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This handy solar panel regulator measures the voltage across your battery or the charging current and will disconnect the panel to stop overcharging. It works with 12V or 24V systems and has a 3.5-digit liquid crystal display. Design by ALAN BONNARD Solar panels are becoming ever more commonplace but the regulators can be pricey items indeed. You do need a regulator otherwise there is a real risk that a permanently connected solar panel will overcharge your battery and cause it to boil dry. This low cost regulator is especially attractive since it has the bonus of the LCD panel to show the voltage or current. There will no longer be any need to dig out your trusty multimet­er to check the state of the battery – it is on display all the time. There are two switches associated with the regulator. The slide switch selects 24V or 12V operation and will only need to be set when the unit is initially connected up. The toggle switch is used to select voltage or current readings on the LCD panel. It will read current up to 5A and voltage up to 30V. There is a 4-way insulated terminal block December DECEMBER 1999  23 1999  23 Whether you have a large or small solar panel, you will need a solar panel regulator to avoid overcharging your battery. This design works at 12V or 24V and includes an LCD to show voltage or current. The solar panel is from BP Solar – Model BP 280F, 80W, $795 rrp. Phone (02) 9454 5111. for the connec­tions to the battery and solar panel and all connections can be permanent, since the regulator’s operation is automatic. A relay disconnects the solar panel when the battery reach­es full charge and a LED comes on to indicate this condition. A series diode prevents the battery from discharging via the panel when the sun goes down. Circuit description Fig.1 shows the circuit diagram and it can be broken down into two parts. First, there is the voltage and current sensing portion which drives the 3.5-digit liquid crystal display (LCD). Second, there is a voltage comparator for controlling the relay which connects the solar panel to the battery bank. Let’s have a look at the voltage and current sensing func­tions for the LCD first. The LCD is driven by IC5, an ICL7106 digital voltmeter chip which is normally set to provide a maximum sensitivity of 2V or 200mV DC, depending on the resistors at pins 32, 35 & 36. In this circuit, the ICL7106 is set to a sensitivity of 1V by trimpot VR1. Ergo, the voltage monitoring circuit should feed no more than 1V to IC5. 24  Silicon Chip The circuit is set to monitor voltage or current. The bat­tery voltage is monitored using a divider network compris­ing resistors R3 and R4. These provide a division ratio of about 93:1 to reduce the voltage to around 130mV for a 12V battery or 260mV for a 24V battery. This divided down voltage is fed to op amp IC1b which is connected as a unity gain buffer to drive the input of IC5, the ICL7106. We will come back to IC5 in a moment. To monitor current, we use IC2, a MAX472 current sensing amplifier. The charging current from the solar panel array is fed via a .01Ω (10 milliohms) shunt resistor connected to the inputs of IC2 and it provides a current output which is proportional to the current being monitored. The output current is then fed to a 1kΩ resistor Main Features • • • • • Suitable for 12V and 24V systems. High efficiency. Voltage and current readout on 3.5-digit LCD. Automatic disconnection and reconnection of battery at set points. Suitable for currents up to 5A. and the result is a voltage which is proportional to the charging current: 100mV for every 1A of current flow through the sensing resistor. Thus for a charging current of 5A, we will have 500mV fed to IC5. The voltage and current signals are fed via switch S2a to IC5, the ICL7106. Since the voltage and current signals are referred to the 0V line in the circuit, the ICL7106 needs a negative supply rail if it is to function correctly. This is provided by IC4, an ICL7660 +5V to -5V converter. This is essen­tially a “charge pump”, which alternately switches two capacitors, C8 and C9, between the voltage rails to produce -5V at pin 5. IC5, the