Electric fence energiser help

[AaronS76]

Hi everyone. New to forum. Lent my electric fence energiser to neighbour. It got saturated and now doesn’t work. Do not want to replace entire board would prefer to learn more and figure out the problem. Hoping for some help.

So everything seems to work. The cap tests ok but voltage doesn’t get from cap to transformer. In between the cap and transformer there is another thing. It has two terminals. It has ST405J written on it. I cannot find anything online about this particular part and electric fence manufacturer was not much help.

Any thoughts? Do I need a photo?

Thanks

Aaron

[KingSolomon]

Do I need a photo?

A photo would certainly help.

[AaronS76]

Ok. Photo attached. It is the red/brown thing with a crack through the plastic in front of the coil.

[twospoons]

Its a 4uf capacitor and I would say its kaput. It may read ok with a meter but be leaking badly at higher voltages. There definitely shouldn't be a giant crack in the body.

[tautech]

I would suspect much of the silicon has been damaged especially if it was powered when it got wet.

[AaronS76]

Awesome. Many thanks.

So, what I don’t understand is why there is this cap between the larger cap and the transformer. What purpose would it serve?

[tautech]

Mains or battery unit ?

Fence units of old produced some horrible EMI and today with much higher power the circuitry is pretty complex, not just to meet modern standards but just for longevity.
Dropping some 40-100 uF @ 325 425V into a transformer every second for 10 KV output is no ordinary application where many failsafes and silicon protection devices need work properly for long service life.
Generally fences get weak after decades as even the pulse grade caps don't last forever.

[AaronS76]

It is a 12v battery unit. Big cap is 12 uf at 900 v. This is 4uf at 250vac. Am I looking at the circuit wrong and this just bumps up 12v to 240 v so it then uses the same gear as the mains units?

[tautech]

It is a 12v battery unit. Big cap is 12 uf at 900 v. This is 4uf at 250vac. Am I looking at the circuit wrong and this just bumps up 12v to 240 v so it then uses the same gear as the mains units?

Much like you think, yes 12V is inverted to higher voltage then rectified where it charges the 900V rated pulse grade cap. Just how high the primary inversion is I could not say but the associated componentry and their ratings will give some clues. 

FYI rectified 230VAC  mains is some 325VDC not 425 as I stated earlier....will edit.

[AaronS76]

Again. Many thanks for the assistance. Suspect I can get this working again and also have a better idea of how these things operate.

Now I just need to find one. ST405J doesn’t appear in google searches. So another 4uf 250vac cap will need to replace it. 

Regards

Aaron

[tautech]

https://www.alibaba.com/product-detail/Polypropylene-Metallized-Film-Capacitor-4UF-250VAC_60837008414.html?spm=a2700.7724857.normalList.41.46db1b20AnFxeE