Thoughts Please on "Electric Fence Circuit"

Thread Starter

Dobbies

Joined Dec 13, 2017
29
Hi

Having built this simple circuit which seems to work fine, I'm looking for you guys to cast an eye over it, especially on the high voltage side ( after IC2 ).

The two diodes I've used ( D2 & D4 ) are the best I could find in "Circuit Wizard" library but I'm open for criticism on both the necessity of use and the values to counter spikes. The MOSFET seems to work well and only gets warm on the heat sink.

I set the "pulses control" to achieve 0.5Hz and the "power control" to about 75% to achieve an output pulse of 4KV

Circuit:
fencer.jpg

Your comments would be appreciated
 
Last edited:

ericgibbs

Joined Jan 29, 2010
10,032
hi Dobbies,
Which types of animals are you planning to contain within the fenced area.?
BTW: should the 4000KV in post #1, read 4KVolt.?
E
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
Hi Eric,

Thanks and well spotted. By the size of my typo error one would expect dinosaurs! :eek:, but of course it should have read 4KV.

My farmer friend keeps sheep and cows.

Best regards
 

ericgibbs

Joined Jan 29, 2010
10,032
hi D,
Sheep are a real pain to keep contained with a electric fence, all that wool and lanolin makes them a good insulator.
A 4KV level will not be enough for sheep, we use old 12V vehicle ignition coils pulsed at about 25KV [ use an old spark plug as a tester] on our farm.
A good Earth return is important, add some protection against lightening strikes

Cows and Pigs are easier to contain.

A good 'trick' is to tie a short red ribbon every few metres along the HV wire, after a few shocks they learn not to touch the wire.

E
 

Dodgydave

Joined Jun 22, 2012
9,160
I would replace the mosfet for an IRF540, and T1 transformer with an ignition coil from an old car, much better output,.
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
hi D,
Sheep are a real pain to keep contained with a electric fence, all that wool and lanolin makes them a good insulator.
A 4KV level will not be enough for sheep, we use old 12V vehicle ignition coils pulsed at about 25KV [ use an old spark plug as a tester] on our farm.
A good Earth return is important, add some protection against lightening strikes

Cows and Pigs are easier to contain.

A good 'trick' is to tie a short red ribbon every few metres along the HV wire, after a few shocks they learn not to touch the wire.

E
Thanks for the advice but it's not my farm. I can up the power as needed when it's up and running.

Any thoughts on the circuit as I'd need to order other parts if the ones I've chosen aren't suitable?

Cheers
 

Dodgydave

Joined Jun 22, 2012
9,160
D2, D4 are not needed really, the IRF540 has built in zener protection, only thing i would change is to put a 1N4148 diode across the 100 ohms resistor R8 Cathode to pin3, this will rapidly shut off the mosfet for a bigger spark.
 

ericgibbs

Joined Jan 29, 2010
10,032
hi,
I use two 555, same as your circuit, one 555 is the dwell time and the other is the interval timer.
So we can set the interval between bursts and the dwell, the period the HV is On.
The output is a 12V relay driver, not a semi, to the ignition coil primary, which is connected via a ballast resistor to limit the primary current to about 4 Amps.

I know this doesn't answer your query, but I thought a little background info might be useful.:rolleyes:

Will look over your circuit.

E
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
D2, D4 are not needed really, the IRF540 has built in zener protection, only thing i would change is to put a 1N4148 diode across the 100 ohms resistor R8 Cathode to pin3, this will rapidly shut off the mosfet for a bigger spark.
Thanks for the info Dave, I've just sent off for an IRF540. I see from the datasheet that it's rated slightly higher than the IRF740.
With regard to the 1N4148 across R8, could you please explain in a little more detail how this works? I don't recall seeing this configuration on any other circuits I've looked over for this purpose.

Many thanks
 

Dodgydave

Joined Jun 22, 2012
9,160
The idea is to turn the mosfet hard on and off fast as possible , so that it doesn't have any resistance between the D/S pins, this will make it into a near perfect switch, and therefore won't get hot.

The 100R resistor R8, will turn it on and will also turn it off, but the gate has a small amount of capacitance and so will take time to discharge before it turns off, the diode makes this much faster.
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
The idea is to turn the mosfet hard on and off fast as possible , so that it doesn't have any resistance between the D/S pins, this will make it into a near perfect switch, and therefore won't get hot.

