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3 ELECTRIC FENCE TESTERS By JOHN CLARKE Do you need to test your electric fence to see if it is working? You could try the wet grass trick but then again you might get a shock. Why not build one of these three electric fence testers instead? MINI MIDI MAXI MAY 1999  37 I t’s all very well having a fancy electric fence installed to keep animals corralled but how do you know if it’s working properly? By the time you discover that the fence has a fault, you could be faced with a real roundup job. Of course, electric fences are not only used to keep ani­mals in a paddock but are often also used to keep animals away from a conventional fenceline. This particularly applies to horses. If the fence uses a large wire mesh, it’s all too easy for a horse to become entangled in the wire, panic and cause itself a serious injury. In fact, unless a trapped horse is released fairly quickly, it can die. One common way of testing an electric fence is to use the wet grass method. The technique is simple: all you have to do is take hold of a short length of wet grass and touch it against the fence. Because the wet grass is somewhat conductive, you’ll get a bit of a belt if the fence is working but the shock won’t be anywhere near as severe as if you touched the fence with your bare hands. The drawback with this method is that it’s a bit hit and miss. Because you’re (hopefully) only getting a “bit of a tingle” through the wet grass, you can’t tell how much “bite” the fence really has. That’s why some hardy souls choose the direct touch method but it’s not one that we recommend. If the fence is func­tioning properly, it will bite like a Northern Territory croco­dile. A far better way is to use one of the three electric fence testers described here. They will quickly indicate whether or not the fence is working and also indicate its effectiveness. For example, although the fence controller itself might be working correctly, there may be problems with the installation that make the fence ineffective. Common faults include poor conduction of the earth stakes, shorts between the high tension (HT) wires and ground, and breaks in the line. Shorts can be caused by long, wet grass brushing against the HT line and/or faulty insulators. Sometimes, the further away you get from the controller, the less effective the fence becomes. This commonly occurs if wet grass is loading down the controller’s output. It’s possible too for one section of the fence to go completely “dead”, due to a break in the line. For this reason, it’s a good idea to check all sections of the fence on a regular basis. Unfortunately, you can’t use a standard multimeter to test an electric fence. This is because the peak voltage on the fence can be as much as 10kV, with each pulse only lasting for 1ms or less. What’s more, the pulses only occur once every second or longer. So while there may be a significant amount of energy in each pulse, the multimeter does not integrate this into any mean­ingful reading. This is particularly true for digital multimeters which have a one or 2-second response time. These three fence testers can be used as more reliable aids for fence maintenance and, best of all, they do not induce an electric shock into the operator. Each contains a “light” which flashes to indicate fence pulse operation. Which one you use depends on what you want to do. We’ve called our three Electric Fence testers the “Mini”, the “Midi” and the “Maxi”. The first unit flashes a neon lamp each time it detects a pulse on the fence, while the second unit can measure the fence peak voltage (up to 10.8kV). The third unit is designed for permanent installation on the fence and flashes periodically if the fence is operating correctly. All three units are powered directly by the electric fence being tested. That way, there are no batteries to replace or leak if the unit has been left unused for some time. OK, let’s take a look at each of our fence testers in turn and find out how they work. WHY AN ELECTRIC FENCE TESTER? This project grew out of necessity: we needed a means of testing the output of the SILICON CHIP Electric Fence Controller, featured in last month's (April ’99) issue. We called for volunteers around the office to act as a tester using the old fingeron-the-fence-and-hope-itdoesn’t-hurt-too-much routine. But there were no takers! (Even Ross Tester refused to live up to his name . . .) So we looked at ways of testing the electric fence with38  Silicon Chip out getting a belt and found that there were several ways to do it – hence the three projects featured here. Incidentally, if all this talk about electric fences and controllers is foreign to you, it’s probably because you missed out on last month’s issue of SILICON CHIP. The high power electric fence controller