Electric fence teardown, trying to understand how it works

[tigrou]

I have open an old electric fence for general inspection. The capacitor connectors are rusty and it probably need to be replaced.

I would like to understand how it works.
Inside there is a big transformer, a capacitor and this PCB :



The white connections on the top left are as follow : mains / capacitor / transformer.
Transformer output goes directly to electric fence output (claimed to be 10000V).

Questions : why is there a need of a big 220V capacitor ? to store energy and release it later, while the transformer is "ON" and doing his job ? Isn't mains enough ?
I would have expected the capacitor to store rectified transformer output, and to release in one go but that's not the case here.

It's also unclear what it the role of the 2 diodes and the 4 mosfet.
Probably to implement a timer circuit (that give a pulse every second). Potentiometers are probably used to adjust this.

I will try do to some tracing and draw proper circuit when if I have some time.

[madires]

I will try do to some tracing and draw proper circuit when if I have some time.

Yep! That's the best way to understand how the circuit works. And it will answer your questions.

[tigrou]

I think I got how it works : the capacitor is charged with DC (that's why you need diodes). Then once it's ready, it get dumped into the transformer using thyristors (so those are not MOSFETs).

I expected the transformer to only work with AC but for a short pulse, it work as well. This is probably similar to "capacitor discharge ignition" used in some vehicles.

[aliarifat794]

In an electric fence controller, storing rectified transformer output directly for later use isn't practical. The high-voltage side needs to be isolated and controlled precisely so that it can create consistent, timed pulses.

[tautech]

I think I got how it works : the capacitor is charged with DC (that's why you need diodes). Then once it's ready, it get dumped into the transformer using thyristors (so those are not MOSFETs).

I expected the transformer to only work with AC but for a short pulse, it work as well. This is probably similar to "capacitor discharge ignition" used in some vehicles.

Correct.

It must be done this way for correct timing and limiting output as their are laws that limit output so to protect innocent individuals from serious injury and/or death.

The earliest units were electromechanical and only ran from LA batteries in the days before semiconductors and required occasional maintenance of the contact points.