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Are you interested in charging batteries from a solar panel? Here is a regulator designed especially for the job. It can be built in two versions (10-amp or 20-amp) & can be used to charge a 12V or 24V battery bank. I N THE SIMPLEST solar panel plus battery setup, all you need is a diode to isolate the panel from the battery. This prevents the battery from discharging via the solar panel when it is not illuminated by the Sun. This is OK for a temporary setup but unless the solar panel is only trickle charging the battery, you will eventually run up against the problem of over-charging. To avoid over-charging the battery you need a regulator circuit so that the panel can charge the battery at its maximum current output until it reaches full charge. At that point, the regulator disconnects the panel from the battery and no further charging takes place. That is the function of the circuit pre­sented here. As depicted in the photos in this article, this regulator is built up on a small PC board with a number of power semicon­ductors which need to be mounted on a heatsink. The board itself would normally be mounted inside a plastic case with the heatsink on the outside. How the circuit works In effect, this circuit works like a switch. If the battery voltage is below 13.6V, the solar panel is connected. Once the battery voltage rises above that point, the solar panel is dis­ connected. A Schottky power diode, used because of its low for­ward voltage loss, prevents the battery from discharging back via the panel when there is no sunlight. To see how the circuit works, have a look at the diagram of Fig.1. Switching regulator for solar panels Design by OTTO PRIBOJ The regulator circuit can be housed in a plastic case but note that the power devices must be mounted on a large finned heatsink to provide cooling. The two indicator LEDs protrude through the lid of the case near one corner of the heatsink. 40  Silicon Chip D1 1N4148 C 10k Q1 BC337 IC1 78L05/7805 OUT IN E GND 100k 22 B BC--B C E 0.1 K A 7805 78L05 OUT IN ZD1 15V GND VIEWED FROM BELOW I GO GDS K A D4 K A D5 LINK FOR 12V K 22 22k 1% 2 +6V 12V OR 24V 2xPBYR1645 ZD2 30V 39k 1% 3 VR1 5k +4V D2 1N4148 8 1 IC2a TLO62 6.8k Q2 BC557 B E D3 1N4148 C 4 4.7k 330k 1% 12k 1% 1k LED1 GREEN 1 8.2k  K A LED2 YELLOW 100k  220k SOLAR PANEL(S) Q3 STP60N05 S G 10k Q4 STP60N05 G S 4.7k A A D D K SOLAR PANEL REGULATOR Fig.1 (above): the circuit is based on comparator IC2a. When the battery voltage is below 13.6V, IC2a’s output is low & so Q2 turns on Q3 & Q4 to connect the solar panel. Conversely, when the battery voltage is above 13.6V, the output devices switch off & the solar panel is disconnected. While the circuit may look a little daunting, it is really quite simple in operation. Notice that the positive terminal of the solar panel connects via Schottky diode D4 (and D5 for a high current version – ie; greater than 10 amps) to the positive terminal of the battery. The negative terminal of the panel connects to the negative terminal of the battery via Mosfet Q3 (and Q4 for the high current version) and it is the Mosfet which is the switching element. It is turned on or off, depending on the charge state of the battery. Op amp IC2a is the heart of the circuit and it is connected as a comparator. It compares a reference voltage produced by a 5V 3-terminal regulator (IC1) at pin 2 with a proportion of the battery voltage at pin 3. When the voltage at pin 3 is above the reference voltage at pin 2, the output at pin 1 is high and transistor Q2 is off. Hence Mosfet Q3 (and Q4 if used) is also off and so the solar panel is effectively disconnected from the battery. Conversely, if the voltage at pin 3 is below the voltage on pin 2, the output The power devices (D4, D5, Q3 & Q4) are connected to the PC board via insulated flying leads. Use heatshrink tubing or plastic sleeving to insulate the leads of these devices, to prevent accidental shorts. January 1994  41 been designed around a TL062 dual low current Fet-input op amp but only one op amp, IC2a, is actually in use. The other op amp is disabled by tying its inputs (pins 5 & 6) low. 24V operation As noted above, the circuit can be used for 24V systems and for this you would need two 12V solar panels in series and a 24V battery (or two 12V batteries in series). When 24V operation is required, the input voltage divider from the battery is changed, to take account of the higher vol­tage. Note the 39kΩ resistor connected to the positive side of the battery. This is in circuit for 24V operation or replaced with a link for 12V operation. Finally, D1, Q1 and ZD1 form a nom­inal 15V regulator to supply op amp IC2a and transistor Q2. For 12V operation, this circuit can be omitted or left in place – the circuit will function either way. The voltage at the emitter of Q1 will be only about +10V for a 12V battery input. Current capacity The assembled PC board is mounted on the lid of the case on 10mm tapped standoffs. Note that a small slot must be cut in the base opposite the terminal block to provide entry for the