shown here was described in detail in that issue. It’s easy to build, costs a fraction of commercial controllers . . . and back issues of the magazine are still available for $7.00 including P&P – a bargain in anyone’s language. “Mini” Electric Fence Tester Fig.1 shows the circuit of the Mini Electric Fence Tester. It is a low-cost unit that’s easily carried in a shirt pocket and can be quickly used to indicate whether or not a fence is work­ing. This is the simplest of the three units and uses just a neon indicator and three 330kΩ resistors. These parts are all mounted on a small PC board and there are two contacts, one at each end. In use, one contact (the finger pad) is held in the fingers and the other is touched onto the electric fence wire. If the fence is operating correctly, the neon indicator will briefly flash each time the HT wire is pulsed. The total resistance in series with the neon indicator (3 x 330kΩ) limits the current flowing from the fence and through your body to ground. In practice, this current is so low that the pulse will not be felt. Note that the light output from the neon indicator is quite low and you may need to shield it from sun­light so that it can be properly observed. By the way, this circuit is somewhat similar to the neon test screwdrivers that are sometimes used to test for mains voltages around power points and light switches. Do not, under any circumstances, use the Mini Electric Fence Tester to check for mains voltages. It’s not designed for this role. Conversely, do not use a test screwdriver to check the operation of an electric fence. This is because they are not rated for electric fence voltages and the resistance in series with the neon indicator may break down. Once damaged, the test screwdriver could present a serious electric shock risk if it is then used on the mains supply. Fig.1 (above): the mini electric fence tester is simply a neon lamp in series with enough resistance to stop you getting a belt! Building it Fig.2 : the PC board layout. Construction is simplicity itself! The hardest part will be soldering the wire loops. Fig.2a shows the assembly details for the PC board (code 11303994, 45 x 20mm). Install the parts as shown and make some wire loops at each end for the contacts. We used paper clip wire for the loops and soldered this directly to the copper pads. Alternatively, you could use small screws and nuts to Fig.2a : you hardly need a PC secure the wire in place. This latter board pattern as it is so simple method will ensure that the copper – but here it is anyway! pads don’t come adrift due to strain from the wire loops. Once the assembly is complete, the PC board can be wrapped in some clear heatshrink tubing, leaving the wire loops exposed. It’s not easy to see any components through the heatshrink but this photo gives an idea of construction. Parts List Mini Electric Fence Tester 1 PC board, code 11303994, 45 x 20mm 1 neon indicator, pigtail type 1 80mm length of 1mm diameter tinned copper wire or paper clip 3 330kΩ 1W resistors 1 45mm length of 25mm diameter clear heatshrink tubing MAY 1999  39 “Midi” Electric Fence Voltage Tester The Midi Electric Fence Voltage Tester is a slightly more elabo­rate instrument than the Mini Tester. It also uses a neon indica­tor but in this case the fence voltage can be read off a cal­ibrated scale after adjusting a single control knob. As shown in the photo, the unit is housed in a small plas­tic case and a small hole in the front panel allows the neon indicator light to be seen when it flashes. As before, the light output is quite low and you need to watch closely to see the flash. Fig.3 shows the circuit details. It’s really very simple and consists of a voltage divider and the neon indicator itself. In operation, the electric fence voltage is applied to a series string of 19 10kΩ resistors which in turn feed a 10kΩ potentiome­ter (VR1). The divided voltage is then tapped off from VR1’s wiper. Why use so many 10kΩ resistors? The answer is that they are necessary to provide a sufficient voltage rating for the divider, which could encounter fence voltages up to 10kV. Fig.3 (left): the midi electric fence tester is essentially a voltage divider across the fence high tension. The neon lamp glows when the fence voltage matches the scale voltage selected by the potentiometer. Fig.4a (above): the component layout on the PC board. Note the comments in the text about reversing the lead connections: you have been warned!!! 