Electric Cow Fence
by G8MNY
 This
is an active cow fence, ideal for those Portable contests.
It is quite safe for cows, but may not be suitable for smaller animals,
or shy ones unless they are used to electric fences!
Although a car ignition coil can put out 30KV (3cm spark) when there
is no load, a 100M or so of electric fence wire 1m high has quite high
capacitance to ground and in practice fence voltages around 1KV (1-3mm
spark) are more normal.
FEATURES: It has 3 modes:-
PULSE mode gives a steady pulse every 2-5 seconds. Ideal when
you are first in the field and the cows are very interested in what is
going on. However BOTH mode may be more appropriate if there is
too much interest.
SENSE mode, sends no regular pulse, but monitors the insulation
resistance of the fence and fires pulses when the insulation changes.
On a permanent low fence resistance it gives up and the sense LED stays
on. This mode saves power and QRM.
BOTH mode, gives the advantage of pulse mode in case of insulation
fault, as well as much higher animal intrusion protection (eg fence being
knocked down between pulses) without the power drain of a much faster
pulse rate.
A Neon gives O/P pulse and connection indication. It glows dimly on
pulses and brightly on heavy loads!
CIRCUIT
The 12v relay with a heavy duty make contact (several contacts paralleled!),
this momentarily powers the ignition coil's primary. On relay release
the coil resonates with the 2u2 cap. The cap must be connected directly
across the coil terminals. This provides the high AC energy to make the
secondary voltage. Having a small neon bulb in series with the fence wire
confirms the output power is there. The relay is driven by T3 (200mA device).
In PULSE mode, T3 turns on when the 220uF has charged up through
the 47K, and is over the 6.2V zener voltage. The 220uF is quickly discharged
through the high voltage diode and 56ê to the high current contact,
ensuring only a momentary on time for high current to flow through the
coil, keeping power needs down. This forms the repeating pulse mode timer.
In SENSE mode, any DC load on the fence under 20Mê will
cause a voltage drop across the 2 power diodes and turn on T1. This
turns on T2 with some filtering to stop mistriggers from RF etc. T2 on
in turn will pulse T3 base on. T2 turning on also lights the SENSE LED.
If there is permanent low resistance being sensed, then the 10uF will
get charged up and T3 will no longer be turned on.
The mode selection has been done with a simple single pole 4 way switch,
but this has meant a germanium diode must be used to maintain the sense
circuit sensitivity when BOTH are selected.
INSTALLING
Keeping the cows away, set-up the insulated fence wire (orange nylon with
2 stainless steel strands in it), I use 1.2m tall GRP rods pushed into
the ground @ about 20m spacing or if tough ground I use a steel rod to
make the hole or with 10cm of garden hose as a top insulator.
To attach the fence wire I use a clove hitch as it is easy to adjust and
does not damage the wire. I suspend the end ball of wire and connect it
to the ignition unit with tinned copper wire. Also using this wire to
a good earth stake. Then connect the car battery.
WHAT A LAUGH
When the cows approach make sure the fence is in PULSE or BOTH mode. I
normally spend the next 30 mins or so falling about laughing at all the
cows as they must all have a lick and jump away in turn. Once this has
happened the cows normally give you a wide birth, and the fence can be
safely put into SENSE mode.
73 De John, G8MNY @ GB7CIP
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