Approximate Characteristics

Period			1S
Pulse Width		5mS
Peak Coil Current	2A
Average Current		4mA


WARNING

Ensure unit has been switched off for at least 1 minute before handling the EHT. C4 can remain charged and power the
circuit for quite some time!


Design Philosophy

A farmer friend of mine had an ancient electro-mechanical electric fence driver that had finally given up the ghost. He
asked me to look at it and see if I could come up with a replacement. The original unit had a complex bi-metal thermal
timer that flicked a contact to a specially made transformer. The whole thing ran from a 6V lead-acid battery that had
to be recharged frequently.

The first decision I made was that I needed to get a readily available HT transformer as I simply didn't have any way to
make one. A car ignition coil seemed to fit the bill perfectly.

My next concern was driving it. Having worked with TVs for a number of years, the obvious choice was a Line Output
transistor. These are designed for reasonably high current, but more particularly very high voltage, as they have to
handle the undamped flyback from the deflection coils.

Experimenting with this quickly showed that not only could I cheaply make a reliable replacement for the farmer, but the
improved efficiency meant average current was so low that rechargeable batteries could be dispensed with.

Eventually, I made several 'pig zappers' that were used for many years by local farmers. Last I heard they were still
working!


Circuit Description

U2c/R1/C3 form a fairly conventional CMOS oscillator, driving the clock input to ripple counter U1. However U2a is used
to reset the counter when both output 2 and 11 are high (U2b acting as a simple NOT gate). This results in a narrow,
well defined pulse at O11. Using gates and a counter in this way produces a very cheap but reliable timer. The
oscillator is running at around 2kHz thus requiring only a small capacitor while maintaining a fairly low impedance, and
hence immunity to stray pickup. The unused gate U2d simply has its inputs tied to 0V.

Q1 is a darlington driver for Q2. R3 buffers U1, and Q1 c-e current is limited by R4. There is a temptation to put R4 in
the emitter and do away with R3 turning it into a straight emitter-follower, but in practice this doesn't seem to work
so well, being more prone to ringing and other parasitic effects. Also, Q1 can't just be cascaded with Q2 as it would
then need the same high collector voltage rating. Furthermore the overall saturation voltage would then be that of Q1+Q2
which at around 2V would represent a major waste of power.

Due to the very short pulse time and hard switching there is no need for a heatsink on Q2

The output from L1 is proportional to the rate of change in the collapsing field generated by cutting the primary
current. Therefore, the faster Q2 can be made to switch off the better. For this reason R5 is provided to rapidly remove
charge from the base when Q1 stops driving it.

R6 along with the fence line capacitance provide a degree of EMC suppression. A better solution is to use proper
resistive EHT cable, which will then also provide some protection in the event of a short circuit in the fence line.

C1/C2/R2 provide all the decoupling needed to ensure spikes from the output don't get back to the oscillator and disrupt
it.

C4 provides low impedance charge storage. Although the average current is very low, there is still a spike of around 2A
which, without this capacitor, would make the battery voltage sag considerably, especially after ageing.

If F1 actually blows you've got a serious problem, but at least it ensures nothing will catch fire!

This unit is intended to be used out in the open, so ideally should be in a box sealed to IP65 standard, also the small
components should be on a PCB sealed with epoxy or silicon jelly, leaving just the connections and the body of Q2
exposed.


Component Choices

The original output transistor (Q2) was a BU105, which was quite expensive. The modern equivalent not only has much
better characteristics but is considerably cheaper, especially when you take inflation into account.

Similarly, Q1 cheaply replaces two separate transistors in the original design.

Almost any standard car ignition coil can be used for L1, although these are becoming slightly less common as more
sophisticated electronic systems feed into the domestic car market.

B1 and B2 are standard 996 lantern batteries. Others could be used but these are cheap, readily available and will last
a very long time.


W Godfrey 1981-2007