I saw this at a garage sale lying in the driveway all abused and instantly fell in love with it, being that I must have anything which makes high voltage. It's an old HOL-DEM Model 68, made by HOL-DEM of Rochester, MN, and rated for 5 miles of fence.
WARNING: This circuit generates voltages and currents which will hurt you very badly and possibly KILL if you are not careful. If you don't know much about working with line (and higher) voltages or if you aren't crazy (like me) then DO NOT attempt to construct this circuit. I CANNOT BE RESPONSIBLE if you electrocute yourself to death! That said, let's have some fun!
A rectifier diode is provided for the control circuitry in case the battery is connected backwards and also helps block some of the back EMF (electromotive force). An astable 555 oscillator circuit (for a more detailed description, see the notes section of the 3-6V Strobe Light) drives the base of a high voltage, high current transistor through a 100R resistor. This lets through enough current to cause the transistor to saturate for an on/off switching action. The timing when the highest power is desired interval at the collector is roughly 1s off and .02s on and can be varied by adjusting the trimmers. The collector is tied to the end of a primary coil of the transformer T1 through a diode.
When the transistor turns on, the power flows through the diode since it is forward biased and begins building magnetic flux in the primary. Flux continues building until either the core saturates or the transistor turns off again. In practice, when the highest power is desired, the 250k trimmer is adjusted so that the transistor turns off just before the core saturates. Turning down the trimmer lowers the amount of stored flux and so reduces the power for shorter fence runs.
Once the transistor is switched off, the stored flux begins to collapse. This collapsing magnetic flux produces a falling off voltage in the windings. After some milliseconds, it crosses zero, reverses and begins to flow in the opposite direction. This flyback voltage is quite high, especially in the secondary winding and is what applies shocks to the erring livestock. The flyback in the primary flows back through the diode and the 220n capacitor which provides some protection for the transistor.
On the output side, there is a 525pF capacitor which provides some
ringing and may absorb extra HV in case the output terminals are
unconnected. A neon indicator and associated current limiting
resistor are also provided to give visual indication of circuit
operation. This resistor is most likely a special high voltage
type.
Most of the notes are written right in the schematic so that they're
there even if this text gets separated from it. There are a couple
things though which aren't said. One is that any automotive ignition
coil can be substituted for the transformer if you don't feel like
winding your own. If you do wind your own, vacuum potting would be one
way to make the transformer almost bulletproof. Whether it's vacuum
potted or not, always make sure there's a way for it to discharge in
case there's no fence connected. Otherwise it will arc internally and
ruin all your hard work. You can do this by having the output
terminals less than 1cm apart. The output capacitor performs this
function somewhat, but you may have difficulties finding one of those
for cheap. It can be left out entirely with no real effect. Also,
you'll have a lot of trouble finding a suitable high voltage resistor
for the neon indicator. A better idea would be to just forget about a
neon and put an LED in series with a 330R resistor and connect them to
pin 3 of the 555 and ground.