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Electric fence with modern devices

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SG61-addict

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Hello,

I'm trying to install an electric fence in my garden to protect my vegetables and trees against boars, deer and other wild game. To be effective I have to put 5 wires from 20cm to 1.5m from the soil and be quite generous with the energy discharge. I will probably build a separate circuit to drive the lower wire, I don't want to kill or over stress the neighbor's cat :)

Basically, a lot of peoples do it with a 12V battery, a car ignition coil and a power transistor driven by a 555 or any pulse generator. The circuits I've found on the web are really old and often use ugly NPN darlington...

basic hv gen.png


In theses designs, there is a big spike at the MOS drain caused by the coil at the opening at the MOS. They try to dump it:
- Adding D2 => works fine but also dump the hv pulse so the whole design is useless
- Adding D3 and/or D4 => in simulation, this have absolutely no effect
- Doing nothing and changing the switching device when dead

When simulating this design with no diodes but a real NMOS model, the Vdrain peak voltage is approximately 10-50V higher than the max Vds voltage specified in the datasheet.

What will be the good protection device if I want to use a NMOS device ?


Some rare designs are using a capacitor in series with the primary coil. The capacitor is initially charged to a medium voltage (100-300V) and then it is shorted across the coil. This more complex design looks to be less RF-noisy and less aggressive for the switching device. The medium-level voltage can be easily generated with a little coilcraft transformer (e.g. DA2032, ...).

capacitive mode.png


In simulation, this works fine with some particularities :
- Compared to the basic schematic, the output energy is much lower when output load decrease
- The primary coil of a car ignition coil is intended to be driven by a 12V signal, not 100V. The output voltage with no load is enormous. The coil insulation may be insufficient.
- Since the energy came from a LC resonant tank, the output waveform is a sine. But ok, a deer will probably not see the difference.

I really would like to build something strong and robust. Not afraid by complex electronics and modern packages.

Professional generators are really expensive. Affordable ones are either not powerful, not robust or I can probably build them for 1/10th of the price.

Any advice will be strongly appreciated :)
Many thanks
 
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Hi,

Upper schematic:

* omit D2 and D3.
* Make D4 a zener. As high as possible, just low enogh to protect the MOSFET.

For sure the MOSFET should be high voltage, too. I can´t find any information on this BSC42.. thingy.

Klaus
 
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You could just remove a car ignition system for the plugs and use resistance plug wire (50kohm/m ??) to limit current in the connection. Barbed wire will extend the wires past fur or hair and arc more easily. How many Joules can annoy a Boar enough that won't kill a cat?

Porcelain insulators will be needed to reduce weather related leakage and unintended corona. I would expect 3kV/mm to creep down to 300V/mm in dusty wet weather or 1/3rd of that from a sharp tip.
 
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An "avalanche rated" MOSFET might be a good way to go. But
you will need to smack it hard or see Miller plateau soak up a
lot of the coil energy (possibly destructively). A big MOSFET will
not be happy without a big driver.

Old timey big bipolars can be pretty breakdown-tolerant (if
you keep pulses sparse enough to respect power limits).

You used to be able to find electronic ignition modules
(a Chrysler, I recall, was popular) that work "standalone"
(bring your own trigger) and these will whomp a coil real
nicely and indefinitely. Check the "DIY EFI" pages (if that's still
a thing) for ignition related stuff.
 
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Hi,

If I´m not mistaken an electric fence need to limit the current as well as the pulse energy to satisfy safety regulations. (maybe even pulse frequency)

Klaus
 
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You would also be able to pick up the EMI on an AM radio when it zaps like a lightning stoke (on an AM radio not to the wildlife). The introduction of current limiting carbon wire to replace metallic wire in cars happened (AFAIR as far as I recall) over 40 yrs ago to reduce AM interference and increase plug lifespan.

there are law and outlaws ... Seek guidance on local electrical codes before installing. Otherwise you may need a solicitor.
1705443557083.png
 
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SG61-addict said:
The primary coil of a car ignition coil is intended to be driven by a 12V signal, not 100V. The output voltage with no load is enormous. The coil insulation may be insufficient.
Click to expand...
The primary of a standard Kettering ignition car coil sees a typical 100-200V spike to generate 100 times that to the plug when the points open (for a typical 100:1 coil), so 100V from the capacitor is not a problem.

That advantage of a CD ignition is that you can increase the capacitor size to get the output energy you need for the fence length (minimum of about 1 Joule per mile of fence).
 
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The pulse need only be 20 - 50uS in the ckt shown, using a 12V source and a 12V car coil is the better way to go - no big series cap needed

on time can be 1-2mS or a bit longer and the turn off zeners can be 220V ( 5W types ) to get the step up - you can also place an 220nF 630VDC rated cap across the zener to get a softer turn off and less radiated RFI from the line.
 
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Hi guys,

Thanks for all your replies.

