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Ahigh voltage
power supply is a very useful source which can be effectively used
in many applications like biasing of gas-discharge tubes and
radiation detectors etc. Such a power supply could also be used
for protection of property by electric charging of fences. Here
the current requirement is of the order of a few microamps. In
such an application, high voltage would essentially exist between
a ‘live’ wire and ground. When this ‘live’ wire is touched, the
discharge occurs via body resistance and it gives a non-lethal but
deterrent shock to an intruder. The circuit is built around a
single transistorised blocking oscillator. An important element in
this circuit is the transformer. It can be fabricated on easily
available ferrite cores. Two ‘E’ sections of the core are joined
face-to-face after the enamelled copper wire wound on former is
placed in it. The details of the transformer windings are given in
the Table.
In this configuration, the primary winding and the feedback
winding are arranged such that a sustaining oscillation is ensured
once the supply is switched on. The waveform’s duty cycle is
asymmetrical, but it is not very important in this application.
Please note that if the oscillations do not occur at the
‘switch-on’ time, the transformer winding terminals of the
feedback or the primary winding (but not both) should be reversed.
The primary oscillation amplitude is about 24V(p-p). This gets
amplified with the large step-up ratio of the transformer and we
get about 800V(p-p) across the secondary. A simple series voltage
multiplier (known as Cockroft-Walton circuit) is used to boost up
this voltage in steps to give a final DC of about 2 kV. The output
voltage, however, is not very well regulated. But if there is a
constant load, the final voltage can be adjusted by varying the
supply voltage.
The present configuration gives 2 kV for an input DC voltage of 15
V. Though higher voltages could be achieved by increasing input
supply, one word of caution is necessary: that the component
ratings have to be kept in mind. If the ratings are exceeded then
there will be electrical discharges and breakdowns, which will
damage the device
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