# A simple electric fence idea

#### crutschow

May 7, 2021
852
A practice I adopted in days of yore was to do every first smoke test of any line-powered gadget with AC supplied from a Variac
Certainly good to do if you have a Variac available.
With only ~1/10th second between 80V and 90V, seems a pretty good reaction time would be needed to use this method!
No slow electronikers allowed.
But it would need to go some above 100V before it starts stressing C2.

#### CircutScoper

Mar 29, 2022
300
Certainly good to do if you have a Variac available.
No slow electronikers allowed.
But it would need to go some above 100V before it starts stressing C2.

200ms then? Practically all day!

#### wingnut

Aug 9, 2012
255
I have an old microwave transformer. When I energise the primary coil with a car battery (the 220V input), on the output high voltage side it gives me a nice kick between the high voltage wire and the chassis of the transformer. Can I use this transformer in place of the car coil?

#### Minder

Apr 24, 2015
3,478
Ensure the pulse is very short, milli-seconds.
.

#### crutschow

May 7, 2021
852
Can I use this transformer in place of the car coil?
The microwave transformer has about a 10:1 turns ratio whereas an ignition coil is usually 100:1, so the fencer output voltage would be reduced to about 800V, which may not be enough to get through the hair on an animal.
My simulation also shows that the large inductance of such a transformer will give oscillations in the circuit.

To use the microwave transformer, it would likely need to be used in a Kettering (inductive) type circuit, with a large pulse current through the primary, such as your battery provided, so it would require a battery or high current, low voltage supply (e.g. 12V) for its operation.
There are many posted inductive type fencer circuits if you want to go that route.

#### CircutScoper

Mar 29, 2022
300
I have an old microwave transformer. When I energise the primary coil with a car battery (the 220V input), on the output high voltage side it gives me a nice kick between the high voltage wire and the chassis of the transformer. Can I use this transformer in place of the car coil?

Well, if your microwave xfmr provides isolation between primary and secondary, why not revisit your original idea of a fence with direct AC excitation and simple impedance limiting to hold the current to a safe level? You might need to provide a grounding surface (e.g., a few square yards of metal screening) depending on how much discouragement the unwanted critters will need to convince them they're truly unwelcome.

If you go this route, it might also be a good idea to put it on a midnight-to-pre-dawn timer to minimize potential (heh-heh) human contact, together with some highly visible warning signs.

#### crutschow

May 7, 2021
852
why not revisit your original idea of a fence with direct AC excitation and simple impedance limiting to hold the current to a safe level?
The impedance should be sufficient to limit the current to below 10mA.
Also to be double safe, the applied voltage should be short duration of perhaps 100ms and interrupted periodically (once a second) to allow a person to let go of the wire and minimize the chance of electrocution.
This can be done, for example, with an SSR in the primary controlled by a 555 timer.

#### CircutScoper

Mar 29, 2022
300
The impedance should be sufficient to limit the current to below 10mA.
Also to be double safe, the applied voltage should be short duration of perhaps 100ms and interrupted periodically (once a second) to allow a person to let go of the wire and minimize the chance of electrocution.
This can be done, for example, with an SSR in the primary controlled by a 555 timer.
Good thoughts.

Ultimately, and depending on the size of the critters you need to discourage, it may come down to a SWAG tradeoff between the likelihood of unexpected human-trespasser contact with your fence when it's live, and just how desperate you are to deter the varmints.

#### crutschow

May 7, 2021
852
it may come down to a SWAG tradeoff between the likelihood of unexpected human-trespasser contact with your fence when it's live, and just how desperate you are to deter the varmints.
Well if you go to the dangerous side of that, I trust you have good attorney to defend you against a wrongful death suit.

#### CircutScoper

Mar 29, 2022
300
Well if you go to the dangerous side of that, I trust you have good attorney to defend you against a wrongful death suit.
Yup. Gone are the days when "Enter at your own risk" could mean what it says.

Now, of course, it only means "Enter at mine."

#### CircutScoper

Mar 29, 2022
300
Yup. Gone are the days when "Enter at your own risk" could mean what it says.

Now, of course, it only means "Enter at mine."

BTW, 10mA of arm-to-arm AC is, as I recall, near the maximum of the lethality-vs-current curve, because it's just right to induce self-sustaining ventricular fibrillation.

Less is harmless, while more will induce total cardiac arrest, which sounds worse but isn't because it will spontaneously convert to sinus rhythm when the juice turns off.

That's the basic principle behind the defibrillator.

Just sayin'.

#### crutschow

May 7, 2021
852
Just sayin'.
What exactly?
Any amount of AC current the body above 10mA is dangerous.
I would not count on a spontaneous recovery of the sinus rhythm from any value of AC current that has caused cardiac arrest.

The defibrillator uses a large DC pulse.

#### CircutScoper

Mar 29, 2022
300
What exactly?
Any amount of AC current the body above 10mA is dangerous.
I would not count on a spontaneous recovery of the sinus rhythm from any value of AC current that has caused cardiac arrest.

The defibrillator uses a large DC pulse.

Defibrillators work by arresting the heart.

#### crutschow

May 7, 2021
852
Defibrillators work by arresting the heart.
Yes.
My point was, they use DC for that, not AC.

#### CircutScoper

Mar 29, 2022
300
Yes.
My point was, they use DC for that, not AC.

True that current (no pun) versions do, but early ones used AC. The principle is the same.

#### wingnut

Aug 9, 2012
255
I am super grateful for the contributions to this thread thus far. I probably will go with the recommended circuit.
BUT....

...at heart I am a simpleton cheapskate and before I go and buy parts, what is your opinion of the circuit below as a simple, cheapskate electric fence. OK it only gives a 160V DC shock. By increasing C1 I can increase the current of the shock. But it would be impossible for this circuit to deliver a significant current except for the initial 160V discharge of the 1u capacitor. And the circuit is insulated from the mains by two caps, a huge resistor and a diode, surely making it safe.

Thank you again.

#### Minder

Apr 24, 2015
3,478
Unless this is sourced from some isolated supply, the circuit is dangerous for anyone coming in contact.

#### CircutScoper

Mar 29, 2022
300
Unless this is sourced from some isolated supply, the circuit is dangerous for anyone coming in contact.

Well, maybe it's not a good idea to get too dogmatic on this subject. It seems that the supposed safety of ANY electric fence design is only a relative thing. Per the old saying: You pays your money and you takes your chances.

Electric fences and accidental death
Michael Burke 1 2, Morris Odell 3 4, Heinrich Bouwer 3 4, Adam Murdoch 5
Affiliations
Abstract

Deaths which occur in association with agricultural electric fences are very rare. In fact, electric fences have undoubtedly saved numerous human and animal lives by safely and reliably keeping livestock confined to their fields and enclosures and thus preventing motor vehicle incidents when livestock get onto roads and highways. Accidental and intentional human contact with electric fences occurs regularly and causes little more than transient discomfort, however, on exceptional occasions, contact with electric fences appears to be directly related to the death of the individual. The precise pathophysiological cause of these deaths is unclear. We present two cases of deaths associated with electric fences, discuss the possible pathophysiological mechanisms in these cases, and suggest a universal approach to the medico-legal investigation and documentation of these deaths.

Keywords: Accidental death; Cause of death; Electric fence; Electrocution; Forensic.

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