ICL7106, provides all the necessary internal cir­cuitry to carry out the conversion from the analog input at pin 31 to drive the liquid crystal display. Well, it drives all the relev­ant pins on the LCD except those for the decimal points and these vary, depending on whether voltage or current is being displayed. So to drive the decimal points we use two gates of a 4030 quad exclusive-OR gate package, IC6. This takes the backplane signal from IC5 and switches it to the appropriate decimal point connection on the LCD, depending on whether current or voltage has been selected by switch S2. Notice that the “test” output, pin 37 of IC5, is used Fig.1: op amp IC1a is set up as a comparator to monitor the battery voltage. If the voltage rises above 13.9V (for a 12V battery) the relay disconnects the solar panel. DECEMBER 1999  25 Parts List 1 PC board, 152 x 72mm 1 3.5-digit liquid crystal display 1 slide switch (S1) 1 DPDT toggle switch (S2) 1 6V SPST relay with 5A contacts 2 40-pin IC sockets 1 4-way PC terminal block 2 TO-220 heatsinks 1 20kΩ horizontal mount trimpot (VR1) Semiconductors 1 LM358 op amp (IC1) 1 MAX472 current sense op amp (IC2) 1 LM336Z 2.5V reference (IC3) 1 ICL7660 +5V to -5V converter (IC4) 1 ICL7106 3.5-digit LCD driver with A/D converter (IC5) 1 CD4030 exclusive-OR gate (IC6) 1 7805 5V regulator (REG1) 2 C9013 NPN transistors (Q1,Q2) 1 red LED (LED1) 1 U840 power diode (D1) 1 1N4004 diode (D2) Capacitors 1 100µF 50VW PC electrolytic 1 22µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 1 1µF MKT polyester 1 0.22µF MKT polyester 1 .047µF MKT polyester 1 .01µF MKT polyester 1 100pF ceramic Resistors (0.25W, 5%) 1 1MΩ 1 22kΩ 1 470kΩ 1 11kΩ 1 110kΩ 4 10kΩ 1 100kΩ 1 5.6kΩ 1 56kΩ 1 2.7kΩ 1 24kΩ 1 2.2kΩ 1 1kΩ 2W wirewound 1 1.2kΩ 4 1kΩ 1 470Ω 1 270Ω 2 100Ω 1 0.01Ω 1W as the ground supply connection for IC6. This is not a mistake as one of the functions of the test output is to serve as the ground supply connection for any decimal point switching circuit. Resistors R25 and R26 are included to pull pins 9 & 12 to 0V when they are not selected by switch pole S2b. Solar panel switching Op amp IC1a and transistors Q1 & Q2 control the relay switching of the solar panel. IC1a is connected as a comparator with IC3 providing a reference voltage to its non-inverting input, pin 3. Pin 2 then monitors the battery voltage via resis­tors R6, R7 & 26  Silicon Chip Fig.2: the parts layout for the PC board. The LCD mounts above the ICL7106 (IC5). Note that the 7805 regulator must be fitted with a heatsink. R8 for the 12V mode and R5, R7 & R8 for the 24V mode, as selected by slide switch S1. In the 12V mode, once the battery voltage rises above 13.9V, pin 1 of IC1a goes low and this turns off Q1 which turns on Q2 and the relay to disconnect the solar panel. LED1 is lit while ever the relay is energised and the panel is disconnected. The battery then discharges over a period of time to 13.1V whereupon pin 1 of IC1a goes high, Q1 turns on, and Q2 and the relay turn off to connect the solar panel again. For the 24V mode, the panel is dis- connected when the bat­tery voltage rises above 27.6V and reconnected when it falls to 25.9V. Another point to note about the circuit is that diode D1 is there to prevent the battery discharging via the solar panel when it is not delivering power (eg, after sunset or during heavy cloud cover). However, there is no protection for the circuit if the battery is connected the wrong way around. Construction All the components of the circuit, including the LCD panel, are mounted Resistor Colour Codes  No.   1   1   1   1   1   1   1   1   4   1   1   1   1   5   1   1   2   1 Value 1MΩ 470kΩ 110kΩ 100kΩ 56kΩ 24kΩ 22kΩ 11kΩ 10kΩ 5.6kΩ 2.7kΩ 2.2kΩ 1.2kΩ 1kΩ 470Ω 270Ω 100Ω 0.01Ω on a PC board measuring 152 x 72mm. The ICL7106 (IC5) and some of its associated components are mounted underneath the LCD panel to conserve board space. The component layout is shown in Fig.2. Insert and solder the links and resistors first, followed by the capacitors, diode D2, trimpot VR1 and the three transis­tors. Take care when mounting the LM336Z (IC3) because it looks like a TO-92 transistor – don’t get it mixed up with the