40  Silicon Chip Housed in a small utility box, the midi electric fence tester is ideal for occasional testing. The probe is as used in a multimeter. VR1’s wiper applies the divided voltage to the neon indica­tor via two 2.2kΩ resistors, while the common side of the circuit is connected to the ground stake on the electric fence. A neon indicator will light when the voltage across it reaches about 90V and so we use this characteristic to calibrate the potentiometer (VR1). If the wiper is wound fully towards the 10kΩ resistors, then the divider ratio is such that the neon will flash when there is 1.8kV on the electric fence. Conversely, as VR1 is wound towards ground, the division ratio increases and so the input voltage from the fence needs to be higher than 1.8kV in order to light the neon indicator. Let’s say, for example, that VR1 is set to its mid-posi­ tion. In that case, the fence voltage needs to be at least 3.6kV to make the indicator flash. One small complication with this circuit is that it will not produce reliable results unless the body of the potentiometer is well grounded. If this isn’t done, the neon indicator conducts the fast rise-time fence voltage into the air and hence shows a small flash, even if the pot is wound fully down. Although the pot body is grounded on the board via a PC stake (and ultimately to the fence ground), the inductance of the ground lead is enough to cause problems with fast rise-time voltages. For this reason, we have specified a metal knob for the pot so that it can also be grounded via your body. In practice, this means that measurements must be made with your hand holding the metal knob, to 99% of the assembly work in this project is soldering resistors! Fortunately, most are the same value. prevent false readings from occurring. When using the tester, the pot is initially wound fully clockwise and gradually backed off until the neon indicator just begins to flash. The fence voltage can then be read directly off the scale. Note that the overall resistance of this tester is 200kΩ, so it shouldn’t load down the fence voltage to any measurable de­gree. leads. These holes should be fitted with small rubber grommets. The pot shaft can now be trimmed to suit the knob, after which the PC board assembly can be mounted on the lid and secured using the pot nut. When fitting the knob, rotate the pot shaft fully clockwise, then tighten the grub-screw with the pointer towards the 10.8kV position. This done, feed the external leads through the grommets and solder them to the PC board. These leads should have good insula­tion to prevent any voltage breakdown between them. Use a green or black alligator clip for the earth wire connection and a red insulated probe for the fence terminal. This will prevent any confusion when you are making the connections to the electric fence. Warning! – if you reverse the connections to this tester, the body of the pot and hence the knob will be at the fence voltage. If the fence is working correctly, this means that you will get a nasty belt as soon as you touch the knob. Get the connections the right way around and you won’t have any problems. Building it Fig.4a shows the assembly details for this fence tester. It’s built on a PC board coded 11303993 and measuring 77 x 47mm. Start the assembly by soldering in all the resistors, then in­ stall PC stakes at the fence and ground inputs, at the three pot terminal positions and at the ground position for the pot’s body. The potentiometer can now be installed by soldering its terminals to the PC stakes and by soldering its body directly to the adjacent ground stake. You will need to scrape away some of the plating from the pot body near the PC stake, using a file or sharp knife, so that it can be soldered easily. The neon indicator has its leads bent at right angles before being soldered into posi­tion. It can be secured to the board with a dob of silicone sealant. The next step is to attach the front panel label to the lid of the case and drill the holes for the pot shaft and for viewing the neon indicator. You will also need to drill two small holes in the sides of the case for the external Figs 4b & 4c: the front panel and PC board artwork, reproduced same size for those who wish to make their own. Parts List Midi Electric Fence Tester 1 plastic case, 82 x 54 x 30mm 1 PC board coded 11303993, 77 x 47mm 1 front panel, 80 x 52mm 1 neon indicator, pigtail type 2 small rubber grommets 6 PC stakes 1 10kΩ 16mm pot. (VR1) 1 black or green aligator clip 1 red instrument probe 1 metal knob 1 1m length of blue or black 250VAC rated wire 1 1m length of red 250VAC rated wire 19 10kΩ 0.5W 1% metal film resistors 2 2.2kΩ 0.5W 1% metal film resistors MAY 1999  41 “Maxi” Electric Fence Voltage Tester Unlike the other two testers, the Maxi Electric Fence Tester uses a high-brightness xenon flash tube although the circuit is only slightly more complicated than before. It uses an internal capacitor to store up some charge from each fence pulse and when this reaches a critical level, the xenon tube emits a bright flash. This cycle is then repeated, with the tube flashing at regular intervals if the fence is operating correctly. As shown in the photos, the unit is housed in a clear plas­tic case and is designed to be permanently attached to the fence. Fig.5: the maxi electric fence tester has a somewhat similar circuit to the midi model but in this case fires a bright Xenon flash tube, the frequency depending on the voltage on the fence. 