This invention relates to improvements and modifications to electric
The electric fence industry is highly regulated with restrictions
being placed on various electrical parameters associated with electric fences.
For instance, there are restrictions on the maximum amount of energy output from
an electric fence energiser, the maximum current allowed as well as maximum voltages
set according to the load under which the energiser is placed.
Unfortunately, these standards do not mimic the operating characteristics
of standard electric fence energisers and therefore electric fence energisers
have not been able to achieve an optimum performance within those standards.
For example, one set of operating standards specify a simple step
function for output voltage according to energiser load. A standard electric fence
energiser has its output voltage varying with load as well, but with a function
which is represented by a shallow curve. In order for a standard energiser to meet
the lower voltage requirements for the lower energiser load (in ohms) the maximum
output voltage from the energiser at the higher loads is significantly less than
that allowed under the standards.
This is of concern, as the applicants have found that an electric
pulse propagates along an electric fence line more readily if there is provided
a higher output voltage. Further, a high voltage pulse is preferable for physiological
reasons as a high voltage pulse can deliver a shock of greater magnitude through
the body of an animal more effectively than a low voltage pulse.
Another problem with electric fence energisers is that occasionally
conductive material may fall on the fence and create a continual current drain
on the energiser causing it to become undesirably hot.
A further problem is that over a period of time, long grass may grow
against the electric fence providing a conductive path. This allows increased
current to flow and causes the output voltage on the fence to drop. This is obviously
undesirable as the effectiveness of the fence pulses has now been reduced.
It is an object of the present invention to address the above problems,
or at least to provide the public with a useful choice.
According to an alternative aspect of the present invention there
is provided an electric fence energiser including an energy storage capacitor,
a controllable switch arranged to control the charge on said storage capacitor,
a control circuit connected to the controllable switch, a sensing means that can
relay information to the control circuit regarding the perceived electrical load
on the output of the electric fence energiser, characterised in that the control
circuit upon receipt of the information from the sensing means operates the controllable
switch so that the charge on the storage capacitor does not exceed a pre-set value
for a particular load on the electric fence energiser.
According to one aspect of the present invention there is provided
a method of operating an electric fence energiser characterised by the steps of:
- a) monitoring the effective load on the electric fence energiser, and
- b) adjusting the voltage on the energy storage capacitor of the energiser in
accordance with the indicated load so the output voltage from the electric fence
energiser is a preset value.
It should be noted that throughout this specification the voltage
and charge on the energy storage capacitor will be used interchangeably as appropriate.
There is an electric fence energiser known that prevents the storage
capacitor from being charged to more than a maximum amount. However, this differs
from the present invention as there is only one possible level of charge that can
be stored on the capacitor and this level is irrespective of the electrical load
on the output of the energiser. Therefore, this energiser does not give optimum
performance when complying with the international standards (such as IEC standards),
particularly if the international standards indicate a step function with respect
to output voltage and energiser load.
Another electric fence energiser is known that does monitor the load
on the output of the energiser. Where this differs from the present invention
is that instead of altering the charge on the storage capacitor in accordance with
the load, the energiser switches in a second output stage when the load on the
output reaches a certain level. The discrete nature of having two output stages
in this energiser is not very satisfactory as the output voltage of the energiser
is, under certain loads, well below the maximum allowable voltage specified by
the standards. Further, the duplication of output stages in the energiser each
having a separate transformer is expensive.
Unlike the prior art, the present invention does not rely on a large
amount of extra componentry, nor is it limited by only having the ability to charge
the storage capacitor only to a single maximum level.
The present invention unlike all previous energisers varies the charge
on the storage capacitor in accordance with the indicated load on the energiser.
The advantages of this are immediately apparent.
