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This solar battery charger uses a compact 6V solar panel to charge 12V sealed lead acid or conventional car batteries. That sounds a little odd but the circuit employs a voltage step-up to extract good efficiency from the panel. Design by Branko Justic* MINI SOLAR BAT T here is any number of applications where this Mini Solar Battery Charger could be put to work. For example, if you have a seldom-used boat which is stored out in the open, whether on a trailer or swinging on a mooring, this solar panel and charger could keep the battery permanently topped up so that it would not risk sulphation. Maybe you have a caravan which spends most of its time unattended? The same comments apply. Or you might use the panel in conjunction with a sealed lead acid (SLA) battery to provide permanent power for a device which is not close to mains power. Why a 6V solar panel for a 12V battery? You might wonder why the circuit uses a 6V panel with voltage step-up rather than a more conventional approach of a 12V panel and a simpler regulator circuit. The reason is that solar panels have their maximum power output at somewhat higher than their nominal voltage. For example, a typical 12V solar panel will deliver its maximum output at around 16V or thereabouts when it is fully illuminated by the Sun. That means that to get the maximum charging efficiency at all times, a fairly complex boost/buck switchmode charging circuit must be used when charging 12V batteries. For similar reasons, a 6V panel will deliver its maximum output at around 8V and it makes sense to double its output to charge a 12V SLA battery. Then if the battery is up 92  Silicon Chip to full charge (say 13.6V), the inclusion of a simple shunt regulator will prevent over-charging. The 6V panel used for this project comes in a sturdy aluminium frame and measures 395 x 160mm, although the active cell area is less than this, at 314 x 123mm. In the photo opposite (where the ratings panel is highlighted), it shows the output is rated at 4W, with a Vmp of 8.5V and Imp of 0.47A. The panel’s open-circuit voltage (Voc) is 10.6V and its short-circuit current (Isc) is 0.5A. So those are the voltage and current parameters we are working with. The panel is coupled to the charger circuit of Fig.1. This circuit is divided into two parts, the DC-DC Converter (voltage step-up) and the Shunt Regulator. The DC-DC Converter comprises a 4093 CMOS quad NAND Schmitt Trigger gate package (IC1), two Mosfets (Q1 & Q2) and five diodes (D1 – D5). IC1b is connected as a square wave oscillator running at around 4kHz, as determined by the 120kW resistor and 2.2nF capacitor. Its output is fed to gates IC1a & IC1c which drive Mosfets Q1 & Q2 in complementary fashion, ie, when Q1 is on, Q2 is off and vice versa. Both gates IC1a & IC1c have RC networks at their inputs to delay the clock pulses from IC1b while diodes D1 & D2 are included to insure that the respective Mosfets are switched off quickly. The inclusion of the RC network components assures that the two Mosfets can never be on at the same time, no siliconchip.com.au TTERY CHARGER matter how short the time may be: This would effectively place a short circuit across the supply and could blow the Mosfets. The output from the complementary Mosfets is used to drive a cascade voltage doubler, also known as a “diode pump” consisting of Schottky diodes D4 & D5 and the two 100mF 35V capacitors, C1 & C2. The voltage developed across C2 would exceed 14V DC but is ultimately limited by the following shunt regulator circuit involving Darlington transistor Q3 and its associated components. The rate of charge depends on the battery under charge but with the 4W solar panel supplied for this design, the charging rate is around 250mA or thereabouts. Shunt regulator The shunt regulator circuit consists of Q3 (the already mentioned TIP117 Darlington transistor), zener diode ZD1 (12V) and diode D6. Nothing happens in the shunt regula- Here’s the back of the solar panel used in this project, with the specifications panel highlighted. Maximum opencircuit voltage is 10.6V while the short-circuit current is 0.5A. siliconchip.com.au February 2008  93 DC-DC CONVERTER SECTION + D4 A D3 1N5817-8-9 100 F 16V SHUNT REGULATOR D5 K A 1N5817-8-9 A D7 A K 1N5817-8-9 K + 1N5817-8-9 K D1 1N4148 120k 5 FROM SOLAR