Tapes for electric fences are available in various qualities and forms, with differing characteristics. They are usually made by interweaving threads in polyethylene, nylon, or some other synthetic material with several strands of wire in stainless-steel, copper, or some other conductor, of relatively small diameter (from one to several tenths of an mm in diameter). For information, a 1 m-long stainless wire has a resistance of around 23 Ω for a diameter of 0.2 mm, and hence 5.75 Ω for a diameter of 0.4 mm. Thus a tape’s linear resistance may vary from a few milliohms to several ohms per meter, depending on the number of strands, their diameters, and the nature of the conductors. But don’t worry, you won’t have to calculate it, just measure it — assuming it isn’t specified by the manufacturer. Some rudimentary tests show that a 2 Ω/m tape carrying a current of 1 A raises the temperature (inside foam pipe insulation) by around 15 °C. Thus in theory, to withstand temperatures down to −15 °C (5 °F), it would be necessary to dissipate 2 W/m inside foam pipe insulation. Thus even with just a simple 50 VA transformer, it is possible to perfectly simply cover 25 m of PE piping (polyethylene, and hence insulating).
Since we have a choice of linear resistance, we can produce a heating tape of a given length while powering it from a safety voltage (less than 50 VAC) with no danger for either us or for animals. So we have
P = V² / R = R × I² = 2
with P in W/m, R in Ω/m and V in V/m. If L is the total length in meters and since VTO.
TAL < 50 VAC, then R < 1250 / L² [Ω/m] and we need I > L / 25 [A]. Knowing that for 2 W/m, V = √(2R) and I = √(2/R), we can work out everything.
However, we do need to take care not to use a current likely to upset the temperature measurement — excessive heating of the driver transistors could disturb the operation of the circuit. The example below will cope with 2 A without any problems. Construction is based, on the one hand, on the use of two IRFR3607 power MOSFETs (RDS(on) 9 mΩ, VDS(max) = 75 V) and on the other, on the LT1172, a thermostat operating at 0 °C (push-pull output, 2 °C hysteresis, ultra-low power consumption of 40 µA maximum @ 5 V, SOT223 package). An LED will indicate that power is present, and another could be put in parallel with the tape. Resistors R2–R5 (SMD 1206 shape) have been used in order to be able to handle the dissipation for the proposed voltage range while ensuring 3 mA in the zener diode, but if the voltage is reduced, the number of resistors can be reduced proportionately. The HYST pin of the LM26 is returned to the 5 V rail in order to select hysteresis of 2 °C. C1 is chosen primarily with respect to the CISS of the MOSFETs — it must be enough to maintain the charge on them without significant loss of gate voltage (5 V here).
On the PCB , the sensor has been kept apart in order to avoid its operation being disturbed by the 0.6 watts dissipated in R2–R5 and the power dissipated by the transistors. The copper planes even out the temperature around the sensor. The board should be pseudo-tropicalized with four coats of transparent varnish, as it is going to be mounted outdoors. The tape has to be prepared, and this is perhaps the most tedious part of the job. Normally, the installation will require a current return conductor — unless you decide to use tape for both feeds and return, either doubling the power or reducing the current by a factor of √2 to compensate. However, you are going to have to unravel the ends of the tape in order to make positive, reliable connections. The tape used is 2 cm (0.8 inches) wide, so you can fix the return wire and insulate it completely where it passes metal elbows and tees using 5 cm (2 inch) wide adhesive ‘duct’ tape, available in any DIY store. A more expensive solution is to use heat-shrink sleeving. To complete the connections, all you need is a soldering iron and some ring terminals and terminal blocks. Then, you still have to attach the tape to the piping… If you’re using automatic drinking troughs, you may need to make a loop under the trough to heat that too; don’t forget to reposition the foam pipe insulation properly. Lastly, position your board outdoors,
high up (2 m / 7 ft.) and preferably horizontal for greater effectiveness. Going about it this way, the piping will be heated up before it is affected by freezing.