The output fence voltage can be kept as high as possible with respect
to the standards set. This ensures optimum pulse propagation along the fence line
and provides better shocking characteristics. Another advantage is that with the
output voltage being directly responsive to load, it is immaterial whether grass
has grown against the fence as the output voltage is adjusted to take into account
Another advantage is that the current flow for the load is optimised
and therefore a cooler unit is achieved as less energy is dissipated. If a conductive
object shorts the fence and doesn't move away, the present invention can be used
to drop the output voltage and hence the energy being dissipated by the electric
Yet another advantage of the present invention is that a transformer
with a higher turns ratio can be used. Previously, an average turns ratio was
1:14. A higher ratio could not be used with previous energisers as under certain
loads, the energiser would exceed the lower of the maximum output voltage standards.
As the charge and hence voltage on the capacitor can be varied by the present invention,
a higher turns ratio, say greater than 1:18 and more likely in the order of 1:25
may be used. This has the advantage that less charge is required on the energy
The sensing means which provides an indication of the load may be
achieved by a number of ways. In one embodiment, current feedback may be used.
A resistor of known value can be placed on the secondary side of the energiser
output transformer. The voltage across this resistor can be measured, from which
the current can be calculated and used to estimate the energiser load.
In an alternative method, a resistor of a known value may be placed
on the primary side of the energiser circuit and the voltage measured across the
resistor. From this the current can be calculated and hence the effective impedance
on the secondary side of the energiser. There are however some disadvantages with
using current feedback as the current tends to change when capacitive loads are
placed on a circuit.
In a preferred embodiment, voltage feedback will be used. For instance,
the energiser load may be estimated from the peak voltage at the secondary side
of the transformer. This can be achieved by a number of ways. The peak voltage
on the secondary side can be measured while maintaining the isolation between the
transformer primary and secondary through a capacitor divider network or a tertiary
winding on the output transformer.
In an alternative method, a constant capacitor voltage test pulse
can test the fence line load and from the measured output voltage, the effective
As an alternative a system of measuring the phase angle between the
voltage and current wave forms may be used.
The charge and hence the voltage on the storage capacitor may be
controlled by a number of means. In a preferred embodiment, a controllable switch
is interposed between the charging circuit for the energiser and the capacitor
and can be switched on and off to control the charge reaching the capacitor as
desired. In one embodiment, this controllable switch may be a triac, but it should
be appreciated that other switching devices, perhaps thyristors, other SCR's, mechanical
devices, optical switches and so forth may be used.
It is envisaged that in a preferred embodiment, micro-processor/controller
technology will be used in the control circuit. The controllable switch may be
connected to one of the ports on the micro-processor. When the micro-processor
receives feedback from the sensing mechanism which gives an indication of the load
on the fence, the micro-processor may then calculate a value (in accordance with
a function) or access a value from a memory means (for instance in an EPROM). This
value is indicative of the voltage which should be on the energy storage capacitor
that gives the desired output voltage on the fence line once the capacitor is discharged
through the energiser transformer. The micro-processor then may open or close
the controllable switch as appropriate to allow the energy storage capacitor to
be charged to that value.
Obtaining the appropriate level of charge on the capacitor may be
achieved by a number of ways. One of the simplest means to ensure that it is charged
to the appropriate level is to monitor the time that the controllable switch is
opened or closed. This is possible as generally the capacitor is fully discharged
for each pulse of the energiser and the charging rate of the capacitor follows
a known characteristic curve. Alternatively, the voltage across the storage capacitor
may be measured and the controllable switch operated as appropriate.
Aspects of the present invention will now be discussed by way of example
only with reference to the accompanying drawing in which:
- Figure 1:
- is a graphical representation comparing energiser output voltages with typical
standards set for electric fence energisers, and
- Figure 2:
- is a schematic diagram of one possible circuit to be used in accordance with
the present invention.
Figure 1 is a semi-log graph of output voltage in kV versus the energiser
impedance in ohms. It should be appreciated that a high energiser impedance represents
an open circuit, that is when no animal or other conductive body is leaning against
the electric fence. A low energiser impedance represents a short circuit, that
is when there is an animal or some other conductive body leaning against the fence.
The lines 1, 2 and 3 delineating the shaded area 4 represent one set of international
standards (IEC) set for output voltage with respect to the energiser impedance.
The present invention can of course be applied to other standards.
