Electric fence energisers - basic principles?

[Chris Green]

We have a mains powered electric fence energiser that is now more than

20 years old. It still works but I suspect it is weakening and I'm wondering if there is any 'main' component whose replacement would rejuvenate it.

Searching for 'How does an electric fence energiser work' is quite hopeless because all you get is a desription of how high voltage pulses are used to keep animals in, not a description of how the pulses are generated.

I am an Electrical Engineer (degree in 1967) so I know the likely ways that the energiser might work and I have actually repaired it once before when it failed completely due to a corroded track on the circuit board. If I remember correctly it looked pretty simple and my

*guess* is that basically it charges a capacitor and then discharges it through a step up transformer to produce the high voltage pulse.

However I'm not sure how the once per second (ish) timing is done or the switching so any specific information (such as circuit diagrams of actual energisers) would be very useful.

I can of course take it apart again and trace the circuit, as I said it's not very complicated, but some initial knowledge would be very helpful.

[Brian Gaff]

Yes why do they pulse, the actual shock does not seem to be that great. I have seen some more sophisticated ones that use an oscillator to drive a transformer and the capacitor is discharged when the voltage set on a little IC is reached by means of a little relay. These tend to be battery powered though, but I guess it could just as easily use a mains psu assuming a suitable damp housing. Other than that though, there has to be a sensor of some kind to detect the fully charged state. Brian

[Colin Bignell]

Have you tried the search term 'how to build an electric fence energiser'? That throws up several videos that might be of help.

[John Rumm]

I would expect most of them are going to be a capacitor bank, and a SCR driven by a timer circuit to dump the output of the cap into a coil of some kind. I expect the venerable 555 gets a fairly common outing!

There is a youtube channel on fixing them - not sure if it is any good:

[John Rumm]

Because you want to give safe warning shock, not electrocute the beasties!

Continuos power risks contracting muscles onto the wire such that you can't disconnect.

[Chris Green]

Yes, well similar, I tried "electric fence energiser circuit diagram", the problem is that they're all 'modern' with transistors and other new fangled devices. As far as I can remember (from when I took it apart) my energiser has passive devices only in it. ... well, plus relays and things like that possibly.

[Chris Green]

Yes, but I suspect that mine dates from before big/cheap SCRs. I don't remember any semiconductors in it.

I think I'm going to have to take it apart and investigate! :-)

[Colin Bignell]

How to build a stone age electric fence energiser? :-)

[maus]

We had one here since 1955 or so. It was kinda a spring operating thing, as I remember, a spring contact, which joined for a moment to a power source. (bicycle batter, as I recall,) and then sprung apart for a while. Made by sunbeam-wollsely, i think. those things can kill, as I remember, a man connected one to the back of a fridge, with the low power power supplied when the fridge was working, turning to high power when it was not. A man was killed with it.

We used to have fun with it when we were jung. A young man could piss across the wire, and cut off the flow quick. men over 30, say, would end in a dribble, and get an amazing shock.

[Rod Speed]

Brian Gaff [e-mail] wrote

Because that works better and is easier to do.

Then you have never used a good one with bare feet and wet grass.

Wrong. Works fine to pulse at a rate such that it is certain to be fully charged.

[Chris Green]

Not 'build', refurbish. It's quite a powerful one, it claims to be able to energise 53km of fence. New ones with this sort of power are expensive, like well over a hundred pounds.

[Thomas Prufer]

ISTR a small device, shaped a bit like a fluorescent tube starter, which contained a spring and contacts and a mechanical oscillating spring disc thing

-- I think...

This would wear out, and be replaceable: unplug old, plug in new.

My google-fu fails me, can't find any reference to it.

Thomas Prufer

[John J]

I seem to recall a circuit which employed an R-C network and a neon in series with a primary coil of a transformer connected across the capacitor which charged up via the resistor until the neon fired. Dumping the capacitor charge through the primary coil created a pulse in the ht winding and hence the output to the fence wire. Once the capacitor charge had been dumped then it began charging again through the resistor. The details are hazy now as it's a long time ago .

[nib]

I have a vague memory from my grandfather's farm decades ago of a battery- powered box attached to the fence with some sort of pendulum system that ticked. I assume it used the magnetic field from an induction coil to keep the pendulum swinging and opening a contact every swing. Like the contacts on an old-fashioned car ignition but self-operating.

nib

[Chris Green]

I think mine is a little more modern than that, I don't think there is any sort of mechanical stuff inside it. As I said I think I just need to bite the bullet and take it apart (again) and look.

[ajh]

That was how our first one worked, it was like a bell circuit but the inertia of the "pendulum" (it was more like a watch balance wheel) set the time between pulses.

[Paul]

No matter the nice operating principle of the primary drive, the main component is a HV transformer, and some insulation can be failing.

The product packaging may have a metal box around it, and where an insulated wire leaves the housing, the insulation can become brittle with age, and energy is being wasted throwing an arc through the crack in the wire.

When an HV transformer has layers of fine wire with insulation between layers, you can have a layer-to-layer failure sapping energy from the thing.

But I would not expect it to stop functioning entirely, it there is a transformer fault. It's just that an aged transformer can lack the "punch" of a fresh transformer.

And that's why the transformers can be "re-purposed" ones. Ordinary transformers off the shelf, would be completely unsuited. CDI transformers, ignition (cylindrical) transformers, TV flybacks, these are the things with pretty-continuous and powerful outputs. And re-purposing such things, is how you inherit good characteristics. Since these items are created in large volumes, that tends to reduce the retail price a bit. Compared to an obscure custom transformer made just for a fencer. They would be made in small lots, and not likely as cheap.

In the lab, we had a couple HV probes for equipment, but there was never a need (most of the time) for such things. Maybe if you were doing HBM testing on a chassis, you might want to check that the output was functioning. A 100:1 or a 1000:1 probe, expects a known impedance on the oscilloscope input, and that's what makes it work in a calibrated fashion.

If the scope is 1 megohm input, then making a 1000 megohm resistive divider could make you a 1000:1 probe. That would take around fifty 22 megohm resistors in series, with suitable corona dope and careful layout so they don't short to anything important in the probe. Since the output is a pulse, you would expect a ringing waveform of impressive amplitude, on a digital storage scope screen.

The fence itself represents a capacitance and would be part of the output circuit. A dry characterisation is likely more stressful for the circuit, than when connected to a length of fence. So if it is going to arc over internally (inside the transformer), it is more likely to do that, unloaded while on the bench.

Paul

[Thomas Prufer]

And I recall, vaguely, that the wheel/spring/contact thing was a plug&play replacement part. But I could be wrong.

I'm sure it has a name, and I can't recall it, and am not sure I ever knew it.. Probably something like "contactor", "pulsor", "energizer", ...

As for fault-finding: I'd go for a close look, maybe a cold solder joint, cracked due to stress, or leaky batteris eating away at something. Old electronics are *fun*: tracks on the PCB hand-drawn, 1/8" wide and up. There's entire R/C SMD circuits that would fit in just the pad of some of those old PCBs...

Thomas Prufer