Electrical Fencing System

We are describing an electrical pulse generator here that has been developed keeping in mind for pulsed electrical fencing. The electrical pick pulse voltage shall be around 1500 volts. Hence, the electrical pulsed shock is injurious but not fatal.

Application for a high voltage spike are numerous; electromagnetic and radio frequency interference studies; electrostatic discharge simulation; investigation of insulation breakdown; strobe effect etc. A DC Power supply or 12V battery is required; and pulse potential may be varied simply by changing the supply voltage. With the 12V DC input the output transformer shall delivers its maximum pulse but a unique multi-vibrator driver circuit makes operation possible even the supply voltage shall be down to 10V, but the output pulse shall be few hundred volts. Its pulse frequency shall be set by potentiometer R8 (2.2Meg) which shall be mounted on the front panel of the high voltage pulse generator enclosure. The range of the frequency shall be from 0.3 Hz to 30 Hz. The output voltage is directly dependent on the fence load Short fencing load shall be much more dangerous. Typical load shall be 400 to 600 Meters depending on the fence wire diameter. Further dirt and moisture shall increase the resistivity of the fence thus decrease the pulse voltage. The return path of the fence is very important.

Two lamps (one incandescent and one neon) on the circuit board shall be mounted in such manner so that these two indicators will be visible from the front panel and those two indicators are the visual indication of the unit’s performance.

PRECAUTIONS: As the output pulse voltage is high enough it might be rarely traumatic, the resulting-reflex muscle contraction could have unfortunate consequences. If a continuous train of pulse causes to involuntarily grasp the high voltage conductor, for instance, the human being might not to be able to let go. On the other hand, if a proper return circuit is not provided, an equally distressing shock could be had by contact with the primary circuit. So, one side of the output transformer should be connected to the earth as mentioned in the circuit (pin no -4 of the O/P transformer) that precaution besides preventing shock by contact with the power leads, also precludes arcing within the power supply itself as the high voltage seeks the shortest return path.

Applying that reasoning to the fence charger options, we can see why a fixed pulse rate is specified, as there is a strong likelihood of accidental human contact with the fence wire: a rate of 60 pulse per minute is less being considered safe. Also, there is a good chance of personal contact with the power or battery leads and a good ground connection is essential as with many electrical fence system. For maximum safety, it is highly essential to use battery supply for the fence charger system. The reverse power supply leads to the circuit, the current limitation lamp lights brightly as warning. However, the equipment must not be allowed to remain this condition for more than few seconds to ensure the permanent damage in the circuitry. It is very essential to include the lamp to limit the excessive surge current that could otherwise occur under some operating conditions which could blow the costly power transistor.

ABOUT THE CIRCUIT: The free running variable multi-vibrator using ‘TR1’ and ‘TR2’ drive Darlington power transistor ‘DT’ which makes and breaks the primary current to transformer ‘T’. Duty or dwell is a few milliseconds and the high voltage pulse is generated and at the end of the period when the circuit disconnected and the field of ‘T’ rapidly collapse through the winding.

An unconventional multivibrator circuit has been developed to provide high saturation current over a wide range of supply voltage. In this circuit both the transistors TR1 & TR2 conduct at the same time and both cut off at the same time. Another unique feature, for safety in the fence charger application in such a manner so that the circuit automatically shut down if the driver transistor TR2 would fail to conduct for any reason (fluctuation of supply voltage and/or intermittent connection etc.).

The series combination of C2 and C3 has negligible effect on the charging rate, which is therefore determined mainly by C4 with the series combination of ‘Rate’ control R8+R9 (or R9 alone, for the fixed frequency version).

Capacitor C4 discharges fully and then begins charging in the opposite direction as its negative terminal rises above zero volts. When TR1 begins conducting and its collector voltage has dropped far enough to start TR2 conducting also, then a positive feedback action is initiated forcing both the transistor into saturation. At the same time power transistor ‘DT’ is turned ON by the current supplied through R3. The Duty Cycle or Dwell is determined by the same constant ‘R5’ and ‘C4’, when the charging current of C4 diminishes below the value that will sustain conduction of TR1, then the regenerative action is again established and time cutting of all three transistors. It is at the moment the high voltage pulse is generated.

