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Simple 12V/24V Regulator for 70V Solar Panels Design by Branko Justic* Want to run 12V lights and accessories independently of any 230VAC mains supply? With a 72W solar panel, this simple regulator and a 12V lead-acid battery, you can run a long string of LED lights and have light in a remote location or in the city – when others are struggling with candles! O transformer T1. The gates of the Mosver the last few years there has available at this voltage. By the way, there is no reason why fets are alternately driven by IC1 and been a growing interest in running 12V lights and accessories this solar panel and regulator could so each half of the primary winding is using a solar panel, a 12V battery and not be used to charge the batteries in fed with the full voltage of the solar not much else. Normally this involves a car, caravan or boat, or even provide panel which can be as high as 90V in using a 12V solar panel and battery 12V power in a remote cabin or when full midday sunlight. IC1 runs at about and often an MPPT (Maximum Power camped in a remote location. It will 100kHz, as set by the 1.5nF capacitor and 6.8k resistor connected to pins Point Tracking) regulator to ensure charge a 12V battery at a useful 5A. The 72V Cadmium Telluride thin 5, 6 and 7 of IC1. that the maximum output of the solar film solar panel measures 1200 x The AC output voltage from the panel is available. The reason for this is that the maxi- 600mm and is quite heavy at about transformer is rectified by two SR1060 mum output from a 12V solar panel is 15kg since it is essentially a large Schottky barrier dual diodes, with each diode pair paralleled to reduce actually delivered at about 17V and piece of glass. By contrast, the step-down regula- their forward voltage. this does not match up well when tor is on a small PCB measuring 145 The effective turns ratio of the transdirectly charging a battery. former can set by links to provide a In this article we look at quite a dif- x 58mm. nominal 12V or 24V DC output to a ferent approach whereby a 72W panel battery. with a maximum output of 90V is fed The circuit The circuit is shown in Fig.1 and Either way, you need to set the outto a step-down regulator to charge a is based on an SG3525A switchmode put voltage using multi-turn trimpot 12V or 24V battery. This achieves much the same result regulator (IC1) driving two IRFB4020 VR1. For a 12V battery, the float voltage as an MPPT regulator working from a Mosfets. Each Mosfet drives one half setting is 13.8V and for 24V it is 27.6V. 12V solar panel but the 72W panel is of the primary winding of step-down VR1 and the 27k resistor connected to the DC output form quite cheap and has a voltage divider which the distinct advantage feeds a portion of the of also being able to output back to pin 1 produce a 24V DC of IC1. output, if required. This is compared to However, we think This panel has an open circuit voltage of 90V DC: 5.1V connected from most people would pin 16 to pin 2, part of probably want to use There is a SHOCK HAZARD at the panel terminals an internal comparator. a 12V output since and on parts of the PCB. When the feedback voltmore LED lights are WARNING ELECTRIC SHOCK HAZARD 70  Silicon Chip siliconchip.com.au It’s all housed on a single PCB but be warned, some of the tracks and exposed metal parts of some components can be at 90V DC at times, which can give you quite a (un!) healthy shock. Ideally, the PCB would be housed in a suitable case, away from prying fingers. Note that this photo is actually larger than life-size, for clarity. The regulator is intended for high voltage solar panels – it won’t work with standard low voltage types. It is designed to suit Oatley Electronics’ 1200 x 600mm CdTe Solar Panel, which puts out around 72W in bright sunlight at about 70-90V. In fact, Oatley Electronics have a special offer for this kit plus the solar panel for $119 (cat K330p) – see www.oatleyelectronics.com * Oatley Electronics age to pin 1 exceeds 5.1V, IC1 reduces the duty cycle of the drive signals to the gates of the Mosfets so that the output voltage is maintained within tight limits. Note that the circuit shows the 27k resistor connected to the output via slide switch S1 when it is in the RUN setting which is the normal mode. It should not be run in SET mode. SET mode has been included to enable the output voltage of the circuit to be set when the solar