Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 138

their built-in, third auxiliary electrode, high-pressure mercury lamps have an ignition
voltage lower than the mains voltage and therefore need no igniter. The ignition volt-
age of most low-pressure sodium lamps is only roughly less than a factor two higher
than the 230 V mains and can therefore be obtained with the aid of a transformer
that increases the mains voltage sufficiently to ignite the lamp. So-called auto-leak
transformers are used that also function as current-limiting ballasts. Low-pressure
sodium lamps produced for use on electronic ballasts have an electronic igniter incor-
porated in the ballast and they are more efficient. Some low-wattage high-pressure
sodium lamps have an internal igniter. High-wattage high-pressure sodium lamps
(HPS) need an ignition voltage peak of some 3000-5000 volts and metal halide
lamps (MH) 600-5000 V. They are normally started using an electronic igniter that
generates a series of high-voltage pulses of the required magnitude. The electronic
circuit is so designed that these pulses cease after ignition has taken place. Because
of differing requirements for ignition voltage, shape of voltage peak and number of
voltage pulses within a certain period, each type of HID lamp (and often also each
different wattage) needs its own type of igniter.

10.4.2 Ballasts for Gas-Discharge Lamps

The ballast of a gas discharge lamp has two functions to perform. In the section
dealing with the operating principle of gas discharge lamps we saw that ionization
of the gas results in an ever-increasing flow of free electrons. To avoid an unlimited
increase in the lamp current, which would ultimately lead to the lamp's breakdown,
a current-limiting device called ballast is needed. The ballast also has to ensure that
the lamp continues to operate despite the fact that twice during each frequency cycle
of the mains voltage (50 or 60 Hz), the current is zero and the lamp off, and thus has
to be re-ignited.
The ever-increasing flow of electrons causes the gas discharge lamp to have a
negative-resistance characteristic (Fig. 10.25 , left). This means that unless something
is done to prevent it, the current will increase uncontrollably (so-called current run-
away). The current is stabilized by introducing an external resistor in the lamp circuit
(Fig. 10.25 , right). The thick-line curve is the result of adding the negative resistance
of the ballast (dotted curve) to that of the series resistor (dotted straight line).
Simple resistors can thus be used as current-limiting ballasts for gas discharge
lamps, but they dissipate a lot of power and are therefore normally not used. Instead,
inductive coils consisting of copper wire wound around an iron core are used. These
have the same effect as a resistor, but at much lower power losses in the ballast. These
traditional, or conventional, inductive ballasts are called “electromagnetic ballasts”
because of the electric and magnetic fields their coils generate. They are sometimes
also referred to as “choke ballasts”.
An inductive ballast also helps to reduce the time the lamp is “off” during each zero
passage of the current—in the ideal case, to zero time off. Figure 10.26 left shows,
for the situation where the mains voltage is in phase with the lamp current, that the