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Build a solar-powered electric fence Electric fences are ideal for controlling livestock or protecting vegetable gardens. This low-cost design runs off a 12V battery & can power fence runs up to several kilometres long. By BRANCO JUSTIC Most electric fence controllers are expensive but this design can be built for between $60 and $130, depending on which options you choose. If you elect to power it from the Sun (via a solar cell array and voltage regulator), you can set it up in a paddock and forget about it. There's no need to worry about recharging the battery at regular intervals. 14 SILICON CHIP When switched on, the control unit delivers short high-voltage pulses to one or perhaps two lengths of bare wire that form the fence. These wires are supported by insulators which can be mounted on the existing fence posts or on temporary stakes driven into the ground. To keep costs down, the design uses an EHT transformer that was origi- nally made for use in monochrome computer m9nitors. It can deliver quite a healthy kick - 2.3kV into an open circuit and ZkV into a soon load. These voltage "kicks" are delivered at 1.5-second intervals and have a pulse . length of 250µs into an open circuit. More importantly, these figures are well within the specifications laid out by the Standards Association of Australia in AS3129. This standard specifies a maximum peak output voltage of 5kV into a lMQ load and a maximum 'pulse length of 0.ls into soon. The period between pulses is specified as greater than 0.75 seconds. Because terminal 2 of the EHT transformer has a relatively low output impedance, the unit can be used to power quite long fence runs if necessary (up to several kilometres). Its average current consumption is about 25mA but this can easily be reduced to about 15mA if the unit is used to power fence runs of less than 1km. How it works Refer · now to Fig.1 for the circuit details. At the heart of the circuit is a DC-DC inverter which steps up the battery voltage to 350V. This voltage appears at the output of bridge rectifier Dl-D4 and charges capacitors C6 & C7. These capacitors are then discharged via the primary of the EHT transformer using a trigger circuit based on UJT1 and SCR1. Let's now look at this is more detail. IC1a, Rl & Cl form a Schmitt trigger oscillator which runs at about 300Hz. Its output appears at pin 11 and drives Mosfet QZ via gates IC2a & ICZb. It also drives Mosfet Ql via gates IC1b, ICZc & ICZd. Because there is an extra stage of inversion in Ql's drive circuit, it is driven 180° out of phase with respect to QZ. Fig.1 (below): the circuit uses a DC-DC inverter (ICl, IC2, Qt & Q2) which steps up the battery voltage to 350V. This voltage appears at the output of bridge rectifier D1-D4 & charges capacitors C6 & C7. These capacitors are then discharged via the primary of the EHT transformer using a trigger circuit based on UJTl and SCRl . Thus, when pin 11 of IC1a goes high, QZ turns on and Ql turns off. Conversely, when pin 11 of IC la goes low, Ql turns on and QZ turns off. Ql & QZ are therefore switched on and off alternatively by the oscillator to provide push-pull drive for step-up transformer Tl. Tl is a 9V centre-tapped mains transformer and is used here back-tofront (ie, the 9V winding is used as the primary). When QZ turns on, the top of the primary is switched to ground and 12V appears across the top half of the winding. This means that 12V also appears across the bottom half of the primary, with Ql's drain going to +24V. Similarly, when Ql turns on, the bottom of the primary goes to ground and QZ's drain goes to +24V. Thus, Ql & QZ alternatively switch each end of Tl's primary winding between +24V and ground. The AC output from the step-up transformer is rectified by diodes D1D4 to produce a DC voltage of about 350V. This then charges C6 & C7 via R10 (180kQ). Actually, the inverter has the potential to charge C6 & C7 to over 600V if it ran continuously but this is not allowed to happen. Voltage regulation To keep the charging voltage to 350V, the inverter output is regulated The lead at the top of the EHT transformer must be cut off & the stub then fitted with heatshrink tubing & covered with a generous amount silicone sealant. using a feedback circuit based on IC1c and voltage divider R2-R4 across the bridge rectifier output. It works like this: when the bridge rectifier output reaches 350V, pins 1 & 2 of IC1c will be at approximately 6V (ie, half the 12V supply rail). The output of IC1c (pin 3) will thus switch low and disable gates ICZa & IC2c. This effectively removes the drive to Ql & Q2 and so the inverter switches off and the output voltage begins to fall. Pin 3 of IC1c then switches high again a short time later when the rectifier output drops below a critical level and this turns the inverter back on again. Thus, the inverter is rapidly .