Hi All,

I'm a newbie - so go gentle on me. I'm not thick - but am not an electronics wizard.....

I have an electric fence to protect my chickens from Mr Fox (And his many, many brothers and sisters that line up along my fence line every day looking for a snack). It works well (if you touch it accidentally you can tell how well it works pretty sharpish....)

The fence is energised by a Daken MT15 - which is a 240V mains energiser that delivers a maximum voltage of 8.9Kv and has a stored energy rating of 1.75J. It pulses as all electric fences do.

The energiser, as it is mains fed, is mounted on the inside of my shed - about 50m away from where the chickens go about their daily business - a cable runs a live and earth underground to a point close to my fence, then it connects to an earth post and an insulated alligator clamp for the live wire then connects to my mesh fence.

The energiser also feeds another part of my property (to project part of the garden from deer that just eat anything they can get to) so it can't be moved closer to the chicken area.

The problem I have is that I can't see if the fence is live - I have to go to the fence and unclip the alligator clamp and check for a jump to see if it's live - or use a tester that only really works when it's dark....(you hang it on the fence and touch a probe to earth - LED's flash up the charger showing the voltage is good)

What I need is a light that pulses each time the energiser pulses to the fence. My plan was to mount this on a post so I can see it from the house.

What I've done is this:-

Bought a Ne2 Neon bulb along with some 10Mohm resistors. ( I was advised that LED's would not work as they would draw voltage away from the fence - there is not a circuit UNLESS something touches the fence and earths it)

My thinking was power = VI -
volts = 8,900 and as joules is watts / second and joules is 1.75 then amps = 1.7/8900 = 0.00019W

V=IR - so R=V/I 8900/0.00019 = 46.8Mohms should be about enough to force the voltage through the fence and not short across the bulb every time drawing voltage from the fence.

Problem is it don't work....

With the circuit

Charged fence -> Ne2 -> earth the bulb flashes but all the flow is through the bulb - the fence is not live - if you touch it you can feel nothing - which means Mr Fox will be in there like, well like a fox in a chicken coop.

With the circuit

Charged fence -> Resistor (10Mohm) -> Ne2 -> earth - as above

With the circuit

Charged fence -> Resistor 10Mohm -> Resistor 10Mohm - > Ne2 -> Earth Nothing flows through the bulb and no light flashes.

The obvious thing for me to do is to try with some smaller resistors - but my question is why doesn't it work with only 20Mohms in the circuit - I would have thought I'd have needed 4 resistors to make sure most of the voltage stayed in the fence?

Or maybe I am thick and I've done something really dumb....

any help would be appreciated...

Cheers
Andy

 

hi Bro,
Welcome to AAC.
The problem is, if you connect any device to the hot fence wire it will pull down the line EHT pulse, your tests have shown this.
Do you have an old portable AM radio receiver that you could use check the fence EM pulse field.?
If yes, set the radio to a low frequency band and off tune from any radio stations, you should hear a strong click on the radio, when near the fence.

I guess you know its a good idea to tie short lengths, say 12 inch long, of Red ribbon on the hot wire every few metres.?
The critters quickly realise after a couple of EHT belts, that ribbon indicates a hot fence and will tend to avoid the fence even when its off.

Is it possible to access the internals of fencer unit and inductively pick a pulse from the unit, that you could relay to the house.?

E

 

Hi Eric,

Thanks for your reply. Sorry I don't have a AM radio - was really looking for a pulsing light so I can see if the fence is live from a distance - particularly in the evening when it's critical that the fence is on. I used to have a solar powered stand alone energiser - and that had a built in flashing light - so I could see from afar if it was hot. This unit, unfortunately, stopped charging recently - the battery must be shot and as I have another "mains wired" unit I've just linked the chicken fence to it via an old extension cable - hence my not being able to see if it's hot problem (first world problem I know - but as I used to have this comfort factor I thought for a few bucks and digging out my old soldering iron would get me back to my previous level of security confirmation.

I'm guessing, as you say, anything that is bridging between the hot fence and earth will attract the flow - as the effective residence between the fence and the earth is infinite unless it is touched by Mr Fox, so whatever I put in the circuit between the fence the light and the earth is better than that. That's why I thought it would work - if I put a resistance close to the max it can be but still trigger the light, but not be "easier" for the flow to choose this route rather than through the fox's nose I thought it would work. Clearly it don't - probably calculated the resistance incorrectly.

