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Design by STEVE CALDER* Battery charger for solar panels Solar panels are coming into wider use every day but getting the most out of a panel is not simply a matter of hooking it directly across your battery. This step-up/step-down battery charger circuit does a much better job. The trouble with solar panels is that their output voltage and current varies widely depending on whether the sun is bright and high in the sky, or clouded over, setting in the west and so on. With the solar panel in bright sun, it may be able to deliver too much current for the battery while at times when the sun is low in the 8 SILICON CHIP sky, its output may be insufficient to charge a battery that badly needs it. What to do? The first approach may be to use a series regulator which at least will stop the battery from being overcharged but you then lose quite a lot of power in the regulator circuitry. Also, the point at which the solar panel stops charging comes quite a bit sooner because of the inevitable voltage loss across the regulator. Apart from those two drawbacks, a series regulator can do nothing about enabling the solar panel to charge the battery when its output is low. For this situation, you need a step-up circuit to increase the panel's output voltage. The circuit presented here does both step-up and step-down, according to whether the solar panel's output voltage is high (say above 15V) or low (below 12V), respectively. In practice, this charger circuit is connected between the solar panel and a 12V battery. It then ensures that the voltage across the battery does not rise above +14.3V, no matter how much the solar panel pumps out. \ The circuit The Solar Battery Charger is mounted on a small PC board which accommodates three transistors, a few diodes and one integrated circuit, ICl. This is the Motorola MC34063, a DCto-DC converter control chip. It is specially designed for this type of application. Its internal circuit diagram is shown in Fig.1. As you can see, its principal sections are a 1.25V reference, a comparator, an oscillator, some gating and a Darlington output transistor. Fig.2 shows a typical application circuit for the MC34063 in a stepdown converter circuit. In this circuit, the internal Darlington transistor is switched on and off at a high frequency set by the capacitor CT, connected to pin 3. The output voltage of the circuit is fed to a voltage divider consisting of 3.6kQ and 1.2kQ resistors and these set the nominal output voltage to 5V. The voltage divider connects to the inverting (-) input of the comparator. at pin 5 while the 1.25V reference is connected internally to the non-inverting (+) input. The operation of the circuit revolves around the comparator. If the output of the circuit is a little high, the inverting input of the comparator will be higher than 1.25V and so the internal Darlington transistor will be off. If The parts for the solar battery charger are all mounted on a PC board which in turn is mounted on an aluminium bracket. The unit enables you to get the most out of your solar panel by always charging the battery at the correct voltage. the output of the circuit is a little low, the inverting input of the comparator will be below 1.25V and so the internal Darlington will be on. The circuit will keep hunting between these two conditions and thereby maintain the output at close to the designated value. .Now let's have a look at the complete circuit of Fig.3. Switch Col lector Drive r r - - - 8 1 - - - - - - - - - - ~ Collector lpk 2 Sense lpk Oscillator Vee ICl drives two transistors, Ql and Q2, while a third transistor, Q3, is connected to the base of Ql. When the input from the solar panel is above 15V, the zener diode conducts and turns on Q3. This pulls down the base of Ql and prevents it from responding to any drive signal from ICl . In this mode, the circuit works as a Switch Emitter Cr 11220 µH 6 Timing Capacitor 1.25V Reference Regulator Comparator 5 Inverting O--+-----~ Input R1 4 1.2 k 3,6 k 470 + Vout 5.0 V/500 mA µF 'I'Co Gnd Fig.I: block diagram of the MC34063 DC-to-DC converter control IC. It includes a 1.25V reference, a comparator, an oscillator & a Darlington transistor. Fig.2: how the MC34063 is used in a step-down converter circuit. In this circuit, the internal Darlington transistor is switched on and off at a high frequency as set by capacitor CT, while Rt, R2 & the comparator set the nominal output voltage. NOVEMBER 1991 9 R1 O.Hl V+v---.