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Queries regarding electric fencing

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renault8gordini

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Hi Everybody, this is a great forum!
First time poster so be gentle!
My Garden has recently been visited upon by a pesky animal called a possum (similar to a squirrel but a bit bigger) who just loves the taste of my herbs, lettuces and flowers!
I have been looking for a simple circuit to power off 12 volts that will deliver a small shock to the animal to dissuade him from his next meal, but not stun him into unconsciousness.
Can anyone suggest a circuit design that would help me?

Thanks,
Julian
 
Hello Julian.
A simple automotive ignition coil fed by 12V AC from a wall transformer works.
 
You can find electric fence circuits on the internet. Basically, they create a high voltage (>10kV) pulses about once a second. Its low current, so its not meant to kill, but to "convince" critters to go away. We have one for our horse pasture. Although building such a circuit is possible, you won't be able to do it more cheaply than buying one. Here in the US, Tractor Supply Co. sells a cheap model for about $20. They even have some solar powered ones. If you're building for the sake of building, then more power to ya :) I think that this would be great.
 
Hello Julian.
A simple automotive ignition coil fed by 12V AC from a wall transformer works.
All too well. A standard ignition coil has a turns ratio of about 100:1 which would generate about 1200V AC from a 12V AC source. An ignition coil has an output resistance of perhaps 5k ohms which can thus supply a current over 200mA. That's well above the lethal range for humans. Thus I wouldn't recommend using an ignition coil in that manner to wire a fence, unless you don't mind dead bodies in your yard.
 
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Hi Renault8Gordini,

as Crutschow already pointed out direct HV generation is lethal. Here is a circuit which delivers timed "shocks" of 15.5, 24 and 31ms. The equivalent power using an ignition coil is 250, 400 and 450mJ, enough to scare the oppossum away but not deadly.

The dual switch is used for circuit safety. If the switch fails the timer circuit might get stuck with a high output, thus causing the MosFet transistor to conduct continuously and frying the transisistor or the ignition coil, most likely both of them.

Boncuk
 

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