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Low
Current, High Voltage Power Supply
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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.
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TABLE
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Details
of Transformer Windings
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Wingdings
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No.
of turns
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Standard
wire gauge(SWG)
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Primary
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50
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12
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Feedback
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12
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31
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Secondary
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1650
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41
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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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