The 100R resistor R8, will turn it on and will also turn it off, but the gate has a small amount of capacitance and so will take time to discharge before it turns off, the diode makes this much faster.
Many Thanks
 

LesJones

Joined Jan 8, 2017
2,639
I think with the 1:50 transformer shown in the schematic and the 20 volt zener diode the output can be no more than 1000 volts. (20 volts x 50) The back emf on the primary of an ignition coil needs to be allowed to reach a few hundred volts to give a high enough votage on the secondary to produce the spark. I think a mosfet with a higher source drain voltage rating than the IRF540 is required and the zener diode needs to be a few hundred volts rating (Several in series.) I think the IRF740 would be more suitable as I think it has a voltage rating of 400 volts.

Les.
 

crutschow

Joined Mar 14, 2008
24,959
An ignition coil typically has a 100:1 turns ratio so a 20V zener on the primary will give about 2000V on the secondary.
I don't think you need more than a few thousand volts on the secondary for effective control of the animals, so a few hundred volts on the primary should not be required.
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
Thanks for all your support on this project. The circuit is now complete taking into account all your "varied" advice .

The transformer in the schematic is just a representation thanks to the limited component catalog within "Circuit Wizard", the program I'm using. The transformer I'm actually using is out of an old fencer unit so I presume it's 1:100 or there about. I've removed both Zener diodes from my original circuit and changed R8 to 10 ohms as advised. I also added reverse power protection.

I've tested the circuit and it seems to work well and produces a healthy spark across a 6/7mm gap but no more. Turning up the power "Pot" only seems to make the MOSFET warmer but doesn't seem to increase the spark strength or jump a bigger gap. The oscilloscope graph at the bottom of the schematic indicates around 4KV spikes at 0.5Hz which I would think is ample. I would like to be able to increase the output power slightly but don't have the necessary knowledge to do so without making the circuit more complicated.

Also and very occasionally when the circuit is running, the single pulse spark will change and become an arc momentarily, but many times. I'm guessing this is something to do with back EMF because when it's happening the MOSFET's heat sink gets much warmer. Then it settles down again to single pulses everything seems stable again. Obviously I need this circuit to be robust and reliable so if someone could talk me through what's happening here I'd appreciate it, and also explain the importance of R8 and it's value?

Thanks again
Bob
MOD: Image expanded.E


Redesigned-DIY-Circuit.jpg
 

crutschow

Joined Mar 14, 2008
24,959
You should have a zener from the MOSFET drain to ground to protect it from any spikes over its voltage rating.
When the transformer secondary is open-circuit, the primary voltage is limited only by the stray capacitances in the circuit.
The MOSFET may be going into some sort of breakdown mode when the arcing occurs.
 

Hymie

Joined Mar 30, 2018
819
The arcing may be the result of electrical noise on the supply line due to the switching of the transformer T1 – causing spurious triggering of the timer circuits.

I would give consideration to filtering the +12V supply line to the 555 timer ICs and their associated components.
 

Thread Starter

Dobbies

Joined Dec 13, 2017
29
hi D,
Sheep are a real pain to keep contained with a electric fence, all that wool and lanolin makes them a good insulator.
A 4KV level will not be enough for sheep, we use old 12V vehicle ignition coils pulsed at about 25KV [ use an old spark plug as a tester] on our farm.
Eric,

I've been playing around with this circuit for a while now, and albeit the original circuit I posted shows T1 as a 1:50 ratio, it should have read 1:100.

In practice I am actually using a brand new car ignition coil, but the output results I'm getting are disappointing. The built in oscilloscope within my "Circuit Wizard" program indicates a peak output voltage of around 4/6KV but in practice the spark looks quite weak and will only cross a gap of around 7/8mm maximum. Whilst testing the circuit in the field, touching the fence only produces a minor shock to me.

I noted that you wrote in the quoted "12V vehicle ignition coils pulsed at about 25KV", far from what I'm getting. Do you have any circuits or advice on how I could boost the output from the coil to get a better result?

I've come across some fencers that have dual Hi/Lo outputs or boosters, but I haven't been able to find a circuit.

Any help appreciated

Thank
Bob
 
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