The curve 5 represents a function of output voltage versus energiser
impedance for a typical electric fence energiser. It can be seen that under high
impedance, the output voltage of a typical electric fence energiser is considerably
below that allowable by the standards indicated by line 3. The reason for this
is apparent when one views the curve 5 when the energiser impedance is at 500 ohms,
that is where lines 1 and 2 intersect. At this loading, the output voltage of a
typical energiser is very close to the standards set for output voltage at impedances
under 500 ohms. The output voltage of a typical energiser at high impedances could
be increased to be closer to the specified standards. However, because of the shape
of the curve 5, the output voltage of the energiser would exceed the specified
standards under lower impedances.
A possible output function for an electric fence energiser operating
in accordance with the present invention is indicated by numeral 6. The function
6 essentially is comprised of four sections 7, 8, 9 and 10.
Section 7 illustrates the high output voltage which can be achieved
under heavy impedances which is close to that specified in the standards. If a
transformer is used with a higher turns ratio than average, then this high voltage
can be achieved without increasing the usual charge on the energy storage capacitor.
The high voltage indicated by section 7 gives good pulse propagation
properties as well as desirable shocking characteristics. It can also be readily
seen by purchasers of an electric fence energiser in accordance with the present
invention that under high impedances the output voltage is considerably more than
that given by standard electric fence energisers.
It should be appreciated that to achieve the straight line of section
7, the energy storage capacitor is required to be charged more for the lower impedances
than for the higher impedances and this is achieved by the adaptive control of
the present invention. Adaptive control allows an energisers output characteristic
to be adjusted to optimise the operation of the energiser to a set of particular
operating conditions. This adaptive control also accounts for the possibility of
long grass growing against the fence.
Section 8 of the function 6 illustrates the transition required for
the electric fence energiser to adapt its output voltage from being close to that
specified by the standards for high impedance to be under the output voltage specified
for low impedances. Depending on the values of the componentry within the energiser,
section 8 may either be achieved by the use of adaptive control as in the present
invention, or may result from the natural effect of decreasing impedance on output
voltage as illustrated by curve 5.
Section 9 of the electric fence energiser is achieved in a similar
manner to section 7 discussed above. Again it can be seen that the output voltage
achieved is very close to that specified by the standards and considerably higher
than that achieved by typical electric fence energisers.
Section 10 represents a drop in voltage which occurs under very low
impedances such as what may happen if a conductive body is left to short the electric
fence. It is thought that by having the voltage drop under this situation, the
energy dissipated by the energiser will be less and hence there will be less power
drain and the energiser will run cooler. It should be noted that the purpose of
providing shocks is to encourage bodies to move away from the fence and therefore
if a body has been on the fence for a period of time, it is unlikely to move away
and hence a drop in voltage is desirable in this situation.
Figure 2 is a schematic diagram of an electric fence energiser in
accordance with one embodiment of the present invention. An energy storage capacitor
11 is connected across the primary 12 of a transformer generally indicated by arrow
13. This secondary 14 of the transformer 13 is connected to an electric fence line
(not shown). A control circuit 15 is connected to an SCR 6 which is operated by
the control circuit 15 to discharge the storage capacitor 11 into the transformer
A controllable switch 17 in the form of triac is situated between
a charging circuit 21 and the energy storage capacitor 11. The triac 17 is also
connected to the control circuit 15 which controls the opening and closing of
the triac 17 and hence the charging of the energy storage capacitor 11.
A resistor 18 of a known value is in the primary side of the energiser
circuit. A sensing means in form of lines 19 and 20 determines the voltage across
the resistor 18. As the resistance of the resistor 18 is known, the current flowing
in the circuitry can be calculated from the voltage. This current is indicative
of the load on the secondary side of the energiser.
With the above information, the control circuit 5 can then open or
close the triac 17 in such a way as to ensure that the voltage on the energy storage
capacitor 11 is as desired.
Aspects of the present invention have been discussed by way of example
only and it should be appreciated that modifications and additions may be made
thereto without departing from the scope of the appended claims.