Hi all,

In order to better understand how electric fence energisers roughly work, I've been trying to model one in Falstad. I tried a simple setup of taking a 5VDC input which is stepped up to around 30V with a buck-boost converter. The 30 VDC is charged in a capacitor, which is then discharged over a 1:100 transformer to create a voltage spike. I've done the discharging with a manual switch in the model, but normally I suppose this is done with a timer or microcontroller of some kind. I've attached the model as an image below, just after starting the discharge of the capacitor over the transformer.

Now I suppose the negative voltage at the transformer side is negative because the coil of the primary side is trying to resist the rise in current due to closing the switch, but I didn't really expect electric fences to work like that. Do they actually operate on a negative voltage at the fence or am I doing something wrong with this model?

I also noticed that the resistance between the two terminals on the secondary side has to be rather large, otherwise no voltage spike will occur. I don't think 1M Ohm is realistic when an animal touches the fence though. Curious to hear what I'm missing here.

 

For mine I used a wasted spark automotive coil. the output is a few Kv.
The primary was pulsed twice at 1 second intervals.

 

Do they actually operate on a negative voltage at the fence

The polarity of the voltage is irrelevant to its effectiveness.
The animal can't tell (or care) which polarity it is.

Have you looked at the many electric fence circuits already designed for that purpose?

 

Probably, since the circuit we see is not the same as any of the fence chargers that I am familiar with, the fence voltage is not the same. As a capacitor is dumped across an inductance the voltage will be a damped oscillation, with both positive and negative peaks. And usually the switching device will be an SCR because it is simpler to trigger, with an automatic switch off by the reversed ring voltage. It seems to work that way for some CD ignition systems as well.

 

Thanks for the replies. @MaxHeadRoom and @crutschow I've taken quite some time to learn more about both DC step up converters and transformers. In the process I've indeed seen quite a few different designs, including the ones using an ignition coil.

@MisterBill2 I've indeed also seen the ones using an SCR. I tried using that in Falstad as well, but for some reason it stayed open. I think the ringing was not enough or perhaps not well implemented in Falstad and the current from the 5V source kept it open. Probably because it can supply an infinite amount of current in the model and there was no resistance included either.

 

REliably shutting off an SCR requires removing the gate drive and often holding the gate low by means of a low resistance. That is one reason why pulse transformer triggering was popular awhile back. Also the pulse transformers provide good isolation between the high voltage AC power and the logic level control circuits.