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3
ELECTRIC
FENCE
TESTERS
By
JOHN CLARKE
Do you need to test your electric fence to see if it is working? You
could try the wet grass trick but then again you might get a shock.
Why not build one of these three electric fence testers instead?
MINI
MIDI
MAXI
MAY 1999 37
I
t’s all very well having a fancy
electric fence installed to keep
animals corralled but how do
you know if it’s working properly?
By the time you discover that the
fence has a fault, you could be faced
with a real roundup job.
Of course, electric fences are
not only used to keep animals in a
paddock but are often also used to
keep animals away from a conventional fenceline. This particularly
applies to horses. If the fence uses
a large wire mesh, it’s all too easy
for a horse to become entangled in
the wire, panic and cause itself a
serious injury.
In fact, unless a trapped horse is
released fairly quickly, it can die.
One common way of testing an
electric fence is to use the wet grass
method. The technique is simple:
all you have to do is take hold of a
short length of wet grass and touch
it against the fence.
Because the wet grass is somewhat conductive, you’ll get a bit of
a belt if the fence is working but the
shock won’t be anywhere near as
severe as if you touched the fence
with your bare hands.
The drawback with this method is
that it’s a bit hit and miss. Because
you’re (hopefully) only getting a “bit
of a tingle” through the wet grass,
you can’t tell how much “bite” the
fence really has. That’s why some
hardy souls choose the direct touch
method but it’s not one that we recommend. If the fence is functioning
properly, it will bite like a Northern
Territory crocodile.
A far better way is to use one
of the three electric fence testers
described here. They will quickly
indicate whether or not the fence
is working and also indicate its
effectiveness.
For example, although the fence
controller itself might be working
correctly, there may be problems
with the installation that make the
fence ineffective.
Common faults include poor conduction of the earth stakes, shorts
between the high tension (HT) wires
and ground, and breaks in the line.
Shorts can be caused by long, wet
grass brushing against the HT line
and/or faulty insulators.
Sometimes, the further away
you get from the controller, the
less effective the fence becomes.
This commonly occurs if wet grass
is loading down the controller’s
output. It’s possible too for one section of the fence to go completely
“dead”, due to a break in the line.
For this reason, it’s a good idea to
check all sections of the fence on a
regular basis.
Unfortunately, you can’t use
a standard multimeter to test an
electric fence. This is because the
peak voltage on the fence can be
as much as 10kV, with each pulse
only lasting for 1ms or less. What’s
more, the pulses only occur once
every second or longer.
So while there may be a significant amount of energy in each pulse,
the multimeter does not integrate
this into any meaningful reading.
This is particularly true for digital
multimeters which have a one or
2-second response time.
These three fence testers can be
used as more reliable aids for fence
maintenance and, best of all, they
do not induce an electric shock
into the operator. Each contains a
“light” which flashes to indicate
fence pulse operation.
Which one you use depends on
what you want to do.
We’ve called our three Electric
Fence testers the “Mini”, the “Midi”
and the “Maxi”.
The first unit flashes a neon lamp
each time it detects a pulse on the
fence, while the second unit can
measure the fence peak voltage
(up to 10.8kV). The third unit is
designed for permanent installation
on the fence and flashes periodically if the fence is operating correctly.
All three units are powered directly by the electric fence being
tested. That way, there are no batteries to replace or leak if the unit
has been left unused for some time.
OK, let’s take a look at each of
our fence testers in turn and find
out how they work.
WHY AN ELECTRIC FENCE TESTER?
This project grew out of necessity:
we needed a means of testing the
output of the SILICON CHIP Electric
Fence Controller, featured in
last month's (April ’99) issue.
We called for volunteers
around the office to act as
a tester using the old fingeron-the-fence-and-hope-itdoesn’t-hurt-too-much routine.
But there were no takers!
(Even Ross Tester refused to
live up to his name . . .)
So we looked at ways of
testing the electric fence with38 Silicon Chip
out getting a belt and found that there
were several ways to do it – hence
the three projects featured here.
Incidentally, if all this talk about
electric fences and controllers is
foreign to you, it’s probably because
you missed out on last
month’s issue of SILICON
CHIP.
The high power electric
fence controller shown here
was described in detail in
that issue. It’s easy to build,
costs a fraction of commercial controllers . . . and back
issues of the magazine are
still available for $7.00 including P&P – a bargain in
anyone’s language.