PANEL IC1b 10 F 16V A K 2 6 2.2nF A 3 S G IC1a 12 IC1: 4093 D2 1N4148 2.2nF 14 1 12k 4 C1 100 F 35V D IC1d Q1 2SJ607 600 8 B A 11 13 9 1N4148 10 G S 7 2.2nF C D6 TO BATTERY K Q2 2SK3812 OR SDP85N03L D IC1c Q3 TIP117 10k C2 100 F 35V K 12k E K ZD1 12V A – – 1N4148 A ZD1 1N5817-8-9 K A A K K SOLAR POWER REGULATOR SC 2008 Q1, Q2 G C TIP117 D B S C E Fig.1: the circuit diagram. We’ve broken it into two sections for ease of understanding – most of the work is done by the DC-DC converter while the shunt regulator only operates when the battery is charged. tor circuit until the voltage across the zener diode is high enough for it to conduct. For that to happen, diode D6 and the base-emitter junction of Q3 also must be forward biased. For that to happen, the total voltage across that string must therefore be 12V + 0.6V + 1.2V = 13.8V. When the voltage across Q3 rises to this level, it effectively becomes a high power zener diode and conducts heavily to prevent any further voltage rise. In other words, Q3 “regulates” the voltage by “shunting off” the excess. Finally, the Schottky diode D7 further drops the voltage being fed to the battery to about 13.6V, by dint of its forward voltage drop of around 0.2V. Diode D7 also serves as an isolating diode and prevents the shunt from operating if 74 HEATSINK CUT FROM 1.3mm ALUMINIUM SHEET 50 24 70 38 Q3 G TIP117 D (UNDER BOARD) 12V + ZD1 100 F 100 F D5 SOLAR PANEL D7 – + S D4 BATTERY IN5819 94  Silicon Chip A152K – + + 100 F G Q1 Q2 (UNDER D BOARD) + D6 10 F + S 4148 12k 2.2nF 4093 D2 31 IC1 D1 2.2nF 2.2nF 4148 12k 120k MOC.SCINORTCELEYELTAO C D3 Fig.2 (left): the component overlay with its aluminium heatsink attached. The photo at right shows exactly the same thing for comparison. siliconchip.com.au PARTS LIST – Mini Solar Battery Charger 1 PC board,74 x 54mm, coded OE-K251A 1 plastic case to suit PC board 1 14-pin IC socket 2 2-way screw terminals with 5mm spacing 2 6mm self-tapping screws 1 3mm diam. 10mm long screw with nut and washer Semiconductors 1 4093 quad Schmitt NAND gate (IC1) 1 2SJ607 P-channel Mosfet (Q1) 1 2SK3812 N-channel Mosfet (Q2) 1 TIP117 Darlington PNP Transistor (Q3) 3 1N4148 signal diodes (D1,D2,D6) 4 IN5817 Schottky diodes (D3-D5,D7) 1 12V 400mW zener diode (ZD1) Capacitors 2 100mF 35V PC electrolytic (C1,C2) 1 100mF 16V PC electrolytic 1 10mF 16V PC electrolytic 3 2.2nF MKT polyester or disc ceramic the battery voltage rises to above 13.8V while it is being charged by other means: alternator, charger etc. Note that shunt regulators are inherently inefficient and in fact, in this circuit, once the battery has come up to full voltage, Q3 dissipates 100% of the boost circuit’s output, ie, is 0% efficient. This also means that Q3 will dissipate about 3 watts and it will need a heatsink. The heatsink is the one “component” not supplied in the kit. You’ll need to find a small piece of aluminium about 75mm or so square and cut it to shape to suit the PC board. While 1.3mm aluminium is specified, if you have slightly thicker, use it. In fact, thicker will make a better heatsink. We wouldn’t go any thinner though. The dimensions are shown on the component overlay. It also needs a hole for the bolt – the quickest way to get its position is to bring the heatsink and PC board together and mark the bolt hole position from the hole in the PC board. At least that way you’ll know they’ll match! Resistors (0.25W 5%) 1 120kW 2 12kW This project was designed by Oatley Electronics, who hold the copyright on the project and PC board pattern. A complete kit for this project, which includes the solar panel, PC board, components, case and 12V 7Ah battery is available from Oatley Electronics for $79.00. Individual components are also available: The solar panel – $36.00; Electronics kit – $18.00; and 12V 7Ah battery – $25.00 Contact: Oatley Electronics Pty Ltd. PO BOX 89, OATLEY, NSW 2223, AUSTRALIA Phone: 02 9584 3563 website: www.oatleyelectronics.com * Oatley Electronics Pty Ltd not you use an IC socket) make sure its notch points to the right, as shown on the component overlay. Otherwise you’ll almost certainly let its smoke out and as we all know, to work projects need the smoke to stay inside. The TIP117 Darlington transistor also requires special mention. Two of its leads, the emitter and base, solder to the PC board in the normal way but its collector