HIGH VOLTAGE GENERATOR CIRCUIT ELECTRONIC PROJECT
HIGH VOLTAGE GENERATOR
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.
CLICK ON THE IMAGE TO ZOOM IN
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.
