ELECTRONIC PROJECT
This
high voltage generator was designed with the aim of testing the electrical
breakdown protection used on the railways. These protection measures are used
to ensure that any external metal parts will never be at a high voltage. If
that were about to happen, a very large current would flow (in the order of
kilo-amps), which causes the protection to operate, creating a short circuit to
ground effectively earthing the metal parts. This happens when, for example, a
lightning strike hits the overhead line (or their supports) on the railways.
This
generator generates a high voltage of 1,000 V, but with an output current that
is limited to few milliamps. This permits the electrical breakdown protection
to be tested without it going into a short circuit state. The circuit uses
common parts throughout: a TL494 pulse-width modulator, several FETs or bipolar
switching transistors, a simple 1.4 VA mains transformer and a discrete voltage
multiplier. P1 is used to set the maximum current and P2 sets the output voltage.
The
use of a voltage multiplier has the advantage that the working voltage of the
smoothing capacitors can be lower, which makes them easier to obtain. The TL494
was chosen because it can still operate at a voltage of about 7 V, which means
it can keep on working even when the batteries are nearly empty. The power is
provided by six C-type batteries, which keeps the total weight at a reasonable
level.
The
2x4 V secondary of AC power transformer (Tr1) is used back to front. It does
mean that the 4 V winding has double the rated voltage across it, but that is
acceptable because the frequency is a lot higher (several kilo-Hertz) than the
50 Hz (60 Hz) the transformer is designed for.
The
final version also includes a display of the output voltage so that the breakdown
voltage can be read. From a historical perspective there follows a bit of
background information.
In
the past a different system was worked out. Every high-voltage support post has
a protection system, and it isn’t clear when the protection had operated and
went into a short-circuit state due to a large current discharge.
Since
very large currents were involved, a certain Mr. Van Ark figured out a solution
for this. He used a glass tube filled with a liquid containing a red pigment
and a metal ball. When a large current discharge occurred the metal ball shot
up due to the strong magnetic field, which caused the pigment to mix with the
liquid. This could be seen for a good 24 hours after the event. After a thunder
storm it was easy to see where a discharge current took place: one only had to
walk past the tubes and have a good look at them.
Unfortunately,
things didn’t work out as expected. Since it often took a very long time before
a discharge occurred, the pigment settled down too much. When a discharge
finally did occur the pigment no longer mixed with the liquid and nothing was
visible. This system was therefore sidelined, but it found its place in the
(railway) history books as the ‘balls of Van Ark’.
