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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. , < :' • '' -.. ' I; • • :· ·-· -~ -t ~ f > (:~ { %. : (.. / i'"·~ · '""~';-:µ :'.;; ) / ~ . 0 0 . 0 .~ \~010 $1 ~ - - SOLAR PANEL LED1 voui-" teDr tED~ 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