panel is not generating much voltage, ie, when it is indoors or maybe it is dark or raining. In this case, the above-mentioned 27k voltage divider resistor is not connected to the DC output of the circuit but to the 5.1V reference (ie, pin 16 of IC1). The voltage at test point TP is then set by trimpot VR1 to 1.885V, to obtain 13.8V (to suit a 12V battery) when the circuit is in RUN mode. Similarly, to set the output to suit a 24V battery, VR1 is adjusted to obtain 0.94V. To repeat, the circuit must not be run in SET mode when it is connected to the solar panel and a battery as the output will be unregulated. Now while the full DC voltage of the solar panel is fed directly to the drains of the two Mosfets, that voltage is far too high to be fed directly to IC1 since siliconchip.com.au it has an absolute maximum voltage of only 35V. Its supply needs to be drastically reduced which is the reason for inclusion of the ancient-looking 2N3055 power transistor, Q1. Why use this antediluvian device? It is not included for its power rating but it does have a high voltage rating for this mode of connection – 95V – so it can cope with the solar panel’s full output. It also offers good heatsinking – without an external heatsink. In fact, Q1 functions as a simple series regulator with a 10V zener diode connected to its base, bypassed by a 100F capacitor. By emitter-follower action, it feeds 9.3V to IC1 – well within its DC ratings. Having said that, the 1.2k 5W resistor connected to the collector of Q1 reduces its dissipation so that no heatsink is required. Finally, notice that there is a 47F 100V electrolytic capacitor connected to the input supply from the solar panel but it is isolated by a series 4.7 1W resistor, to reduce the output impedance of the panel supply. It was found that if no 4.7 resistor was incorporated into the circuit, it had the potential to blow the fuse on a multimeter if it was used to check the short-circuit current of the panel. Furthermore, in isolated cases it also PARTS LIST – Solar Panel Regulator 1 PCB, code K326-3, 145 x 58mm 2 ferrite core halves 1 prewound transformer bobbin 2 transformer clips 1 SPST slide switch, PCB mounting 2 2-way screw terminal blocks, PCB mounting 1 DIL 16-pin IC socket 4 M3 10mm screws, nuts and washers 2 TO220 heatsinks Semiconductors 1 2N3055 power transistor 2 IRFB4020 Mosfets 1 SG3525 SMPS IC 2 SR1060 dual Schottky diodes 1 10V 400mW Zener diode Capacitors 3 100F 16V electrolytic 2 47F 100V electrolytic 4 68nF monolithic ceramic 1 1.5nF metallised polyester 2 560pF 200V disc ceramic Resistors (0.25W 5%) 2 27k 1 6.8k 2 1001W 3 10 1 4.71W 2 0 1 1.2k 5W 1 20k 10-turn potentiometer September 2013  71 + INPUT 4.7 1W 1.2k 5W 27k 0 Q1 2N3055 C 10 E 100F 16V B 15 16 +5.1V + Q2 IRFB4020 D 13 2  72V SOLAR PANEL 100F 16V 5 7 K ZD1 10V 11 10 12 4 9 S 4 S G 1 560pF D Q3 IRFB4020 10 8 T1 14 L1 5T 23T 12 5T11 L2 23T 5T10 L3 1 560pF G IC1 SG3525 100 1W 10 14 6 100F 16V 47F 100V 5T 7 L4 8 100 1W 68nF A 6.8k 0 1.5nF 68nF 68nF –INPUT 72v SOLAR panel BATTERY CHARGER/REGULATOR K C Fig.1 is the complete circuit diagram. It suits a high voltage (~72V) solar panel, also available from Oatley Electronics. It will not work with the more common low voltage panels. them – it’s too easy to make a mistake! Note that there are two 0 resistors to be placed – these are on the top left of the PCB. The lone 5W resistor (1.2k) must be mounted standing vertical off the PCB. Next to go in are the low-profile capacitors, followed by the electrolytic capacitors. The marking for two electrolytics might confuse you: on the PCB screen overlay, they’re labelled as 22-100F. In the kit, they’re almost Construction All components mount on the top side of the PCB so construction is relatively simple. Start by checking the PCB for any defects and if all is OK, commence construction by soldering in the resistors. Use the resistor colour code table at right and/or check the values with a digital multimeter before you place INPUT 560pF S1 100 SR1060 4.7 47F – 68nF 68nF 100 SR1060 560pF IC1 SG3525 68nF TP 14 27k Q3 6.8k 10V 1.5nF ZD1 T1 D2 100F 10 68nF 1 K326-3 + Q1 2N3055 47F + + 100F IRFB4020 10 10 IRFB4020 0 0 Q2 + 27k – 1.2k 5W + 100F + Fig.2 shows the component placement on the PCB. All components as such mount on the top side of the board, as shown here, but you will need to place two links on the underside of the board (as shown in Fig.3 opposite) to set up your charger to suit a 12V or 