“Mini” Electric Fence Tester
Fig.1 shows the circuit of the
Mini Electric Fence Tester. It is a
low-cost unit that’s easily carried
in a shirt pocket and can be quickly
used to indicate whether or not a
fence is working.
This is the simplest of the three
units and uses just a neon indicator
and three 330kΩ resistors. These
parts are all mounted on a small
PC board and there are two contacts, one at each end. In use, one
contact (the finger pad) is held in
the fingers and the other is touched
onto the electric fence wire. If the
fence is operating correctly, the
neon indicator will briefly flash
each time the HT wire is pulsed.
The total resistance in series
with the neon indicator (3 x
330kΩ) limits the current flowing
from the fence and through your
body to ground. In practice, this
current is so low that the pulse
will not be felt. Note that the light
output from the neon indicator is
quite low and you may need to
shield it from sunlight so that it
can be properly observed.
By the way, this circuit is
somewhat similar to the neon test
screwdrivers that are sometimes
used to test for mains voltages
around power points and light
switches. Do not, under any circumstances, use the Mini Electric
Fence Tester to check for mains
voltages. It’s not designed for this
role.
Conversely, do not use a test
screwdriver to check the operation of an electric fence. This is
because they are not rated for electric
fence voltages and the resistance in
series with the neon indicator may
break down. Once damaged, the test
screwdriver could present a serious
electric shock risk if it is then used
on the mains supply.
Fig.1 (above):
the mini electric
fence tester is
simply a neon
lamp in series
with enough
resistance to stop
you getting a belt!
Building it
Fig.2 : the PC
board layout.
Construction is
simplicity itself!
The hardest part
will be soldering
the wire loops.
Fig.2a shows the assembly details
for the PC board (code 11303994, 45 x
20mm). Install the parts as shown and
make some wire loops at each end for
the contacts. We used paper clip wire
for the loops and soldered this directly
to the copper pads. Alternatively, you
could use small screws and nuts to
Fig.2a : you hardly need a PC
secure the wire in place. This latter
board pattern as it is so simple
method will ensure that the copper
– but here it is anyway!
pads don’t come adrift due to strain
from the wire
loops.
Once
the
assembly is
complete, the
PC board can
be wrapped in
some clear heatshrink tubing,
leaving the wire
loops exposed.
It’s not easy to see any components through the
heatshrink but this photo gives an idea of construction.
Parts List
Mini Electric Fence Tester
1 PC board, code 11303994, 45 x 20mm
1 neon indicator, pigtail type
1 80mm length of 1mm diameter tinned copper wire or paper clip
3 330kΩ 1W resistors
1 45mm length of 25mm diameter clear heatshrink tubing
MAY 1999 39
“Midi” Electric Fence Voltage Tester
The Midi Electric Fence Voltage Tester is a slightly more
elaborate instrument than the Mini Tester. It also uses a
neon indicator but in this case the fence voltage can be read
off a calibrated scale after adjusting a single control knob.
As shown in the photo, the unit is housed in a small
plastic case and a small hole in the front panel allows the
neon indicator light to be seen when it flashes. As before,
the light output is quite low and you need to watch closely
to see the flash.
Fig.3 shows the circuit details. It’s really very simple
and consists of a voltage divider and the neon indicator
itself. In operation, the electric fence voltage is applied to a
series string of 19 10kΩ resistors which in turn feed a 10kΩ
potentiometer (VR1). The divided voltage is then tapped off
from VR1’s wiper. Why use so many 10kΩ resistors? The
answer is that they are necessary to provide a sufficient
voltage rating for the divider, which could encounter fence
voltages up to 10kV.
Fig.3 (left): the midi electric fence tester
is essentially a voltage divider across the
fence high tension. The neon lamp glows
when the fence voltage matches the scale
voltage selected by the potentiometer.
Fig.4a (above): the component
layout on the PC board. Note the
comments in the text about
reversing the lead connections:
you have been warned!!!
40 Silicon Chip
Housed in a small utility
box, the midi electric
fence tester is ideal for
occasional testing. The
probe is as used in a
multimeter.
VR1’s wiper applies the divided voltage to the neon
indicator via two 2.2kΩ resistors, while the common
side of the circuit is connected to the ground stake on
the electric fence.