transis­tors. Both the power diode (D1) and the 7805 3-terminal regulator require a heatsink although our prototype did not have a heatsink fitted to the regulator. A small U-shaped heatsink is adequate if 12V operation is all that is required but a somewhat larger U-shaped heatsink will be required to cope with 24V battery opera­tion. The four small ICs (IC1, IC2, IC3 & IC4) can be soldered directly into the PC board but IC5 and the LCD panel should be installed in sockets. For this reason, two 40-pin IC sockets are required. One of the 40-pin sockets must be cut in half and both halves installed to provide the socket for the LCD. Next, install the 4-way insulated terminal block, the relay, LED1 and the slide switch. Switch S2 is connected via a 6-way length of ribbon cable. Setup and testing Once all components are installed we are then ready to set up the regu- 4-Band Code (1%) brown black green brown yellow violet yellow brown brown brown yellow brown brown black yellow brown green blue orange brown red yellow orange brown red red orange brown brown brown orange brown brown black orange brown green blue red brown red violet red brown red red red brown brown red red brown brown black red brown yellow violet brown brown red violet brown brown brown black brown brown not applicable lator. The first step is to connect a variable DC power supply to the battery input connections. This should be set to around 12V. Now check that +5V is present at the output of REG1, at pin 8 of IC1 and IC4, at pin 1 of IC5 and pin 14 of IC6. You should also be able to measure +2.5V at the positive connec­ tion of IC3 (middle terminal). This is best measured at the junction of R9 & R10. On our prototype, we measured the 2.5V reference at 2.4864V (on a Tektronix 4.5-digit DMM). Now measure the supply voltage with your digital multimet­ er and adjust trimpot VR1 so that the LCD gives the same reading. Next, set the slide switch for 12V operation (slider away from relay) and slowly wind up the supply voltage. The relay should click and LED1 should light as the voltage rises above 13.9V. Now wind the supply slowly down and observe that the relay clicks again and LED1 goes out as the supply voltage goes below 13V. Note that the exact voltages are not crucial; anywhere between 13.7V and 14V is fine for the disconnection point, while the reconnection point should be around 13V. If the relay is not operating as it should, check the components around Q1 & Q2. You can also check whether pin 1 of IC1 switches low for supply voltages above 13.8V and low for voltages below 13V. If you are going to charge a 24V battery, set the slide switch for 24V operation (slider close to the relay) 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown brown brown black orange brown brown black black orange brown green blue black red brown red yellow black red brown red red black red brown brown brown black red brown brown black black red brown green blue black brown brown red violet black brown brown red red black brown brown brown red black brown brown brown black black brown brown yellow violet black black brown red violet black black brown brown black black black brown not applicable Capacitor Codes  Value      1µF   1u0  105 0.22µF  220n  224 .047µF   47n  473 .01µF   10n  103 100pF  100p  101 IEC Code EIA Code and wind up the supply. The relay should click and LED1 should light as the voltage rises above 27.6V. Similarly, the relay should click again and LED1 should go out as the voltage drops below 25.9V. Once all is correct, connect your solar panel and the bat­ tery to the regulator. The current from the panel can then be monitored and the orientation can be optimised for maximum SC bat­tery charging. Where To Buy The Kit The design copyright for the Solar Regulator is owned by Futurlec who can supply the kit. The complete kit is available for $44 plus $5 packing and postage within Australia. Orders many be placed via the website at www.futurlec.com and payment may be made via Bankcard, Visa Card or Mastercard. Alternatively, orders may be sent with a credit card authorisation, cheque or postal money order to Futurlec, 24 William St, Paterson, NSW 2421. DECEMBER 1999  27