Power supply of lighting circuits

A source of comfort and productivity, lighting represents 15% of the
quantity of electricity consumed in industry and 40% in buildings. The
quality of the lighting (light stability and continuity of service) depends on
the quality of the electrical energy thus consumed. The supply of electrical
power to lighting networks has therefore assumed great importance.
To help with their design and simplify the selection of appropriate
protection devices, the authors present in this document an analysis of the
different lamp technologies and the main technological developments in
progress. After summarizing the distinguishing features of lighting circuits
and their impact on control and protection devices, they discuss the options
concerning which equipment to use.

The different lamp technologies
Artificial light
Artificial luminous radiation can be produced from
electrical energy according to two principles:
incandescence and electroluminescence.
Incandescence
This is the production of light via temperature
elevation. The energy levels are plentiful, and in
consequence, the emitted radiation spectrum is
continuous. The most common example is a
filament heated to white state by the circulation
of an electrical current. The energy supplied is
transformed into the Joule effect and into
luminous flux.
Luminescence
This is the phenomenon of emission by a
material of visible or almost visible luminous
radiation.
c Electroluminescence of gases
A gas (or vapors) subjected to an electrical
discharge emits luminous radiation.
Since this gas does not conduct at ordinary
temperature and pressure, the discharge is
produced by generating charged particles which
permit ionization of the gas. The spectrum, in the
form of stripes, depends on the energy levels
specific to the gas or vapor used. The pressure
and temperature of the gas determine the length
of the emitted rays and the nature of the
spectrum.
c Photoluminescence
This is the luminescence of a material exposed
to visible or almost visible radiation (ultraviolet,
infrared).
When the substance absorbs ultraviolet radiation
and emits visible radiation which stops a short
time after energization, this is fluorescence. Not
all the photons received are transformed into
emitted photons. The best efficiency rating for
existing fluorescent materials is 0.9.
When the light emission persists after
energization has stopped, it is phosphorescence

Incandescent lamps
Incandescent lamps are historically the oldest
(patented by Thomas Edison in 1879) and the
most commonly found in common use.
They are based on the principle of a filament
rendered incandescent in a vacuum or neutral
atmosphere which prevents combustion.
A distinction is made between:
c Standard bulbs
These contain a tungsten filament and are filled
with an inert gas (nitrogen and argon or krypton).
c Halogen bulbs
These also contain a tungsten filament, but are
filled with a halogen compound (iodine, bromine
or fluorine) and an inert gas (krypton or xenon).
This halogen compound is responsible for the
phenomenon of filament regeneration, which
increases the service life of the lamps and
avoids them blackening. It also enables a higher
filament temperature and therefore greater
luminosity in smaller-size bulbs.
The main disadvantage of incandescent lamps is
their significant heat dissipation, resulting in poor
light output, But, they have the advantage of a
good Color Rendering Index (CRI) due to the
fact that their emission spectrum is fairly similar
to the eye’s reception spectrum.
Their service life is approximately 1,000 hours
for standard bulbs, 2,000 to 4,000 for halogen
bulbs. Note that the service life is reduced by
50% when the supply voltage is increased by 5%.

Fluorescent lamps

This family covers fluorescent tubes and
compact fluorescent lamps. Their technology is
usually known as “low-pressure mercury”.
Fluorescent tubes
These were first introduced in 1938.
In these tubes, an electrical discharge causes
electrons to collide with ions of mercury vapor,
resulting in ultraviolet radiation due to
energization of the mercury atoms. The
fluorescent material, which covers the inside of
the tubes, then transforms this radiation into
visible light.
This technology has the disadvantage of a
mediocre CRI due to the fact that the emission
spectrum is discontinuous. However, nowadays
there are different product families which meet
the many needs of CRI, for example so-called
“daylight” tubes.
Fluorescent tubes dissipate less heat and have a
longer service life than incandescent lamps, but
they do need an ignition device called a “starter”
and a device to limit the current in the arc after
ignition. This last device called “ballast” is usually
a choke placed in series with the arc. The
constraints affecting this ballast are detailed in
the rest of the document.
Compact fluorescent lamps
These are based on the same principle as a
fluorescent tube. The starter and ballast
functions are provided by an electronic circuit
(integrated in the lamp) which enables the use of
smaller tubes folded back on themselves.
Compact fluorescent lamps were developed to
replace incandescent lamps: they offer
significant energy savings (15 W against 75 W
for the same level of brightness) and an
increased service life (8,000 hrs on average and
up to 20,000 hrs for some).
Standard compact fluorescent lamps take a little
longer to ignite and their service life is reduced
according to the number of times they are
switched on. So, if the ignition frequency is
multiplied by 3, the service life is reduced by a
ratio of 2.
Lamps known as “induction” type or “without
electrodes”  start instantaneously
and the number of switching operations does not
affect their service life. They operate on the
principle of ionization of the gas present in the
tube by a very high frequency electromagnetic
field (up to 1 GHz). Their service life can be as
long as 100,000 hrs.

