electric fence noise interference problem...

[dacflyer]

i have a friend that has a new home theater system, and lives on a farm, problem is. his 6joule fence charger is interfering with the H.T.S. (home theater system) each time the fence fires, it makes a really loud pop on his system,, the fence charger is a pulse type, and on a separate power meter. and the fence is about 80 ft from the house at closest point and its powering about 14 miles of fence. 5 strands on each pole controlling 3 pastures..

anyone have any suggestions of a cure ?

[sofaspud]

Wow... I could probably ask a long series of questions about each system.
This industrial-grade fence works by zapping grounded livestock to keep them in, correct?
If the noise is truly coming in through the power lines (i.e. it's not RF transmitted) I'd think there's nothing to do but block/filter it out.
Precisely what/where/how 2 B determined.

[MrAl]

Hi there,


He can try a line filter, two actually, one on the home system and one on the fence charger.

If it is coming in through RF though it may be more difficult than that. Many times a strong spark RF can punch through the input section of an audio amplifier where the input stage acts like a crummy but effective detector. Maybe grounding the home system a little differently may help or it may make it worse. Maybe a large sheet metal plate grounded placed between the nearest fence section and the home system, like right behind the home system box, might help too.

[Bob Scott]

Hi dac,

I am not familiar with the circuit of an electric fence, but I think that uses inductive discharge like a flyback transformer in a TV. The TV's transformer has a capacitor associated with the primary of the flyback so that the LC combination creates a tuned circuit with a resonant frequency several times higher than the horizontal frequency.

I'd suggest try adding a small capacitor to the primary of the fence transformer, so that it resonates at a frequency lower than radio frequencies. Try to find the right value of capacitor value high enough so that it no longer causes interference yet low enough so that high voltage is still created.

[dacflyer]

i am sure the transformer inthe fence charger is more like a high voltage transformer, rather than a flyback type.. the power to the coil is pulsed, sort of the same way a ignition coil works on a car,
i have seen some old old models that ran off a 6 volt battery, and had a mechanical way of working. but they were just as effective as a modern type. connect a 6 volt truck battery to the unit, and it would work for months.

[Bob Scott]

i am sure the transformer inthe fence charger is more like a high voltage transformer, rather than a flyback type.. the power to the coil is pulsed, sort of the same way a ignition coil works on a car,
i have seen some old old models that ran off a 6 volt battery, and had a mechanical way of working. but they were just as effective as a modern type. connect a 6 volt truck battery to the unit, and it would work for months.

Yes, but the cap should still work if connected across the primary winding, no matter what type of transformer. I don't see your point.

[dacflyer]

ok, thought you ment across secondary...

[dacflyer]

PROBLEM SOLVED >>>
found out that the RCA cable from TV to the AMP was defective, and it was a new cable..go figgure, THANKS CHINA

[haklesup]

AV cable! Time to upgrade to HDMI

Still interested in how the noise did couple into that wire presumably so far from the source. I suppose reversing polarity on the fence zapper would just be equivelent to the starting point.

[MrAl]

PROBLEM SOLVED >>>
found out that the RCA cable from TV to the AMP was defective, and it was a new cable..go figgure, THANKS CHINA

Hi dac,


Good to hear, hope it stays that way this time

[Ken1]

Hi, A modern electric fence charger works by charging up a capacitor typically with a rating of 20 to 30 uF 900 VDC and then using an SCR to dump the charge into the primary of a step up pulse transformer to produce a pulse of several thousand volts which is then applied to the fence. The units can be either 120 VAC or 12 VDC powered and use an oscillator to charge up the capacitor to around 600 volts. The SCR is tripped using a zener diode and a few resistors to set the trip point. A heavy diode is placed across the SCR just like a damper diode across a horizontal output transistor in a CRT tv to prevent the SCR from being destroyed by the back emf. The unit generates a high voltage pulse about every 1.5 seconds. I have repaired many of these over the years for the local livestock farmers around here.

[dacflyer]

i think the RCA cable had a bad ground, the RCA was for the audio, not video.

what have you found that typically goes bad on the fence chargers ?
i like the battery (electro mechnical types) old technology...

[Ken1]

Most common failure is the SCR shorts out. Occasionally the damper diode will short. Also occasionally the charging capacitor loses capacity or opens. On battery powered units, the switch transistor for the oscillator fails occasionally. It's also common for these units to get hit by lightning as fences are long and are easy targets. As long as the board doesn't get burned or components burned beyond recognition, changing out all the semiconductors gets them going again. Today's high powered units cost upwards of $300 and there are only a few semiconductors in them and they are all common and easy to obtain.

[dacflyer]

thanks for the info

[mgscarbo]

Anyone know of a source for the a tranformer like the one Ken1 is talking about? My charger is a Zareba A5. Where can I get a suitable step up pulse transformer to replace my faulty one? Zareba only sells the whole circuit board as far as I can tell. They wouldn't tell me the specs for the transformer either.