Electric fence energiser help - Page 1
Forum section: electronics / beginners

author: AaronS76
author_rank: Newbie
timestamp: May 22, 2020, 01:28:17 am
content: 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?ThanksAaron

author: KingSolomon
author_rank: Contributor
timestamp: May 22, 2020, 02:23:01 am
content: Quote from: AaronS76 on May 22, 2020, 01:28:17 amA photo would certainly help.

author: AaronS76
author_rank: Newbie
timestamp: May 22, 2020, 03:46:59 am
content: Ok. Photo attached. It is the red/brown thing with a crack through the plastic in front of the coil.

author: twospoons
author_rank: Frequent Contributor
timestamp: May 22, 2020, 03:57:14 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2020, 04:00:26 am
content: I would suspect much of the silicon has been damaged especially if it was powered when it got wet.

author: AaronS76
author_rank: Newbie
timestamp: May 22, 2020, 04:03:28 am
content: 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?

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2020, 04:12:29 am
content: 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 @ 325425V 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.

author: AaronS76
author_rank: Newbie
timestamp: May 22, 2020, 04:19:01 am
content: 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?

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2020, 04:25:05 am
content: Quote from: AaronS76 on May 22, 2020, 04:19:01 amMuch 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.

author: AaronS76
author_rank: Newbie
timestamp: May 22, 2020, 04:30:18 am
content: 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.RegardsAaron

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2020, 04:57:23 am
content: https://www.alibaba.com/product-detail/Polypropylene-Metallized-Film-Capacitor-4UF-250VAC_60837008414.html?spm=a2700.7724857.normalList.41.46db1b20AnFxeE