Understanding Electric Fence Tape Composition
Electric fence tapes come in diverse qualities and configurations, each with distinct characteristics. Typically, these tapes are crafted by interweaving threads of polyethylene, nylon, or other synthetic materials with multiple strands of wire, such as stainless steel or copper. These wire strands have relatively small diameters, ranging from one to several tenths of a millimeter. For context, a 1-meter-long stainless wire with a 0.2 mm diameter exhibits a resistance of approximately 23 Ω, increasing to 5.75 Ω for a 0.4 mm diameter. Consequently, a tape’s linear resistance can fluctuate from a few milliohms to several ohms per meter, contingent on factors like the number of strands, their diameters, and the conductor materials. While calculating these values isn’t necessary, measuring them becomes crucial unless provided by the manufacturer.
Temperature Considerations for Electric Fence Tapes
Essential tests indicate that a 2 Ω/m tape carrying a current of 1 A can elevate the temperature (inside foam pipe insulation) by approximately 15 °C. In theory, to endure temperatures as low as -15 °C (5 °F), it is essential to dissipate 2 W/m within foam pipe insulation. Remarkably, with a basic 50 VA transformer, it’s feasible to effectively cover 25 m of polyethylene piping, known for its insulating properties. This simplicity underscores the practicality of the solution for various applications.
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.
Ensuring Temperature Stability in Tape Heating System
Maintaining precise control over the current used is crucial to prevent any disturbances in the temperature measurement and subsequent circuit operation. In the provided example, the circuit can handle up to 2 A without encountering issues. It employs two IRFR3607 power MOSFETs (with RDS(on) of 9 mΩ and VDS(max) of 75 V) and the LT1172 thermostat. The LT1172, designed to operate at 0 °C with a push-pull output and 2 °C hysteresis, ensures stable performance.
LEDs serve as power indicators, with the possibility of adding an additional LED in parallel with the tape. Careful consideration has been given to resistor selection (R2-R5, SMD 1206 shape) to manage dissipation across the proposed voltage range while maintaining 3 mA in the zener diode. The hysteresis value of 2 °C is selected by connecting the HYST pin of the LM26 to the 5 V rail. The choice of capacitor C1 is primarily influenced by the CISS of the MOSFETs, ensuring sufficient charge retention without significant gate voltage loss.
PCB Design and Tape Preparation
In the PCB layout, careful segregation of the sensor is employed to prevent interference from the power dissipation in R2-R5 and the transistors. Copper planes help in temperature uniformity around the sensor. The board should be pseudo-tropicalized with four coats of transparent varnish, given its outdoor installation. The tape preparation, albeit tedious, is vital. Typically, a current return conductor is necessary, unless you opt for using the tape for both feeds and return, either doubling the power or reducing the current by a factor of √2 to maintain balance.
Ensuring reliable connections involves unraveling the tape ends and making secure connections using a soldering iron, ring terminals, and terminal blocks. Using adhesive ‘duct’ tape or heat-shrink sleeving provides insulation where the tape passes through metal elbows and tees. Additionally, for automatic drinking troughs, creating a loop under the trough ensures effective heating. Mounting the board outdoors, preferably at a height of 2 m (7 ft.) and in a horizontal position, guarantees efficient heating, preventing freezing before the pipes are adequately warmed.