Neon light transformer, with primary switched on/off every second, to create a "pulsed" electric fence charger?

FredFlintstone

Joined Apr 30, 2022
23
Hello,

I've been thinking about using a small, cheap neon light transformer (something like this one which is 3kV 30mA) to charge an electric fence around my chicken coop at night to discourage raccoons. Since the transformer would be "always on" if I plugged it directly into the mains, would it make sense to use a timer circuit to switch the mains connection on and off, for example once per second, so that the transformer output becomes "pulsed"? Or would this constant on/off switching of the transformer cause it to burn out prematurely? Or might there be other issues that make such a plan impossible or undesirable?

Joined Feb 20, 2016
4,134
I would not go that way. It would be very dangerous and could fail permanently on.
The capacitor discharge method would be a lot safer. This would only deliver pulses and could not feed continuous power down the wire.

olphart

Joined Sep 22, 2012
107
How about an automotive ignition coil? Pulse it at X volts (<=12) and N Hz to get the "attention" of critters.
It's built for frequent short spikes. Could drive with a 555 feeding a big MOSFET, just mem to diode clamp the back EMF.
The MOSFET has a clamp, but I'd put a good schottky in to be sure.
Good Hunting <<<)))

crutschow

Joined Mar 14, 2008
29,779
As dendad mentioned, a better (and safer) approach is to pulse a vehicle ignition coil to generate a high voltage pulse.
A Google search will show many circuits to do that.

FredFlintstone

Joined Apr 30, 2022
23
Thank you for the comment dendad, I will keep it in mind.

Note that if it "fails permanently on" it would only happen at night since I will plug it in every night when I lock up the chickens, and unplug it every morning when I let them out. Also note that the electric fence wire itself is located 8 feet above the ground (I keep my chickens in a very tall coop) so only nocturnal climbing animals might come in contact with it, and so far those animals have been limited to rats and raccoons.

FredFlintstone

Joined Apr 30, 2022
23
Hello olphart and crutschow,

I bought this kit from Gary Chemelec before he died, but because I did not build it when I received it many years ago, I seem to have misplaced it. I'm sure I put it "in a safe place" but after looking for a day and a half and not finding it, I am now considering alternatives, which is the reason why I created this thread.

I understand that my lost/misplaced Chemelec kit will be a better and safer solution, so I hope I find it. But if not, I may still proceed with the neon light transformer approach. Having said this, I would like to ask:

Is there a relatively simple and/or inexpensive way to reduce the charge on the electric fence wire, perhaps by inserting a current limiter in between the transformer and the fence wire?

ThePanMan

Joined Mar 13, 2020
367
I would certainly advise against using the Neon Transformer. The voltage is just too high and dangerous. And depending on the amount of current it could carry - you could be creating a high voltage heater that may burn down the coup.

Certainly an ignition coil would work. So would a regular transformer being switched on and off from a DC source. Yes, I said DC; not AC. With a small transformer that could fit in the hand, one that is 10:1 (actually 1:10, just using the secondary as a primary with a 12V DC source switched on and off would produce a nice kick in the bricks for anything touching it and ground at the same time. Not likely to harm any animals either. Of course this is the same thing as an automotive coil, but even old coils can be expensive. A small transformer out of an old clock radio could be big enough to send critters scurrying for the cover of the brush.

Let me explain it a little further just in case my explanation is not clear enough:
A 12 volt car battery that is charged during the day. That 12V powers a 555 timer that produces a brief pulse every second. That pulse switches on a transistor that provides ground for the secondary side of the transformer. The other lead of the secondary is connected to 12V DC. Since the transistor is off - it should be using no power. But at night when switched on - the battery provides a brief pulse via the transistor to the secondary. The primary side is - one lead grounded, the other lead suspended and insulated from accidental contact with ground or anything that could conduct to ground - say for excepting a critter you want gone. Just be sure to put a suppressor diode across the secondary side of the transformer so you don't blow your transistor. A small 2n2222 should be big enough for a small transformer with a 1n4007 diode. Why a 4007? Because you don't know how high a voltage is going to be kicked back through the transformer secondary.

Joined Jul 18, 2013
25,202
I used a waste spark automotive coil for a electric fence to deter deer eating the plants in the garden, I used a pulse every second, generated by a 8 pin micro and a Mosfet drive.
Any similar timer should work.

ThePanMan

Joined Mar 13, 2020
367
R1 & R2 set the time C1 takes to charge up. R2 sets the time C1 takes to discharge. Therefore R1 needs to be a higher value than R2. The time circuit R1C1 (sets the period) R2 sets the discharge period. When pin 3 goes high it turns the 2N2222 on and the transformer builds up a magnetic field. When pin 3 goes low the magnetic field collapses and sends a high energy pulse down the Critter Deter Cable (CDC). The 1N4007 prevents Back EMF from harming the transistor. All you need to do is decide on how frequently you want the pulse to go out on the CDC. A small transformer should be easy enough to find in some scrap electronics. Even an old doorbell transformer will produce quite a knick to the knuts.

FredFlintstone

Joined Apr 30, 2022
23
"I would certainly advise against using the Neon Transformer. The voltage is just too high and dangerous."