--------+------------------------0+ 12V R1 390k 4093 01-04 4x1N4004 T2 r-------7 C1 .022+ 1 EHT +12V ~ GDS PLASTIC SIDE m E B2O81 ':' R7 1.SM A OUTPUT VIEWED FROM BELOW KAO ELECTRIC FENCE cs• 1+ ':' GND ':' ':' APRIL 1993 15 ~3cJY1ACT As an option, this battery can be kept topped-up by the low-cost solar charger described in last month's issue (the 4-cell array would do the job quite nicely). Note that the unit is not designed for use with a plugpack supply. It is designed fur battery operation only. Construction FENCE TERMINALS 12V BATTERY + Fig.2: this wiring diagram shows the fence controller PC board (top) plus the optional solar panel regulator board described last month. Be sure to use heavy duty cable for the output connections & do not apply power to the unit until it is installed in a case, as high voltages are present during normal operation. switched on and off by the feedback circuit so that it produces a regulated 350V output. This regulated output charges C6 & C7 to virtually the full' 350V in less than half a second via current limiting resistor Rl0. The inverter thus operates only as necessary to maintain this charge. Unijunction transistor UJTl is employed as a simple relaxation oscillator. In operation, C5 charges via R7 towards the +12V supply rail. When the voltage on its emitter exceeds 0.67 x 12V = 8V, the UJT conducts and Cs quickly discharges via the E-B1 junction. This cycle then repeats itself at 1.5-second intervals, as set by C5 & R7. Each time the UJT conducts, it produces a positive trigger pulse at its B1 terminal and this triggers SCRl via R9. C6 & C7 now discharge via SCRl and the primary winding ofEHT transformer T2 which produces the output voltage for the electric fence. Note that the output is taken from terminal 2 of the EHT transformer (not from the EHT lead which emerges from the top of the transformer). Power supply Power for the circuit is derived from a 12V lead-acid or sealed gel battery. Most of the parts for the fence controller, including the EHT transformer, are accommodated on a PC board measuring 14 7 x 71mm. This board and the voltage regulator board are supplied as a single piece and must be separated using a hacksaw. Fig. 2 shows the wiring details. Before mounting any of the parts, the first step is to cut off the EHT lead where it emerges from the top of the transformer. The top of the "stub" must then be insulated by applying a generous coating of neutral cure silicone sealant. Apply several coatings if necessary and leave the unit to cure for at least 24 hours. The various parts can be mounted on the board in any order, although it's best to leave Tl, T2 and the two 0.47µF capacitors until last. Take care with the orientation of the two ICs and don't forget to install the wire link that runs adjacent to IC2. A second wire link is installed between Rl and ICl (a trimpot is shown on the board silk-screening but is not used), while a third link is installed in the Link A position on the PC board. Note that the Link B position marked on the board must be left vacant. Although IC sockets were used on the prototype, these are not really necessary. In fact, the circuit will be more reliable without them. RESISTOR COLOUR CODES 0 0 0 No. 2 .o 0 0 0 0 16 1 3 1 SILICON CHIP Value 4-Band Code (1%) 5-Band Code (1%) 10MQ 1.5MQ 390kQ 330kQ 180kQ 47.Q 10.Q brown black blue brown brown green green brown orange white yellow brown orange orange yellow brown brown grey yellow brown yellow violet black brown brown black black brown brown black black green brown brown green black yellow brown orange white black orange brown orange orange black orange brown brown grey black orange brown yellow violet black gold brown brown black black gold brown shown in Fig.2. The EHT transformer (T2) is secured using its integral bracket assembly - don't forget to solder its pins on the underside of the board. The PC board assembly can now be completed by connecting flying leads to the supply inputs and to the output terminals. Use heavy-duty insulated cable for the OUTPUT & GND leads (eg, mains cable or heavy-duty automotive cable). The supply leads can be run using medium-duty cable. Do not apply power to the board at this stage. That step comes only after the board has been mounted inside its case to prevent the possibility of severe electric shock (see warning panel). Solar charger . ' The control board is installed in the case after the baseplate has been installed, to allow access to the baseplate mounting screws (see text). Delete the regulator board if you don't wish to use a solar panel to recharge the battery. If you intend using the controller with short fence runs (less than 11cm), you can leave one of the 0.47µF capacitors out of circuit. This will reduce the average current consumption to about 15mA while still giving the fence plenty of "bite". The two Mosfet transistors (Ql & Q2) are installed with their metal tabs towards the adjacent edge of the PC board. Push them down onto the board 43 as far as they will comfortably go before soldering their leads. The same goes for SCRl - its metal surface also goes towards the adjacent edge of the board. Transformer Tl is now temporarily secured to the PC board using