Electric fences are derivatives of the first design which used an ignition coil from a car. The pulsed dc comes from the sudden interruption of current in the low voltage side of the coil or transformer.
As the electric fence becomes longer, the impedance (resistance, capacitance and inductance together) act to reduce the voltage along the line.

It occured to me that if the electric fence was continuously energised with dc, instead of pulsed, then the length of line would have less effect. The energy would be stored in a small capacitor at the end of the line.

So the wire is permanently at high voltage, and when touched, the capacitor discharges.

Over the next 1 second, the capacitor is re-charged (If there is some problem with danger during the charging cycle, a second capacitor can be pre-charged and alternately connected).

Some advantages are that there is no radio interference; the current leakage can be easily measured; the voltage along the line is constant and the line can be longer.

(The housing estate where I live has suddenly become a target for break-ins: 5 in one night, and 1 or 2 every night since. And it doesn't matter if the owners are present or not).
-- Ling, Oct 06 2005

Another effect would be that when you touched the wire, your muscles would contract around it, gripping tighter. Unlike a conventional electric fence, you wouldn't have the opportunity to remove your hand before the next pulse. This is what make the DC power rails on the underground so lethal.
-- wagster, Oct 06 2005


No, the capacitor would discharge when the line is touched.
-- Ling, Oct 06 2005


Thought this was going to be like the Berlin Wall, but on the Potomac. Something to keep the politicians in.
-- coprocephalous, Oct 06 2005


Oh yeah. Not woken up yet! Would there not be a similar voltage drop as the capacitor discharges? After all, it is still providing a dc pulse...
-- wagster, Oct 06 2005


Yes, there is a falling dc voltage, much like a pulse, when the line is touched; but if the line is not touched, there is a constant voltage.

If the line is very, very long, then capacitors could be used at intervals.
-- Ling, Oct 06 2005


I²R heating would be a bitch. Also, you'd still have voltage loss along the line; that's why AC is used for high-voltage power distribution.
-- angel, Oct 06 2005


Angel, the current is extremely small. Volts jolts, and that's all we need to send Mr. Burglar running. Volt drops in the normal systems are usually due to inductance and capacitance, which effect a pulsed system, but not a steady state system.

High voltage transmission is used to reduce the current, for the same power, and hence reduce the i2R losses.

Btw, if you want to know why ac distribution systems are used, check out some Tesla web sites. It's easy to change ac HV to ac LV using a transformer.

An electric fence normally runs at anything from 1,000V to say, 4,000V, i.e. it is already a high voltage transmission system.
-- Ling, Oct 06 2005


Ok [Ling], you haven't convinced me that there is actually a problem with electric fence design, but you've convinced me that if there is then this is the solution.
-- wagster, Oct 06 2005


Electric fences, especially long ones, typically have a number of partial shorts along their length. The capacitor you describe would probably be in a permanently discharged state, and your electric fence unit would be constantly pumping power to earth (through wet grass, errant tree branches, dodgy insulators etc.).

Workable though if you could ensure perfect insulation. Good point regarding the radio interference, although after a while you tend to tune out (awful pun) the clicking.
-- Texticle, Oct 06 2005


Texticle and fghwgads, thanks for all your comments. All good points, and well taken. All high voltage systems require good insulation; but for the dc system, the capacitor would slowly discharge and then require re-charging - effectively becoming a slowly pulsed system. I suppose counting the rate of re-charges could indicate the insulation resistance.

fghwgads, I'm glad you put an isolation transformer in your system. One must always think about the possible failure modes...
Electric fences are normally rated in Joules, which is the energy content in the pulse. A good belt would be 1J, as far as I have seen, although more is required for longer wires (in a pulsed system).

Thanks for the offer to build something, but since I spend so much time on the HB, for our housing estate I would only consider a proprietary unit.

wagster, erm, thanks...I think.
-- Ling, Oct 07 2005



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