Incidentally, I put the Ne2 light directly inline between the Live alligator clip and the fence - in series effectively with the fence, and it flashed - and the fence appeared to be hot - not sure how hot though - I'm guessing I will need to test the fence with this in circuit to see what voltage it's giving (I can use the tester I have for that check). Not sure that a Ne2 will last that long though with 8.7Kv passing through it every 1.5 seconds....

Yep - the chicken fence has tape flapping off it - but the foxes are wily crafty creatures. They come back and try frequently - I've had one chew partially through the second bottom line recently - but I'm guessing it wasn't prepared to keep at it as it only broke one or two strands....

Cheers
Andy

 

hi Andy.
Unfortunately the fences I made were for our farm are over in South Africa, I am in the UK, so I cannot try out some ideas.

We do get urban foxes in the gardens, so have I PIR's, they work well with fox size animals.
I use a one of the units that you can record a message on and it plays back the message when the PIR's are triggered,
I cannot post what the verbal message is as I would get Banned.! but the foxes/cats get the message and run like hell.!
Fun to watch their reactions on security CCTV videos.

E

 

HI Eric,

I'm English but have lived here in Australia for the last 16 years (Melbourne) and the foxes are a bit of a problem. We have 20 acres (as do all the properties around us) so there are a lot of foxes about. Everyone has chickens, cows, goats, etc and there are kangaroos everywhere. It's only the foxes that are a problem - I've got lights that are motion triggered - hadn't thought about a noise - but the area fenced off is about 20m X 5m - so really the fence is the best bet...

Cheers
Andy

 

First. The result of your calculation is not watts, it is coulombs. I think that with the resistor in series with the neon the flash is too short to see. With the neon directly across the output it is clamping the voltage to about 80 volts so the stored energy in the inductor in the charging unit takes longer to be dissipated and also the current through the neon will be higher. You could try connecting the neon in series with the connection to the fence. The stray capacitance and leakage to ground will probably be enough to make the neon. This will subtract about 80 volts from the pulse to the fence but you will still get a shock if you touch the fence flash. (And the flash from the neon will be brighter.) If this does not work then I think you will need a unit with it's own power supply.(Battery) You would need a resistor of a few hundred meg ohms from the fence to the gate of a mosfet. You would also need a zener diode (About 15 volt) between the gate and source of the mosfet to limit the gate voltage. There would be an LED and resistor in the drain circuit of the mosfet. (A capacitor could also be added to lengthen the flash of the LED. For this solution you would have to know if the pulse to the fence was positive or negative to decide weather to use a P channel or N channel mosfet.

Les.

 

Thanks Les,

Ref the calc - I'd put Watts when I actually meant Amps!! So you pointing out that it should have been coulombs means I'm twice as daft as I thought I was !!!! But thanks for that anyway.

Ref the options you point out - I think you are correct and rather than me trying to be clever and having a different circuit to fire the light between the fence and earth, the easiest solution is to link the light between the energiser and the fence, in series with no resistor. If you are saying this will only drop the voltage to the fence by 80V then it's not an issue (the energiser puts out 8.9KV so taking 80 away is not going to make that much difference to Mr Fox - it's still going to remind him that my chickens are not for dinner.

Will a Ne2 light stand up to this high voltage and pulsing source over a prolonged time? I don't want it to blow and then, as it is in series to the fence, effectively stop the fence from energising and therefore not allow the fence to do it's job.

The other option sounds pretty complicated for someone like me - some of the terms you've used in describing the potential solution are a little beyond my capability (which is based on A Level Physics taken in the mid 80's) - so I will go with your first option - but as I say - do you think I'm at risk of blowing the Ne2 ?

Regards
Andy

 

Presumably the fence energiser has something internally to limit the fence current in the event that the fence gets short-circuited? Providing the current limit is a good bit less than the maximum rated current of the neon then the neon would not be harmed.

 

Presumably the fence energiser has something internally to limit the fence current in the event that the fence gets short-circuited? Providing the current limit is a good bit less than the maximum rated current of the neon then the neon would not be harmed.

Thanks Alec - there is very little current flowing (by design as 9,800V and any amount of current would result in BBQ fox rather than irritated fox) so if the Ne2 will stand high voltage low current for an extended period this looks like my easiest solution -i.e. Energiser -> Ne2 -> Fence

Regards
Andy