leads to the battery & to the solar panel. at pin 1 is low and so Q2 turns on Q3 (and Q4 if used) so that the solar panel is now connected. The rest of the circuit really amounts to a few frills. LED 1, at the output of IC2a, indicates “float/full charge”. It turns on when the solar panel is disconnected. LED 2, driven by transistor Q2, is turned on while ever the solar panel is connected to the battery. It indicates “on charge”. We should note that the circuit has As noted above, the circuit can be configured to handle the output of panels rated up to 10 amps with one Mosfet (Q3) or increased to 20 amps with two Mosfets (Q3 & Q4). If two Mosfets, are used then two Schottky diodes will also be required. (D4 & D5). If more than one solar panel is used, then an alternative arrangement of one Schottky diode in series with each panel should be used. Construction All parts with the exception of Mos­ RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 2 1 1 1 2 1 1 2 1 42  Silicon Chip Value 330kΩ 220kΩ 100kΩ 39kΩ 22kΩ 12kΩ 10kΩ 8.2kΩ 6.8kΩ 4.7kΩ 1kΩ 4-Band Code (1%) orange orange yellow brown red red yellow brown brown black yellow brown orange white orange brown red red orange brown brown red orange brown brown black orange brown grey red red brown blue grey red brown yellow violet red brown brown black red brown 5-Band Code (1%) orange orange black orange brown red red black orange brown brown black black orange brown orange white black red brown red red black red brown brown red black red brown brown black black red brown grey red black brown brown blue grey black brown brown yellow violet black brown brown brown black black brown brown 8.2k 0.1 4.7k 1uF D3 100k 12k 6.8k 1 IC2 TLO62 VR1 330k 100k 39k PARTS LIST 4.7k 10k ZD1 ZD2 LED2 D2 22uF 22k 220k SOURCES Q3, Q4 SOLAR CELLS ANODES D4, D5 22uF Q1 D1 DRAINS Q3, Q4 BATTERY CATHODES D4, D5 LED1 IC1 1k Q2 GATES Q3, Q4 10k Fig.2: install the parts on the PC board as shown in this wiring diagram. The connections to the power devices (D4, D5, Q3 & Q4) are made via flying insulated leads which are soldered directly to the pins of the terminal block on the back of the PC board. 1 PC board, code OP-004 1 PC mount 4-way terminal block 1 heatsink (see text) 1 5kΩ multiturn trimpot (VR1) Semiconductors 1 78L05 3-terminal 5V regulator (IC1) 1 TL062 Fet-input op amp (IC2) 1 BC337 PNP transistor (Q1) 1 BC557 PNP transistor (Q2) 1 STP60N05 N-channel Mosfet (Q3; add Q4 for 10A version) 1 PBYR1645 Schottky diode (D4; add D5 for 10A version) 3 1N4148 diodes (D1,D2,D3) 1 BZX79C15 15V Zener diode (ZD1) 1 BZX79C30 30V Zener diode (ZD2) 1 green LED (LED1) 1 yellow LED (LED2) Capacitors 2 22µF 35VW PC electrolytic 1 1µF 35VW PC electrolytic 1 0.1µF monolithic Resistors (0.25W, 1%) 1 330kΩ 2 10kΩ 1 220kΩ 1 8.2kΩ 2 100kΩ 1 6.8kΩ 1 39kΩ 2 4.7kΩ 1 22kΩ 1 1kΩ 1 12kΩ The power devices must be insulated from the heatsink using suitable mica washers & insulating bushes. Smear all mating surfaces with thermal grease before bolting the assemblies together, then use your multimeter to confirm that each device is indeed correctly isolated. fets and Schottky diodes are mount­ed on a small PC board measuring 78 x 54mm. A 4-way insulated terminal black is mounted at one end for the four external connections to the battery and solar panel. Trimpot VR1 is a multi-turn top adjust type which gives easy and precise setting of the “end-of-charge” battery voltage. The 3-terminal 5V regulator may be a 7805 or a 78L05 type, although the latter is preferable since its current drain is lower which could be important in this application. Two prototypes are depicted in the photos accompanying this article. One is shown as a board only, with the power semiconduc­tors attached by flying leads. They will need to be mount­ed on a suitable heatsink with the usual insulating bushes, mica washers and thermal grease. The second prototype is shown with the PC board mounted in a plastic box and the two LEDs have been taken off the board and mounted so that they protrude through the lid of the case. Where to buy the parts Short form kits for this project are available only from the designer, Otto Priboj. The kit consists of the regulator PC board plus components and is priced at $54. Additional components to make a 20A version are priced as follows: STP60N05 $8; PBYR1645 Schottky diode $5.00; postage & packing, $4.00. Mail orders with a cheque or money orders should be sent to Otto Priboj, PO Box 362, Villawood, NSW 2163. Phone (02) 724 3801. Setting up To set the circuit up you will need a power supply to sub­stitute for a solar panel and a 12V battery (since the circuit will not work unless a battery is connected). Turn on the power supply and wind up the voltage. Note that no current will flow until the power supply exceeds the battery voltage. Turn up the supply voltage so that it is a few volts higher and measure the voltage across the battery. Adjust trimpot VR1 so that the battery vol­tage does not exceed 13.8V while on charge. For a 24V system, the approach is the same except that the cutoff voltage SC is adjusted to 27.6V. January 1994  43