Concerning the regulation, it is hard to know exactly what is allowed or not (the full official document is more expensive to buy than a commercial HV generator)...

What is known is that the pulse repetition rate must be >1s and the energy should be less than 5J for 50-500ohm loads. 5J is definitely higher than what I need. I have only 5 // wires, 0.1mile long. 1J/mile is the standard requirement to overpass leaks, so I need only 0.1J.

The voltage is important to arc and break the resistive part of animals (skin + hair). Considering that, a sheep need a higher voltage than a boar. But a boar probably need higher energy than a sheep to be well educated :)

I tried some simulations with a 300V MOS protected by a 200V zener. Works well. Energy is low for low loads (down to 3mJ at 50ohm !).

With 12V input voltage, the output energy is essentially limited by the primary coil resistance (0.5 ohm in my simulations, 0.2 to 3 ohm according to the documentation I've found). I'm going to buy a random car coil and hope its primary R will not be too high...
 
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SG61-addict said:
I have only 5 // wires, 0.1mile long. 1J/mile is the standard requirement to overpass leaks, so I need only 0.1J.
Click to expand...
The suggested energy of 1J/mile is for the capacitance of the wire, which is the primary load.
So 5 || wires, 0.1 mile long, would require 0.5J.
SG61-addict said:
the output energy is essentially limited by the primary coil resistance
Click to expand...
For a Kettering type circuit, the output energy is limited by the coil inductance and saturation current, not the resistance.

I have the design for a simple, line-powered CDI type circuit that delivers about 2J.
If interested, I can post it.
 
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Okay, below is the LTspice sim of the circuit:
The circuit is directly powered from the line (be carful when working on it and only plug it to a GFCI socked) so the required isolation to the fence is provided by a dual-output (lost spark) type coil which is isolated from primary to secondary (standard coils are not).

C1 is a low-cost, motor-run film type capacitor whose AC impedance efficiently limits the charging current through the rectifiers to the storage capacitor C2.
C1's value charges up C2 to the 100V trigger voltage in a little over a second.

C2 is an 560µF electrolytic, that stores a little over 2J of energy.

When C2's voltage reaches 100V, Zener D4 turns on the transistor to fire the SCR and rapidly discharge the capacitor through the coil's primary to generate the spark.

One caveat is that the circuit has only been simulated, not built, so some tweaking may be necessary to get a real circuit to work as desired.

1705506260515.png
 
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crutschow said:
The suggested energy of 1J/mile is for the capacitance of the wire, which is the primary load.
So 5 || wires, 0.1 mile long, would require 0.5J.

For a Kettering type circuit, the output energy is limited by the coil inductance and saturation current, not the resistance.

I have the design for a simple, line-powered CDI type circuit that delivers about 2J.
If interested, I can post it.
Click to expand...
Old points-ignition cars I have had, all had a high power resistor in the coil primary - points - IGN path.

That is the "backstop" for key on engine off (KOEO) condition. Point dwell sets the coil current by Volt-seconds in normal operation at higher RPM.
 
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@crutschow - standard practice is to put a 10A10 diode in inverse parallel with the SCR to recover energy on an unloaded line

560uF is a little large, 250uF at 200V is 5 Joules - which is getting up there according to the regulations

Also the coil has to be mains rated in the circuit shown, i.e. 2500Vac pri to sec for 1 min.

with that design the coil will saturate pretty early on and the peak currents in the SCR can be well in excess of 500A ( we design Tx's for Elec fence companies )
--- Updated ---

@dick_freebird the series resistor for the coil is an aid to starting, ( when the batt volts typically drop ) the series R is shorted out at starting by the key - and then unshorted at key release to limit coil current under driving.
 
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Easy peasy said:
560uF is a little large, 250uF at 200V is 5 Joules
Click to expand...
The circuit charges to 100V, not 200V.
I don't want more than 10kV on the secondary.
Easy peasy said:
Also the coil has to be mains rated in the circuit shown, i.e. 2500Vac pri to sec for 1 min.
Click to expand...
Since the coil has a normal primary to secondary voltage of 10kV, that should not be a problem.
 
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"The circuit charges to 100V, not 200V." - this is not guaranteed - if the ckt is left to charge longer it can reach 200V


I don't want more than 10kV on the secondary.

Easy peasy said:
Also the coil has to be mains rated in the circuit shown, i.e. 2500Vac pri to sec for 1 min.
Click to expand...
"Since the coil has a normal primary to secondary voltage of 10kV, that should not be a problem."

a normal car coil has a link from LV to HV - i.e. it is a 3 terminal device - it has no pri to sec guaranteed safety rating at all if you manage to disable that link.
 
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I never saw more than 102V on the Cap in my simulation with no load. XFMR is 8mH N=0.01
Output does have some loading effects but it pulses 10kV continuously every few seconds.

Various RLC loads of just the wire can produce some resonance at high impedance but the mean of 10kVpk is very consistent.

1705712773536.png
 
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