|
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 |