machine screws, nuts & star washers. Orient the transformer so that its lowvoltage windings face towards the ICs and terminate the leads at the points 71 HOLES: 3.5mm DIA. C, "' a, "' 145 155 MATERIAL: 1.5mm ALUMINIUM OR PCB BLANK DIMENSIONS IN MILLIMETRES Fig.3: this diagram shows the dimensions of the baseplate & the locations of its mounting holes. Not shown are the mounting holes for the two PC boards. The optional solar charger circuit consists of two assemblies: (1) a 4-cell solar panel array; and (2) a switching regulator circuit. These should be assembled exactly as described in last month's issue. Note that for short fence runs, with only one 0.47µF capacitor installed on the control board, a 2-cell array would probably be adequate for charging the battery. However, the 4-cell array would still be advisable in areas of frequent cloud cover. Case assembly Once the two boards have been completed, they can be installed in a waterproof plastic case. The recommended case is a "Bopla" wall-mounting box that's currently available from Jaycar for $14.95 (Cat. HB-6040). It is made from high-impact plastic and features a hinged transparent door with a rubber seal. The case is divided into two sections: (1) an upper section about 115mm deep with integral slots for a PC board; and (2) a lower section about 47mm deep located behind a removable front cover. As with the lid, this cover is fitted with a rubber gasket to ensure a waterproof seal. The two PC boards are mounted on an insulated baseplate which is in turn secured to integral tapped holes in the back of the case. A piece of blank PC bqard material measuring 155 x 95mm was used as a baseplate in the prototype but Perspex would do the job equally as well. Note that you will have to make a 12mm square cutout at each corner of APRIL 1993 17 Use a waterproof plastic case to house the circuit boards, to ensure long-term reliability. The case specified in the parts list is ideal for the job & comes with rubber gaskets to seal the lid & the sub-panel blow it. the baseplate to clear the mounting columns inside the box (see Fig.3). Drill mounting holes along the edges of the baseplate as shown in Fig.3, then temporarily install the baseplate in the case. Control board mounting The fence control board is mounted on the baseplate using three 6mm spacers plus an additional nut between each spacer and the baseplate. 1\vo of these spacers sit directly under transformer Tl's mounting holes REGULATOR BOARD CASE RIB 18 SILICON CHIP (just remove the mounting screws that were previously fitted). The third mounting point is drilled in the top right-hand corner of the controller board, immediately beneath the "G" of the "Danger High Voltage" warning. Drill this hole to 3mm, then install the controller board inside the case and use it as a template for marking out its mounting holes on the baseplate. The baseplate can now be removed from the case and the extra holes drilled to 3mm. This done, fit a 12mmFig.4: mounting details for the regulator PC board. Note the nut under the 6mm spacer where the board is secured to the case rib. long screw and nut at each board mounting location. At this stage, you will also have to drill two 3mm mounting holes in the regulator board - one in the corner near the input terminals for the solar panel and the other in the diagonally opposite corner (be careful not to break the continuity of the earth track). This board is mounted by securing one corner to the baseplate and the other (nearest the solar panel terminals) to a rib moulded into the back of the case - see Fig.4. The trick is to first attach the regulator board to the baseplate using a 6mm.spacer, screw, nut and washer. The assembly can then be slid into the case (regulator board first) and the extra mounting hole marked and drilled in the plastic rib. At the back of the case, behind the rib, is a narrow channel about 3mm wide and 5mm deep. You will have to file the head of the mounting screw slightly so that it fits into this chan- WARNING! PARTS LIST The internal wiring of the Electric Fence Controller is highly dangerous. There are potentially lethal voltages present on the PC board when the electric fence is running and, if there is a fault, these can remain even after the power has been switched off. In particular, be wary of the two 0.47µF capacitors. These capacitors are dangerous when charged and can only be regarded as "safe" when the voltage across them is below 20V. · As a safety precaution, never apply power to the board while it is outside its c~se. If you do have to work on the board, disconnect the power and check that the voltage across the two 0.47µF capacitors (C6 & C7) is below 20V before removing the board or touching any of the parts. You can check this voltage by measuring between the anode (centre terminal) of ScR·1 and link A. (Note: if the trigger circuit is operating correctly, it will discharge C6 & C7 as the circuit "powers down"). You should also take care with the installation of the unit. It should not be installed where members of the public are likely to come into contact with it and any installation should be clearly identified with large warning signs at regular intervals. The control unit should be moµnted in a position where it is free from mechanical damage and any wiring should be kept well away from any electrical or telephone cables. 