42  Silicon Chip The maxi fence controller, housed in a see-through and weatherproof plastic case. The Xenon flash tube is clearly visible through the case so this can be left permanently connected to the fence. We used the small plastic clips on the top of the case and cable ties to secure this tester to a suitable fence post. Fig.5 shows the circuit details. It uses a string of 18 820Ω resistors to provide current limiting and these drive a bridge rectifier consisting of diodes D1-D4. The output of the bridge in turn is connected to the xenon tube and to a parallel 0.47µF 630V polyester capacitor. The trigger pulse for the xenon tube is derived by connecting its trigger (T) terminal to a point higher up the resistor string. In operation, each fence pulse charges the capacitor by 10-40V, depending on the pulse amplitude. When the voltage across the capacitor reaches 200-300V, the xenon tube is ready to fire. It then fires when the next fence pulse takes the trigger input sufficiently high. When the xenon tube fires, the 0.47µF capacitor quickly discharges. The capacitor now recharges on each successive elec­tric fence pulse until the breakover voltage of the xenon tube is reached again. The flash rate depends on the fence voltage. The circuit draws about 0.5mJ per pulse from the electric fence which does not affect normal operation. This is why the circuit can be left perma- nently connected to the fence. Building it A PC board coded 11303992 and measuring 77 x 47mm accommo­dates all the parts – see Fig.6a. Begin by installing PC stakes at the two external wiring positions, then fit the resistors and diodes. Make sure the diodes (D1D4) are all correctly oriented. The capacitor is installed on the copper side of the PC board. Bend its leads at right angles so that the body of the capacitor can lie flat against the board before soldering it into position (see photo). This is necessary to allow the PC board assembly to fit into the specified case. The leads of the xenon tube must also be bent at right angles before mounting it on the board. Use needle-nose pliers to hold the leads adjacent to the glass body before bending them – if you don’t do this, you could crack the glass tube. This done, solder the tube into position and don’t forget the trigger lead. You will need to drill two holes in the sides of the case for the external leads. Fit these holes with rubber Front (above) and rear (right) views of the completed PC board. Note that the 0.47µF discharge capacitor attaches to the copper side of the board. grommets, then pass the leads through and solder them to their respective PC stakes on the PC board. It’s a good idea to use a red lead for the HT connection to the fence and a blue or green lead for the fence ground connection. As with the previous design, these leads should have good insulation, to prevent any high-voltage leakage between them. The PC board is designed to clip into the case against the integral side pillars. If necessary, you can lightly file the sides of the PC board so that it is a neat fit. Because it will be exposed to the weather, it’s necessary to seal the wire entry holes and the case lid using silicone sealant. Before doing this, however, it’s a good idea to test the circuit to make sure it works correctly. That way, if you do have a fault, you can easily remove the board from the case and check for missed or bad solder joints, or incorrect component place­ment. Finally, you will have to figure out some way to mount this unit. This may involve fashioning a suitable clamp or you can do what we did and fit a couple of small plastic clips so that the unit can be tied to a convenient fence post using tie-wire. The HT lead can be attached to the electric fence using a suitable electric fence joiner, while the ground lead can be attached directly to a ground SC stake. Figs 6a & b: follow the PC board overlay (left) and you should have no problems assembling the board. The full-size PC board pattern above can be used to etch your own board or to check commercial boards before assembly. Parts List Maxi Electric Fence Tester 1 clear plastic plastic case, 82 x 54 x 30mm 1 PC board, code 11303992, 77 x 47mm 1 straight 32mm-long xenon flashtube 2 fence clips 2 PC stakes 1 1m length of green or blue 250VAC rated wire 1 1m length of red 250VAC rated wire 2 electric fence wire joiners 4 1N4936 1A fast diodes (D1-D4) 1 0.47µF 630V polyester capacitor 1 220kΩ 0.5W 1% metal film resistor 18 820Ω 0.5W 1% metal film resistors MAY 1999  43