A neon indicator will light when the voltage across it
reaches about 90V and so we use this characteristic to
calibrate the potentiometer (VR1). If the wiper is wound
fully towards the 10kΩ resistors, then the divider ratio
is such that the neon will flash when there is 1.8kV on
the electric fence. Conversely, as VR1 is wound towards
ground, the division ratio increases and so the input
voltage from the fence needs to be higher than 1.8kV in
order to light the neon indicator.
Let’s say, for example, that VR1 is set to its mid-posi
tion. In that case, the fence voltage needs to be at least
3.6kV to make the indicator flash.
One small complication with this circuit is that it
will not produce reliable results unless the body of the
potentiometer is well grounded.
If this isn’t done, the neon indicator conducts the fast
rise-time fence voltage into the air and hence shows a
small flash, even if the pot is wound fully down.
Although the pot body is grounded on the board via a
PC stake (and ultimately to the fence ground), the inductance of the ground lead is enough to cause problems with
fast rise-time voltages. For this reason, we have specified
a metal knob for the pot so that it can also be grounded
via your body. In practice, this means that measurements
must be made with your hand holding the metal knob, to
99% of the assembly work in this project is soldering
resistors! Fortunately, most are the same value.
prevent false readings from occurring.
When using the tester, the pot is
initially wound fully clockwise and
gradually backed off until the neon
indicator just begins to flash. The
fence voltage can then be read directly
off the scale.
Note that the overall resistance of
this tester is 200kΩ, so it shouldn’t
load down the fence voltage to any
measurable degree.
leads. These holes
should be fitted
with small rubber
grommets.
The pot shaft can
now be trimmed to
suit the knob, after
which the PC board
assembly can be
mounted on the lid
and secured using
the pot nut. When
fitting the knob,
rotate the pot shaft
fully clockwise,
then tighten the
grub-screw with the
pointer towards the
10.8kV position.
This done, feed the external leads
through the grommets and solder them
to the PC board. These leads should
have good insulation to prevent any
voltage breakdown between them.
Use a green or black alligator clip
for the earth wire connection and
a red insulated probe for the fence
terminal. This will prevent any
confusion when you are making the
connections to the electric fence.
Warning! – if you reverse the
connections to this tester, the body
of the pot and hence the knob will
be at the fence voltage. If the fence
is working correctly, this means that
you will get a nasty belt as soon as
you touch the knob. Get the connections the right way around and you
won’t have any problems.
Building it
Fig.4a shows the assembly details
for this fence tester. It’s built on a PC
board coded 11303993 and measuring
77 x 47mm. Start the assembly by
soldering in all the resistors, then in
stall PC stakes at the fence and ground
inputs, at the three pot terminal positions and at the ground position for
the pot’s body.
The potentiometer can now be
installed by soldering its terminals
to the PC stakes and by soldering its
body directly to the adjacent ground
stake. You will need to scrape away
some of the plating from the pot body
near the PC stake, using a file or sharp
knife, so that it can be soldered easily.
The neon indicator has its leads bent
at right angles before being soldered
into position. It can be secured to the
board with a dob of silicone sealant.
The next step is to attach the front
panel label to the lid of the case and
drill the holes for the pot shaft and for
viewing the neon indicator. You will
also need to drill two small holes in
the sides of the case for the external
Figs 4b & 4c: the front panel and PC
board artwork, reproduced same
size for those who wish to make
their own.
Parts List
Midi Electric Fence Tester
1 plastic case, 82 x 54 x 30mm
1 PC board coded 11303993,
77 x 47mm
1 front panel, 80 x 52mm
1 neon indicator, pigtail type
2 small rubber grommets
6 PC stakes
1 10kΩ 16mm pot. (VR1)
1 black or green aligator clip
1 red instrument probe
1 metal knob
1 1m length of blue or black 250VAC
rated wire
1 1m length of red 250VAC rated
wire
19 10kΩ 0.5W 1% metal film
resistors
2 2.2kΩ 0.5W 1% metal film
resistors
MAY 1999 41
“Maxi” Electric Fence Voltage Tester
Unlike the other two testers, the
Maxi Electric Fence Tester uses a
high-brightness xenon flash tube
although the circuit is only slightly
more complicated than before.
It uses an internal capacitor to
store up some charge from each
fence pulse and when this reaches a
critical level, the xenon tube emits
a bright flash.
This cycle is then repeated,
with the tube flashing at regular
intervals if the fence is operating
correctly.
As shown in the photos, the unit
is housed in a clear plastic case
and is designed to be permanently
attached to the fence.