24V system. certainly 47F 100V. Now it’s time to place the various semiconductors. Be careful not to confuse the Mosfets and Schottky diodes – they do look the same but are clearly identifiable – nor get them around the wrong way (see the circuit diagram and component overlay). The Schottky diodes have heatsinks fitted but these can be left until last as they will get in the way. The IC must be inserted in the right SET led to failure of one of the Mosfets. E RUN 2013 A + OUTPUT SC  2N3055 B ZD1 7 8 c oatleyelectronics.com VR1 20k FLOAT ADJ. D1 (TOP VIEW) 72  Silicon Chip siliconchip.com.au Resistor Colour Codes VOUT S1 RUN SET D1 SR1060 24V A1 12V K A2 12V A1 OUTPUT TO BATTERY 27k K 24V A2 D2 SR1060 TP VR1 20k 47F 100V 68nF 0V (GND) SR1060 A1 K IRFB4020 K G A2 D D S way around too – identify the notch at one end which goes towards the top. The collector of Q1 connects to the copper track underneath by means of a nut and bolt. Ensure there is good connection between the pad and the screw head – it might pay you to place a layer of solder on the pad first. After soldering in the switch (S1), the two terminal blocks and the trimpot (VR1), all that is left is the transformer. This is supplied as a pre-wound No. Value o   2 o   1 o   2 o   3 o 1 o 2 4-Band Code (1%) 27k red violet orange brown 6.8k blue grey red brown 100 brown black brown brown 10 brown black black brown 4.7 yellow violet gold brown 0 single black stripe coil on a bobbin, two ferrite cores and two clips to hold it all together. It’s vital that the primary side, which has only three connections, goes to the left-hand side looking at the board as shown below. Both sides have seven pins to solder to the board; the primary has three of these connected while the secondary has five. Pin 1 on the transformer is clearly identified – ensure it goes into the top left PCB hole. Finally, fit the heatsinks to the two Schottky diodes using 3mm nuts and bolts. A smear of heatsink compound on the metal of the diodes wouldn’t go astray. Links That almost completes assembly. All you have to do now is solder two links on the bottom (copper side) of the PCB which determine whether you have a 12V or 24V system. It’s quite easy to “bridge” between the appropriate pads with solder – just be sure you get the right pads! If you have any problem with solder not taking, use an offcut from a resistor or capacitor to make the small wire links. Setup After checking your component placement, you need to set up the charger. This can be done with a solar 5-Band Code (1%) red violet black red brown blue grey black brown brown brown black black black brown brown black black gold brown yellow violet black silver brown single black stripe panel connected or not. If you have a solar panel connected and it’s producing power (ie, it’s sunny!), disconnect the battery being charged and switch S1 to the “run” position. Measuring VOUT with a DMM, adjust VR1 to give 13.8V for a nominal 12V system and 27.6V for 24V. Leave S1 in the “run” position. If you don’t have a solar panel connected or if it isn’t producing power (eg, it’s dark!) you could simulate one by connecting, say, a 50V DC supply in series with a 50 resistor. Alternatively, without any input (solar panel or simulated), leave the battery in position and switch S1 to the “set” position. Connect your DMM to the test point (TP) and adjust VR1 so that it reads 1.885V (for 12V) or 0.94V (for 24V). Don’t forget to switch back to “run” when finished. Cabling All cabling should be run in a gauge not only heavy enough for the current but also with insulation more than capable of handling the ~90V which this panel can produce during bright sunlight. It’s more likely that your panel will produce less than this – say 7080V – unless it is tracking the sun, is kept scrupulously clean and is never SC shaded by trees or even posts. Where d’ya geddit? This kit is available exclusively from Oatley Electronics who hold the copyright on the design and PCB. A Solar Panel Regulator Kit and a FS272 CdTe High Voltage Solar Panel (as discussed in this article) are available for the special price of $119 plus freight, which varies according to your location (the panel is quite heavy!). Email branko<at>oatleyelectronics.com.au for a freight quotation. You can order online – www. oatleyelectronics.com – or by phone from Oatley Electronics during business hours (9am-4.30pm Mon-Fri) on (02) 9586 3564 siliconchip.com.au September 2013  73