-cell panel is
used, or at 48mA if a 4-cell panel is
used.
SC
Resistors (0.25W, 5%)
2 4.7kn
1 1oon
11.5kQ
2 22Q
21 kQ
Where to buy the parts
A kit of parts for this project is
available from Oatley Electronics,
PO Box 89, Oatley, NSW 2223,
Australia. Phone (02) 579 4985.
This kit includes four solar cells
plus all the parts for the regulator
& is priced at ·$42 plus $4 for
packing & postage (aluminium
sheet metal, silicone sealant &
hookup wire not included). Note:
copyright © of the PC board is
retained by Oatley Electronics.
The solar cells are wired by soldering connecting leads to small pieces of
copper foil which are held in place by spring clips. Use your multimeter to
identify the positive & negative terminals before installing the wiring. Note that
the 2-cell array is shown here; the 4-cell array is wired as shown in Fig.3.
MARCH
1993
23
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