Fig.5: the maxi electric
fence tester has a
somewhat similar circuit
to the midi model but in
this case fires a bright
Xenon flash tube, the
frequency depending on
the voltage on the fence.
42 Silicon Chip
The maxi fence controller, housed in a see-through and weatherproof plastic
case. The Xenon flash tube is clearly visible through the case so this can be left
permanently connected to the fence. We used the small plastic clips on the top
of the case and cable ties to secure this tester to a suitable fence post.
Fig.5 shows the circuit details. It
uses a string of 18 820Ω resistors to
provide current limiting and these
drive a bridge rectifier consisting of
diodes D1-D4. The output of the bridge
in turn is connected to the xenon tube
and to a parallel 0.47µF 630V polyester
capacitor. The trigger pulse for the xenon tube is derived by connecting its
trigger (T) terminal to a point higher
up the resistor string.
In operation, each fence pulse charges the capacitor by 10-40V, depending
on the pulse amplitude. When the
voltage across the capacitor reaches
200-300V, the xenon tube is ready to
fire. It then fires when the next fence
pulse takes the trigger input sufficiently high.
When the xenon tube fires, the
0.47µF capacitor quickly discharges.
The capacitor now recharges on each
successive electric fence pulse until
the breakover voltage of the xenon
tube is reached again.
The flash rate depends on the fence
voltage. The circuit draws about 0.5mJ
per pulse from the electric fence which
does not affect normal operation. This
is why the circuit can be left perma-
nently connected to the fence.
Building it
A PC board coded 11303992 and
measuring 77 x 47mm accommodates
all the parts – see Fig.6a. Begin by installing PC stakes at the two external
wiring positions, then fit the resistors
and diodes. Make sure the diodes (D1D4) are all correctly oriented.
The capacitor is installed on the
copper side of the PC board. Bend its
leads at right angles so that the body
of the capacitor can lie flat against the
board before soldering it into position
(see photo). This is necessary to allow
the PC board assembly to fit into the
specified case.
The leads of the xenon tube must
also be bent at right angles before
mounting it on the board. Use needle-nose pliers to hold the leads adjacent to the glass body before bending
them – if you don’t do this, you could
crack the glass tube. This done, solder
the tube into position and don’t forget
the trigger lead.
You will need to drill two holes in
the sides of the case for the external
leads. Fit these holes with rubber
Front (above) and rear (right) views of
the completed PC board. Note that the
0.47µF discharge capacitor attaches to
the copper side of the board.
grommets, then pass the leads through
and solder them to their respective PC
stakes on the PC board. It’s a good idea
to use a red lead for the HT connection to the fence and a blue or green
lead for the fence ground connection.
As with the previous design, these
leads should have good insulation,
to prevent any high-voltage leakage
between them.
The PC board is designed to clip
into the case against the integral side
pillars. If necessary, you can lightly
file the sides of the PC board so that
it is a neat fit.
Because it will be exposed to the
weather, it’s necessary to seal the wire
entry holes and the case lid using
silicone sealant. Before doing this,
however, it’s a good idea to test the
circuit to make sure it works correctly.
That way, if you do have a fault, you
can easily remove the board from
the case and check for missed or bad
solder joints, or incorrect component
placement.
Finally, you will have to figure out
some way to mount this unit. This may
involve fashioning a suitable clamp
or you can do what we did and fit a
couple of small plastic clips so that the
unit can be tied to a convenient fence
post using tie-wire.
The HT lead can be attached to the
electric fence using a suitable electric
fence joiner, while the ground lead
can be attached directly to a ground
SC
stake.
Figs 6a & b: follow the PC board
overlay (left) and you should have no
problems assembling the board. The
full-size PC board pattern above can
be used to etch your own board or
to check commercial boards before
assembly.
Parts List
Maxi Electric Fence Tester
1 clear plastic plastic case, 82 x
54 x 30mm
1 PC board, code 11303992, 77
x 47mm
1 straight 32mm-long xenon
flashtube
2 fence clips
2 PC stakes
1 1m length of green or blue
250VAC rated wire
1 1m length of red 250VAC rated
wire
2 electric fence wire joiners
4 1N4936 1A fast diodes (D1-D4)
1 0.47µF 630V polyester
capacitor
1 220kΩ 0.5W 1% metal film
resistor
18 820Ω 0.5W 1% metal film
resistors
MAY 1999 43
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