The circuit is intended
for carrying out harmless experiments with high-voltage pulses and
functions in a similar way as an electrified fence generator. The p.r.f.
(pulse repetition frequency) is determined by the time constant of
network R1-C3 in the feedback loop of op amp IC1a: with values as
specified, it is about 0.5 Hz. The stage following the op amp, IC1b,
converts the rectangular signal into narrow pulses. Differentiating
network R2-C4, in conjunction with the switching threshold of the
Schmitt trigger inputs of IC1b, determines the pulse period, which here
is about 1.5 ms. The output of IC1b is linked directly to the gate of
thyristor THR1, so that this device is triggered by the pulses.
The requisite high voltage is generated with the aid of a small
mains transformer, whose secondary winding is here used as the primary.
This winding, in conjunction with C2, forms a resonant circuit.
Capacitor C3 is charged to the supply voltage (12 V) via R3.When a pulse
output by IC1b triggers the thyristor, the capacitor is discharged via
the secondary winding. The energy stored in the capacitor is, however,
not lost, but is stored in the magnetic field produced by the
transformer when current flows through it. When the capacitor is
discharged, the current ceases, whereupon the magnetic field collapses.
This induces a counter e.m.f. in the transformer winding which opposes
the voltage earlier applied to the transformer.
This means that the direction of the current remains the same.
However, capacitor C2 is now charged in the opposite sense, so that the
potential across it is negative. When the magnetic field of the
transformer has returned the stored energy to the capacitor, the
direction of the current reverses, and the negatively charged capacitor
is discharged via D1 and the secondary winding of the transformer. As
soon as the capacitor begins to be discharged, there is no current
through the thyristor, which therefore switches off. When C2 is
discharged further, diode D1 is reverse-biased, so that the current loop
to the transformer is broken, whereupon the capacitor is charged to 12 V
again via R3. At the next pulse from IC1b, this process repeats itself.
Since the transformer after each discharge of the capacitor at its
primary induces not only a primary, but also a secondary voltage, each
triggering of the thyristor causes two closely spaced voltage pulses of
opposite polarity. These induced voltages at the secondary, that is, the
230 V, winding, of the transformer are, owing to the higher turns
ratio, much higher than those at the primary side and may reach several
hundred volts. However, since the energy stored in capacitor C2 is
relatively small (the current drain is only about 2 mA), the output
voltage cannot harm man or animal. It is sufficient, however, to cause a
clearly discernible muscle convulsion.
Author: P. Lay
Copyright: Elektor Electronics