1 PC board (Oatley Electronics) 1 9V CT mains transformer (T1) 1 EHT transformer (T2) 1 weatherproof plastic case · (Jaycar Cat. HB-6040) 2 front panel labels 1 solar charger kit - switching regulator plus solar cells; see March 1993 issue (optional) 2 large insulated screw terminals 1 cordgrip grommet 5 6mm-long spacers 5 3mm x 12-mm long screws 9 3mm nuts 5 lockwashers ELECTRIC FENCE and the latter secured using screws, nuts and lockwashers. Final wiring CONTROL BOX + METAL STAKE DRIVEN INTO GROUND II V \ ,i " Fig.5: conneciion details for the fence controller (power supply & solar panel option not shown). The GND terminal is connected to a metal stake that's driven into the ground. nel. This done, install the screw, secure it with a nut, and fit a 6mm spacer - see Fig.4. The regulator board and baseplate assembly can now be secured to the case. Finally, three 6mm-long spacers can be dropped into position on the baseplate for the fence controller board The external w1rmg can now be completed as shown in Fig.2. This involves the wiring to the solar panel, power supply and fence terminals. Run colour-coded leads for the battery and solar panel through a cordgrip grommet in the bottom of the case and fit them with a short length of plastic sleeving where they pass through the grommet to ensure a waterproof seal. The fence output leads are connected to solder lugs and terminated on two large insulated screw sockets mounted on the sub-panel. You can use the label as a drilling template for the holes for the screw terminals. Note that this label should be sprayed with a hard-setting clear lacquer to protect it from the weather. After that, it's simply a matter of fitting the external leads with suitable connectors to go to the battery and solar panel. You will also have to make up suitable leads to connect to the fence. These will typically be terminated with eyelet connectors at one end to connect to the control box and alligator clips at the other. Sleeve all connections with heatshrink tubing for weather protection. Note that the GND connection goes to a metal stake that's driven into the ground, while the "+" terminal goes Semiconductors 2 4093 quad Schmitt trigger NANO gates (IC1 ,IC2) 2 MTP3055 Mosfets (01 ,02) 1 2N2646 unijunction transistor (UJT1) 1 C1060 SCR (SCR1) 4 1N4004 silicon diodes (01 -04) Capacitors 1 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 2 0.47µF 250VAC polyester 1 .022µF metallised polyester • 2 .0033µF ceramic Resistors (0.25W, 5%) 2 10MQ 1 180kQ 11.5MQ 347Q 1 390kQ 1 10Q 1 330kQ Miscellaneous Medium & heavy-duty hook-up cable, alligator clips, battery connectors, heatshrink tubing. Where to buy the parts Short form kits for this project are available from Oatley Electronics, PO Box 89, Oatley, NSW 2223, Australia. Phone (02) 579 4985. Prices are as follows: Fence controller board plus onboard components ............... $40 Solar regulator board plus onboard components ............... $10 Four: 6V 1W solar cells (does not include mounting plate) ........ $32 The Bopla weatherproof case (Cat. HB-6040) is available for $14.95 from Jaycar Pty Ltd, PO Box 185, Concord 2137. Note: copyright of th~ PC boards is retained by Oatley Electronics. APRIL 1993 19 Fig.6: here are the full-size front panel artworks for the fence controller. The top label can be attached to the inside of the lid, while the bottom label is attached to the sub-panel on the bottom of the case. Make sure that the latter is suitably we~therproofed (eg, by spraying the label with a clear hardsetting lacquer). ELECJRIC FENCE CONTROLLER POWER SUPPLY: 12V DC, 25mA (BATTERY ONLY) WARNING! HIGH VOLTAGES INSIDE REFER SERVICING TO QUALIFIED PERSONNEL ONLY TERMINALS BENEATH COVER rn TO SOLAR PANEL FENCE TERMINALS GND + + + TO BATTERY to the fence wire. Fig.5 shows the details. Testing The output leads should be fastened together using plastic cable ties & the output terminal connections sleeved with heatshrink tubing., 20 SILICON CHIP To test the unit, connect it as shown in Fig.5 and apply power. Make sure you get the supply polarity correct, to prevent damage to the circuit (do not fit a reverse polarity protection diode in series with the battery leads, as it will prevent the battery from charging). If the unit is working correctly, you will hear a faint "ping" in the wire at about 1.5-second intervals. Finally, take care with the installation of the unit. It should not be installed where people are likely to come into contact with it and any installation should be clearly identified with warning signs at regular intervals. The control unit should be mounted in a position where it is free from mechanical damage and any wiring should be kept well away from any electrical or telephone cables. SC