----4p--.------.~5Mw~......- - - - - - + - - - - , 1k SOLAR 2200 PANEL 25VW + 2200 _ 25VW IC1 MC34063 + _ 2 6800 1.5M 100 + 16VW _ 470pF V-0--....__ _ _ _-+_ _ _ _....__ D2 MR851 1k 6.2k BATTERY _,.__ _ _....__~,___---4....,__ _.__ _ _ _ _ _ _.__.Q QJ BC547 E 10k B EOc VIEWED FROM BELOW SOLAR BATTERY CHARGER Fig.3: the final circuit uses ICl to drive two transistors, Ql & Q2. In the stepdown mode, Q3 turns Ql off & Q2 is switched at a 200kHz rate. In the step-up mode, both Ql & Q2 are switched simultaneously at a l00kHz rate & the energy in the inductor charges the lO0µF output capacitor via D1 & D2. step-down converter or, if you like, as a simple switching regulator. It works exactly like the circuit of Fig.Z, described above. There is one difference though and that involves transistor QZ. Whereas the circuit of Fig.Z uses no external transistor, the circuit of Fig.3 uses QZ to boost the output of the internal transistor. The transistor is switched on and off at about ZOOkHz, with the "on time" of the transistor being varied depending on the charge state of the battery and the output voltage from the solar panel. Step-up mode When the voltage from the solar panel falls below 15V, the operating mode of the Solar Battery Charger circuit changes quite markedly. Because the zener diode no longer conducts , Q3 turns off and this allows transistor Ql to respond to voltage signals from ICl. The chip is now in "step-up " mode whereby the voltage from the solar panel is boosted to a level which will continue to charge the battery. In this mode, both Ql and QZ are turned on simultaneously by ICl. This effectively places inductor Ll directly across the supply voltage from the solar panel. Ql and QZ stay on just long enough for the current through the inductor to build up to saturation, whereupon they both turn off simultaneously. The energy stored in the inductor is then fed to the lOOµF output capacitor via diodes Dl and DZ. So just how does the inductor deliver its stored energy via the two diodes? It is not easy to visualise but look at it this way. When a current flowing through an inductor is suddenly interrupted, the collapsing magnetic field around the inductor tends to maintain the current flow in the same direction. So what happens is that the current which previously was going through Ql is now diverted via Dl. Similarly, the current previously Where to buy the kit The Solar Battery Charger kit is available from Jaycar Electronics, PO Box 185, Concord, NSW 2137, or from any one of their retail outlets. Jaycar also have a selection of 12V solar panels and sealed lead acid batteries. Note: copyright of the PC board associated with this project is retained by Jaycar Electronics. * Hycal Electronics. Phone (02} 633 5477. 10 SILICON CHIP BCE passing through QZ is now diverted via DZ. So the energy stored in the inductor is discharged by means of a current pulse delivered to the lOOµF capacitor. Ql and QZ then turn on again and the cycle repeats itself, effectively stepping up the voltage from the solar panel. In this step-up mode, the transistors switch on and off at a lower frequency than QZ is switched in the step-down mode. Typically, in the step-down mode, the frequency of operation is around ZOOkHz but in the step-up mode it is around lOOkHz. Depending on the amount of voltage and current being delivered by the panel, the charger circuit may become audible due to a pulsed oscillation mode it can run in. The maximum output voltage of the charger circuit is set by the voltage divider resistors connected to pin 5 of ICl; ie, 6.ZkQ, 68kQ and 1.5MQ. By using the exact values specified and with the internal reference voltage of ICl exactly 1.Z5V, the output voltage is set at 14.3V. In practice, the internal reference voltage can vary between 1.18V and 1.3ZV: Also, the 6.ZkQ and 68kQ resistors are specified at 1 %, which means that the final battery voltage may vary between 13.3V and 15.3V for the worst case combinations of reference voltage and resistor tolerance. Typically, the final battery voltage should be close to 14V. If not, it is possible to tweak the circuit by chang- Fig.4: install the parts on the PC board exactly as shown here & note that Ql & Q2 are both oriented with their metal tabs facing outwards. The inductor (Ll) consists of 200 turns of0.4mm enamelled copper wire on a ferrite potcore. ing the value of the 1.5MQ resistor. To increase