The neon transformer I linked to actually puts out 40% LESS voltage than the smallest commercial fence charger I can find (here is one of them, but there are several others with similar specs). Also, from my understanding, a car ignition coil puts out about 8kV, which again is much higher than the specified neon transformer.

I am not trying to be difficult here, but I am definitely trying to understand why you say that 3kV is more dangerous than 5kV or 8kV, because it seems to me that the opposite would be true.

Or is it actually the Amperage that is the real danger? This is what I have always been taught ...

Or is it the PULSE DURATION (or the possibility of being "constantly on") that is the real danger?

ThePanMan

Joined Mar 13, 2020
367
Couple things here: Since I don't know what Neon Txfmr you're in possession of - I can only guess. Suppose it's putting out 1 amp at 3kV. That's a LOT of wattage. Dangerous for sure. Ignition coils are also something I don't know specifics about but they're not going to be putting any where near the amperage your NT will be outputting.

With a small transformer as I've illustrated, the pulse is high but weak. It will certainly influence an animal to stay away while it does no harm other than give that animal nightmares - if they have them. The NT has way too much kick behind it. If you're planning on using the NT powered by 110VAC through some sort of switching system, during the "ON" time that transformer is going to be producing lethal currents. Even if the switch is on for 1/10 of a second. During that time the current changes direction 12 times and allows the transformer to launch an animal over the moon. Suppose that switch mechanism fails in the ON position: Anything that comes in contact with that wire is likely going to die. And if the animal is not able to release from the wire - it's going to be cooked come breakfast time. And I think with all that fur - it's going to stink to high heck as well.

You want a DC based system that even if the switch gets stuck on - the transformer being used to generate the high voltage pulse can't produce an output. It's only when the magnetic field collapses that you see that high voltage pulse. And there's no chance an animal can get fried for breakfast. Neither can some unsuspecting human. Let someone wander onto your property and touch the output from your NT - if they live - you're going to get sued. If they die - you're going to jail.

I would continue to urge you not to use the Neon Transformer. Seriously!

FredFlintstone

Joined Apr 30, 2022
23
Wow, nice schematic and great explanation ThePanMan!

Your circuit is much simpler than my (misplaced and never to be found) Chemelec kit. Have you or someone built and tested this circuit? If not, I can possibly be the first, unless I run out of time and have to buy one of those cheap low power commercial fence chargers that output only 0.1 Joule.

Do you or others here have a favorite online source for the components I would need to build your solution? I have not sourced any electronic components in the USA since Radio Shack had them several decades ago.

FredFlintstone

Joined Apr 30, 2022
23
Since I don't know what NT you're in possession of - I can only guess. Suppose it's putting out 1 amp at 3kV.
I don't even own one yet, but I found this relatively low-powered one on Amazon for quite cheap. It outputs only 30mA, not 1A, which does not seem all that powerful to me ... but what do I know?

And if the animal is not able to release from the wire - it's going to be cooked come breakfast time.
I had this problem several years ago with, yes, a commercial fence charger. It was a small one, but it was old and did not produce any pulsing. Before this fencer failed it killed a few rats, a baby opossum, and an armadillo --- because none of them could release from the wire so they got fried. When it finally failed I took it apart and there were only two components inside:

1- a small transformer, probably like the one you suggested above
2- a capacitor

I think I could have fixed it by replacing the capacitor, because when it failed it simply started producing such a weak charge that I could barely feel it. Or maybe the transformer was faulty rather than the capacitor. Either way I did not want to keep killing small animals with a non-pulsed charger, so I ordered the Chemelec kit which is a pulsed unit. Then we got rid of the chickens for a few years so I had no need to repel wild critters any more ... until this year.

Joined Feb 20, 2016
4,134
You do need to go with a pulsed fence exciter. DO NOT use a constantly powered one. It is dangerous and as @ThePanMan stated, you may well kill someone and end up sued and in jail. And well deserved too!
The Neon power supply is not to be used under any circumstances. Everyone here is saying the same thing. Do you think the advice is worth taking or not?
A capacitor discharge system driving an ignition coil or transformer s a good way to go as the charge in the capacitor controls the strength of the zap. A larger cap will give a higher powered shock. And, it cannot produce a continuous charge under a fault condition. That is VERY important.
You say a previous system killed some rodents. That should not be how the thing works. If it can kill beasties, do not use it!

FredFlintstone

Joined Apr 30, 2022
23
Thanks everyone!

I will NOT use a neon sign transformer. Instead I will build the one ThePanMan posted, or I will buy a cheap commercial unit. Or if by some miracle I actually find where I stashed my Chemelec kit, I will build that one.

Joined Jul 18, 2013
25,202
If you have ab Auto Wrecker nearby this is a cheap source of coils.

FredFlintstone

Joined Apr 30, 2022
23
If you have an Auto Wrecker nearby this is a cheap source of coils.
Actually I already have a brand new coil that I purchased several years ago for my Chemelec Fencer-2 kit. I know where the coil is, I just wish I could find the fencer kit. It came with the custom PCB and all the solder-on components. I also have a small 12V 1.3Ah battery that, according to Gary Chemelec, will be sufficient to power the fencer.

FredFlintstone

Joined Apr 30, 2022
23