connects to the copper via the small bolt and nut which holds it and the heatsink in position. The easiest way to make sure that the hole in the transistor lines up with the hole in the PC board is to mount it, with its heatsink underneath, onto the PC board with its Board assembly All the components for this project are mounted on a PC board measuring 74 x 54mm and coded OE-K251A. The major point of interest about the PC board is that the two Mosfets (Q1 & Q2) are surface-mount devices which have their bodies soldered directly to the underside. This is the first step in the assembly. You can do this by first tinning the leads with solder. Then hold the Mosfets in place with a clothes peg or similar spring clamp. They’re pretty small so you must make sure you get them into the right spot before soldering and that they don’t move during soldering. You might need to apply a little extra solder to ensure the leads are actually soldered to the PC board. Once these are done, proceed as you would for any project: solder the lowest components such as the 1N4148 diodes and work your way up to the tallest capacitors. Leave the 4093 IC until last just to make sure you don’t damage it. When you’re ready to solder it (and whether or siliconchip.com.au This oscilloscope screen shot illustrates the operation of the DC-DC converter section of the circuit. The top trace (yellow) is the oscillator output from pin 4 of IC1, running at 7.6kHz. Traces 2 & 3 (magenta and cyan) show the delayed gate drive signals to the complementary Mosfets, Q1 & Q2. Finally, the green trace shows the waveform at the commoned drains of Q1 & Q2 and this drives the diode pump circuit involving D4 & D5. February 2008  95 The regulator with its heatsink mounted inside the plastic case supplied with the kit (left). Above the same case is shown in its closed position. Provision is made for cables to emerge from the bottom of the case (handy when used outdoors!). bolt and nut. You only need to do the nut up finger-tight just now. Bend the base and emitter leads down at the appropriate point so they will go through their holes in the PC board. If you have a brass bolt and nut, it’s a good idea to solder the nut to the PC board copper to make absolutely sure it’s making good contact. The same can be done for steel nuts but usually these have a nickel coating which makes them difficult to solder. (Don’t know if you have a steel or brass nut? Try a magnet – if it picks up it’s steel!) Mounting in the case If you are building the project from the complete Oatley kit, it will come with a small case (as shown above) which is a perfect fit for the PC board and heatsink (you’d almost think they were designed that way . . .). Even the two mounting holes on the bottom of the PC board line up with mounting pillars inside the case. Using it If everything has been assembled correctly, it should work properly first up. There are no adjustments or controls to worry about. Connect the solar panel and battery with polarity-marked cable – polarised figure 8 is ideal – to the appropriate terminal blocks (again, watch the polarity – make sure + goes to + and - goes to -). Measure the voltage in from the solar panel and compare it to the voltage out across C2 (if you measure the output terminals you’re likely to be reading the battery voltage). If the panel voltage is ~6-8V and the voltage out is >12V, the circuit is working correctly. After a full day’s charge in the sun you should find the heatsink gets quite warm, indicating that the shunt regulator section is also working correctly. SC Expertise From SILICON CHIP 160 PAGES 23 CHAPTE RS Completely NEW projects – the result of two years research & development • Learn how engine management systems work • Build projects to control nitrous, fuel injection and turbo boost systems • Switch devices on and off on the basis of signal frequency, temperature and voltage • Build test instruments to check fuel injector duty cycle, fuel mixtures and brake & temperature Mail order prices: Aust. $A22.50 (incl. GST & P&P); Overseas $A26.00 via airmail. Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 96  Silicon Chip From the publishers of Intelligent turbo timer I SBN 095852294 - 4 9 780958 522946 $19.80 (inc GST) NZ $22.00 (inc GST) TURBO BOOST & nitrous fuel controllers How engine management works siliconchip.com.au