the final battery voltage, you can either increase the value of the 1.5MQ resistor or leave it out altogether. To reduce the final battery voltage, reduce the value of the 1.5MQ resistor, to say 1.2MQ or lMQ. The efficiency of the circuit can run as high as 85% although more typically it would run around 70%. Note that if the solar panel you are going to use with this charger circuit has a series protection diode (and most do), you can gain a further improvement in efficiency by shunting the diode. This is possible because diode Dl in the charge circuit effectively stops the battery from discharging via the panfll. As presented, the charger circuit will handle currents ofup to around 2 amps or so, making it suitable for use with solar panels of up to around 25 watts. Construction The charger circuit is built onto a small PC board measuring 75 x 50mm (see Fig.4). The two transistors , Ql and Q2, should be mounted on a small heatsink which can also double as a mounting plate for the board. The assembly process is quite simple and should take less than an hour, including winding the coil. Let's discuss winding the coil. It is quite straightforward and only requires one winding to be placed on the plastic former. Wind on 200 turns of 0.4mm enamelled copper wire and terminate both start and finish at the same point on the bobbin. This done, assemble the bobbin and the two ferrite core halves and secure them with electrical tape. The assembled transformer can then be affixed to the board using contact or epoxy adhesive. The wire ends can be stripped of enamel and soldered to their respective points on the board. The remaining components can PARTS LIST 1 PC board, 75 x 50mm 2 Philips 18/11-3B7 ferrite potcores (4322 020 21500) 1 single section bobbin for above (4322 021 30270) 1 right-angle aluminium heatsink bracket 2 T0-220 mounting hardware sets 4 9mm tapped PC standoffs MICA INSULATING WASHER ~,jl SCREW r ftllllill(3 -.__CASE 1 T0220 DEVICE Fig.5: transistors Ql & Q2 must be isolated from the metal bracket using mica washers & insulating bushes. After mounting each transistor, use your multimeter to confirm that its tab is correctly isolated. now be mounted on the board, taking due care with polarity of transistors, diodes and electrolytic capacitors. Note that diodes Dl and D2 need to be mounted "end on" and the leads of transistors Ql and Q2 should be left at full length to allow them to be suitably bent and then mounted to the heatsink panel. Do not make a mistake by inadvertently swapping Ql and Q2 otherwise the circuit won't work and you will probably damage both transistors. When mounting the transistors on the heatsinks, you will need a mica washer and insulating washer for both. Smear a little heatsink compound on the mounting tab and the heatsink mating area. Fig.5 shows the mounting details. You will need four leads terminated to the board, two for the battery and two for the solar panel. The negative leads from the battery and from the panel can both be black while the positive lead to the battery can be red and the positive lead from the panel can be, say, blue or orange. When all the assembly work is finished, check your work carefully Semiconductors 1 MC34063 DC-to-DC converter controller (IC1) 1 BO649, TIP121 Darlington NPN transistor (01) 1 BO650, TIP126 Darlington PNP transistor (02) 1 BC547 NPN transistor (03) 2 MR851 fast recovery diodes (01 ,02) 1 15V 1W zener diode (ZD1) Capacitors 2 2200µF 25VW electrolytic 1 100µF 16VW electrolytic 1 470pF ceramic Resistors (0.25W, 5%) 1 1.5MQ 1 6.2kQ 1% 1 68kQ 1% 2 1kQ 1 10kQ 1 6800 1 0.1 Q 5W wirewound Miscellaneous 2 metres of 0.4mm enamelled copperwire, insulated hookup wire, heatsink compound, screws, nuts, lockwashers, solder. against the wiring diagram and the circuit. To test the unit, you need a power supply capable of at least 18V DC or thereabouts, a lkQ 1 watt resistor and your multimeter. Connect the lkQ resistor to where the battery would normally be terminated and then connect the solar panel inputs to your power supply. Vary the output of the power supply between +lOV and +18V and check that the output voltage across the lkQ test resistor is constant and close to+ 14.3V. If the voltage is too high, say about +14.7V, you will need to reduce the value of the 1.5MQ resistor. Alternatively, if the output voltage is less than +14 V, you will need to increase the value of the 1.5MQ resistor or omit it altogether. SC NOVEMBER 1991 11