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Electric fence tester.

KOEZE

New Member
Hello,

I've fenced my garden using an electric fence to keep out cats (street) and keep in our pure bread cats. As garden vegetation touch the wires shorts are created lessening the functionality of the fence. Is there a simple way to measure the voltage between the ground pin and the fence.
I've been told that (allthough very low current (specially suited equipment for small animals)) the voltage is around 7000 volts.

I have no idea if this is true but a simple test with an old multimeter (surplus) simply blew the fuse. 8)

If you guys have any ideas I'd be glad to hear about them.

EJK
 

lavenatti

Member
I used my kid, when he jumped and started crying I knew the fence was working ok.

Or... use a small neon bulb. Tie one lead to the fence and run a wire from the other end to about a half inch from the ground. It should flash when the fence fires.
 

stevez

Active Member
After 40 yrs my little brother can still recall the day I convinced him to pee on the electric fence. The expression on his face was a rather dramatic indicator of the voltage.

Some ideas:

1. Not sure what the purpose of the test is but the ability to jump a measured gap might give an approximation of the voltage. You might set the gap when you know the fence is working ok - if, when you run the test the gap isn't jumped you have at least some indication.

2. Make a simple voltage divider that might total 100 meg ohms. Was thinking of 9 10 megohm resistors with the 10th position made up of several resistors that would total 10 megohms - so that you end up measuring 100 volts or so. 1/8 watt or 1/4 watt resistors are cheap.

They do make high voltage probes but this is less expensive.
 

Optikon

New Member
stevez said:
After 40 yrs my little brother can still recall the day I convinced him to pee on the electric fence. The expression on his face was a rather dramatic indicator of the voltage.

Some ideas:

1. Not sure what the purpose of the test is but the ability to jump a measured gap might give an approximation of the voltage. You might set the gap when you know the fence is working ok - if, when you run the test the gap isn't jumped you have at least some indication.

2. Make a simple voltage divider that might total 100 meg ohms. Was thinking of 9 10 megohm resistors with the 10th position made up of several resistors that would total 10 megohms - so that you end up measuring 100 volts or so. 1/8 watt or 1/4 watt resistors are cheap.

They do make high voltage probes but this is less expensive.
The arcing gap would be too hard to measure at only 7000V. 3KV can ionize and arc 1mm of air at 50% humidity. It would be highly dependent on the air conditions etc.. not to mention a possibly wobbling fence.

The resistor divider is an excellent solution. Measure the input impedance of your multmeter on the highest range. You can do this with a resistor divider and an external source (battery).. then make a string of resistors for each multimeter lead. Use your multimeter as the other half of your divider. Put a series string in each lead and shrink tube it. That way, you can just grab your meter, connect your megaohm strings to the leads and go and measure it again. This time you will get a reading that you can back calculate to find out the KV on the line. Oh, and the fuse wont blow. Use the highest range on the meter for this.
 

Klaus

New Member
A couple of tips to the answers above: try to use decent size resistors, perhaps 1 watt types, for the voltage divider string. The tiny 0.25W ones do not have a high enough voltage rating.
Since the output is quite a short pulse rather than a steady wave form, it may be difficult to measure that on a meter. Just a jumble of numbers on a digital one unless it has a peak hold facility. A quick kick on an analog meter's needle does not give much of a reading either but gives a clue if the unit works.
I like the neon indicator idea best, you find a ready made one inside the little screwdriver like mains testers. Yes, I know, they are not rated for 7000volts but the high internal resistance and miniscule current passing through the tester should minimise the 'feel' on the other end of the tester, as you hold the tip of it to the wire.

I'm surprised that the electric fence works for cats, thought they were clever enough to learn to just to jump over it :)
Klaus
 

KOEZE

New Member
Thanks for the tips guys.

:idea: I'll try the ideas, starting with the neon light bulp and maybe the voltage divider if I can figure out how to measure the impendance of my meter. Maybe I'll buy one cheap and use that for that purpose only. They I don't have to disconnect anything.

The tips with the kid touching the fence and the pissing on it are less appealing. :lol: :lol: :wink:

I'll give the results here at the forum.

Klaus, our purebreads are like sacks of beans. They have very little interest in wandering outside our garden but it just to be sure. As for normal cats. They have to climb a fence about 1.8 meters high with 3 wires (electric) on top. Most cats tend to jump onto something and then jump over. This is a one time occasion. Most have learned and in the 3,5 years we have the fence none of our cats escaped and no outside cats have come into our garden.

EJK
 

mechie

New Member
Electric Fence Tester

I have a gizmo to convert my multimeter (cheapo digital, no peak-hold or anything).
A simple resistor divider wouldn't work for me, I ended up with a passive integrator (a couple of resistors and a capacitor.
The cap charges up by about 1 volt per kV on the fence, the meter then indicates volts instead of kV.
I will note the component values tonight and post tomorrow? (Friday).

The gizmo I made requires the fence to fire several times before a good reading is obtained.

Neon testers are available for farm fences (£5 to £10 GBP), a series of four or five neons flash with each fence pulse (very breif - 10uS?- hard to read in daylight!)
Also little kilovoltmeters with digital displays are commercialy available but pricey!
http://www.gallagherusa.com/testers.htm
 

KOEZE

New Member
Mechie,

That would be exactly what I need. I'd be very gratefull if you could post a little schematic with the values too.

EJK
 

mechie

New Member
Its a deal ?

Consider it done - tomorrow (I'm away from home at the mo...) :wink:
 

mechie

New Member
Overdue - Sorry !

At last, the circuit you wanted...
This is exactly as I built mine, the resistor ratio may not make mathematical sense but it reads correctly for two different energisers of mine.
Note I show an earth connection - I use a 16" length of 6mm stainless tube to provide an independent earth for the meter -
Make sure you connect earth first !

I use two different digital multimeters, one is auto-ranging, both work OK.
Expect a reading of 1V for every 1kV from the energiser.
The capacitor takes a few fence pulses to charge up, maybe 5, you can see the meter reading peak higher each time. It will never be stable as the cap voltage will decay between pulses but you will get as clear an indication as any commercial meter will give.
 

Attachments

KOEZE

New Member
:D Thanks Mechie,

I'll try your schematic asap. It looks very usefull. I'll buy me an simple digital mm if it works and build the circuit into one of the leads. I'll bend the probe into a hook so that I can hang the thing on one of the wires.

EJK
 

crutschow

Well-Known Member
Most Helpful Member
Don't you need a compensated voltage divider for this, like a scope probe?
Those value resistors are low enough that you don't need compensation.
 

dr pepper

Well-Known Member
Most Helpful Member
Might be an old thread, but its a new question.
The capacitor is on the o/p of the circuit, the spec is 1v per 1000v, so if your fence makes 50kv the o/p will be 50v, so you need a 50v cap.
However seeing as the circuit is meant for hiogh voltage in general a 400v cap is advised.
I knew an old farmer that had an interesting way of checking the fence, not legal nowadays.
 

Beau Schwabe

Active Member
If you just want to know if a fence is ON ... Just grab a 1/4 inch wide 2 inch long blade of grass and lay one end on the fence while you hold the other end. What you feel through the blade of grass will lessen the blow than if you were touching the fence directly.
 

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