Capacitors C2 and C3 form a voltage divider that ensures rapid cutoff of transistor TR1, while C2 acts as a bypass to prevent TR1 from being retriggered by pick-up of the high voltage pulse. The duty cycle must be long enough to permit the field around transformer to be fully developed to its steady state condition under all anticipated conditions of loading. Although the period is not critical it may be set for optimum results with a particular transformer.

A higher capacitor value of C4 is specified for a fixed frequency fence-charger version for reasons of safety. It allows the use of a lower resistance value for R9reducing the stunting effect of dirt and moisture which might otherwise cause a significant increase in the repetition rate. For this it is suggested to use the axial type capacitor of C4 so that its pads in PC board are widely spaced than a radial type.

POWER AMPLIFIER: Because the field of Transformer as might be supposed collapses through the primary as well as secondary the inductive surge comprises a positive pulse on the collector of Darlington transistor ‘DT’, the capacitor C6 is required. Nevertheless, that the reactive voltage reaches several hundred volts and it is necessary to take the advantage of it to light the Neon indicator. Each flash verifies the integrity of the power amplifier circuit.

If no arc is drawn the positive pulse on the collector of DT is followed by a negative going excursion. Transistor DT is used as that has been designed for inductive load and that contains a shunt diode which prevents that backswing from being applied to the base through the base collector junction. The diode also protects transistor DT if the power supply leads are accidentally reversed.

Automotive lamp ‘L’ limits the surge current occurring as a result of various normal operating conditions as well as accidents such as reversal of power supply polarity and it absorbs the energy of backswing.

TRANSFORMER: For the fence charging system one commonly available 12V/ 1Amp to 230V transformer which has been made with good quality CRGO core materials can be used. The 230V winding end will be put up as the secondary (i.e. at the output stage of the fence charger) and the 12V winding end will be set up as the primary. The rapid collapse of its field when DT cuts off as compared to the relatively slow 50 Hz sine wave for which it is designed explains how several thousand volts can be developed across the 230V winding (E = L. di/dt). That winding will typically be found to measure 30 to 120 ohms DC while the 12V winding will have a resistance of around 1 Ohm. It is very much important to keep in mind that most of the commonly available transformer has been rated for a break down voltage of 1500 volts rms. But selection of transformer should be such that the break down voltage of the transformer should be more than 5-kilo volts. Therefore, at the time of manufacturing of the transformer sufficient insulation and impregnation of the winding is very much important as the selection criteria of the fence charger system.

POWER TRANSISTOR: Further a transistor type (MJE 5742) is specified for DT. The transistor is rated at 400 volts under heavy inductive load. The transistor could be substituted by another one transistor which shall operate on 400 volts and withstand on heavy inductive loads.

Potentiometer R8 for the variable pulse generator project can be any 2.2 to 2.5 Meg ohms linier type but for the better convenience the potentiometer would be connected in counter clock wise end. The time constant network (C4 & R5) determines the ‘dwel’ or ON time. But, it is very important that the capacitor should be a very good quality part with an excessively high equivalent series resistance (ESR) then only dwel may turn out to be greater than necessary to serve the needs of the transformer. Therefore, to be in the safe side a tantalum type capacitor (C4) shall work more perfectly.

LAMP: The main purpose of using the incandescent lamp in series with the circuit is that, normally the lamp will not glow at all but, if any short circuit occurs on the fencing instantly the lamp will glow. The reason of explanation is that, the merit of an incandescent lamp as a protective device lies on the dependence of its resistance (filament resistance) on the current flow. The filament resistance in cold or normal condition is of only ½ ohms but in case of current surge or accidental short circuit its resistance quickly rises to a hot value of around 6 to 8 ohms sparing power amplifier DT from the devastation requirement of breaking an excessive current into an inductive load. For the better spark at high frequencies it is suggested to select the lamp that the resistance in hot condition shall be approximately 4 ohms. In this case it shall be better to use a double filament lamp in place of single filament lamp.

Bill of material and Circuit Schematic

Electronic Circuits – Simplified