Electric fence energiser question  - Page 1
Forum section: electronics / projects

author: atgeek
author_rank: Contributor
timestamp: April 16, 2017, 07:56:31 pm
content: Would be possible to build an electric fence energiser using those cinese high voltage transformer? The input is 3-6V and the output should be about 400kV. It has a built-in oscillator circuit

author: tautech
author_rank: Super Contributor
timestamp: April 16, 2017, 08:19:00 pm
content: There are legal requirements for electric fence energisers. They shouldn't exceed 10KV and pulse rates should be ~1/s.These are so they are not lethal to stock or people.Don't use that please.

author: cowana
author_rank: Frequent Contributor
timestamp: April 16, 2017, 08:34:14 pm
content: In addition, electric fences tend to have leakage to earth - I suspect that module wouldn't be able to supply enough current to keep the fence powered.In either case, those modules are made for ignition rather than continuous operation, so it will likely overheat/wear out within an hour or so if you decided to use it.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2017, 09:36:43 pm
content: With an electric fence, one side of the energiser goes to the fence and the other to earth.As mentioned above those cheap, high voltage boost converters are designed for ignition systems, not fences. Another thing to bear in mind is they're not designed to work open circuit and if you try, the internal insulation will breakdown and destroy the unit.

author: atgeek
author_rank: Contributor
timestamp: April 16, 2017, 09:57:47 pm
content: How do they leak to ground?

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2017, 10:44:17 pm
content: Quote from: atgeek on April 16, 2017, 09:57:47 pmAtmospheric moisture and capacitive coupling (since the fence is normally driven from pulses).

author: atgeek
author_rank: Contributor
timestamp: April 16, 2017, 11:07:15 pm
content: But in order of few milliamps or lower, right?

author: DaJMasta
author_rank: Super Contributor
timestamp: April 17, 2017, 01:19:38 am
content: but a few milliamps at 4kV is tens of watts on the output side of your voltage boosting circuit, it's a decent amount of power especially when trying to use a small transformer.

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2017, 09:16:58 am
content: Electric fences are limited as to the pulse energy you can have per pulse, typically 8J of energy total per energiser. They are limited to output voltage so it is a warning and non lethal shock, and they typically have a closed loop design that has a source, the length of insulated fence, and a sink and measurement unit, typically a lot of 1kV varistors in series with a LED lamp at the base to give a light pulse on each energy pulse. This light pulse is then used to detect the current flow ( and thus the energy) and the light intensity is stored with a sample and hold and used to show leakage and contact as the touching thing shunts some energy to the grounded fence wires. Low enough and it triggers the fence alarm, saying there is either a leakage or short. Also detects cut wiring as an open circuit does not give a path back.Typical energisers use a capacitor ( around 12uF) charged to 300VDC and discharged through the primary of a step up transformer by a pretty high power SCR, the secondary being connected to the fence. Transformer is around 50VA in size, so you will be building one about that size in any case. Currents on the primary side are non trivial, and the capacitors used are designed for pulse discharge application, a regular film unit here will blow up after a few hundred cycles of use from either heating or the foil self healing to nothing.

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2017, 09:37:17 am
content: There have been some big advances in energisers in the last decade or so Sean. NZ makes some of the more powerful units on the market, many with output energy in excess of 50 Joules.One on my bench has 6 x 30uF 900 VDC pulse grade caps paralleled for 180uF. The output pulse is short usually ~1ms.Depending on local soil moisture content the return path can be the soil to a # of 2M ground earth pegs and/or via an "earth return" system were earth/ground wire are run parallel to the hot/live wire but require more ongoing maintenance to ensure separation between live and ground wires.Earth return systems are used where there is low soil moisture to ensure good animal control at long distances from the energiser.

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2017, 02:42:09 pm
content: Here I do not deal with agricultural fences, but with ones that are the most common, residential units almost every property has as a first line of defence, which are limited in power output. Yes i can get the farm units, but it is illegal to install them in a suburban environment.Can't do the Eskom method, which is to declare the fence area as a hot cage and wire it to 33kV via a 20A recloser, and SCADA monitor current and recloser resets, along with leaving the recloser in full auto reclose with no timeout on retries.