Discharge lamps
The light is produced by an electrical discharge
created between two electrodes within a gas in a
quartz bulb. All these lamps  therefore
require a ballast to limit the current in the arc.
The emission spectrum and the CRI depend on
the composition of the gas and improve as the
pressure increases. A number of technologies
have therefore been developed for different
applications.
Low-pressure sodium vapor lamps
These have the best light output, however the
color rendering is very poor since they only have
monochromatic radiation, which is orange in color.
Applications: tunnel, motorway lighting.
High-pressure sodium vapor lamps
These produce a white light with an orange tinge.
Applications: street lighting, monuments.
High-pressure mercury vapor lamps
The discharge is produced in a quartz or ceramic
bulb at pressures of more than 100 kPa. These
lamps are called “fluorescent mercury discharge
lamps”. They produce a characteristically bluish
white light.
Applications: car parks, hypermarkets,
warehouses.
Metal halide lamps
The latest technology. They produce a color with
a broad spectrum.
The use of a ceramic tube offers better luminous
efficiency and better color stability.
Applications: stadia, retail premises, projectors.

LEDs (Light Emitting Diodes)
The principle of light emitting diodes is the
emission of light by a semi-conductor as an
electrical current passes through it. LEDs are
commonly found in numerous applications, but
the recent development of white or blue diodes
with a high light output opens new perspectives,
especially for signaling (traffic lights, exit signs or
emergency lighting).
The average current in a LED is 20 mA, the
voltage drop being between 1.7 and 4.6 V
depending on the color. These characteristics
are therefore suitable for an extra low voltage
power supply, especially using batteries.
A converter is required for a line power supply.
The advantage of LEDs is their low energy
consumption. As a result, they operate at a very
low temperature, giving them a very long service
life. Conversely, a simple diode has a weak light
intensity. A high-power lighting installation
therefore requires connection of a large number
of units in series.
These diodes are used particularly where there
is little power available.

Lamps for special applications
The types of lamp mentioned in this sub-section
only have, with the exception of the last two, a
single application. Their electrical power supply
should always be designed according to the
special technical information provided by their
manufacturers.
Special incandescent lamps for 3-color traffic
lights.
Their service life is increased and their special
mounting helps them resist vibrations.
Special mercury vapor lamps
These produce a uniform beam of blue-white
light designed for reproduction graphics, screenprinting
or jewelers’ decorative lighting.
Lamps producing white light with a radiation
around 655 nm
These are designed to accelerate
photosynthesis in plants. Applications include
florists’ shops, entrance halls, industrial
greenhouses.
Germicidal lamps
These produce ultraviolet in the 253.7 nm wave
length. Applications include purification,
sterilization of air, water and instruments in the
pharmaceutical industry, hospitals, treatment
plants or laboratories. These lamps produce
radiation that is dangerous to the eyes and skin.
UVA lamps
These are used for tanning and light therapy.
Black light lamps
These generate ultraviolet emission in the long
wavelengths which has the effect of activating
fluorescent pigments. Applications include
finding defects in industry or counterfeit items
(notes, pictures, etc) as well as use in show
business.
Special halogen lamps
Used for projection of images (slide viewer,
overhead projection, microfiche reading), their
heat radiation onto the film is reduced by 60%
compared to a conventional lamp.
Lamps adapted to projection in film studios
and theaters
Their color temperature is 3200° K. Their power
rating can be as high as 5000 W.
These lamps have better luminous efficiency and
more luminous flux but a reduced service life
(12 hrs, 100 hrs, 500 hrs).
Heating lamps
These generate a short infrared heat energy
beam. Certain types are designed for farming,
others for drying and curing paintings, heating in
industrial processes or zone heating by
radiation.

Lamps with direct power supply
Constraints
Due to the very high temperature of the filament
during operation (up to 2500° C), its resistance
varies greatly depending on whether the lamp is
on or off. As the cold resistance is low, a current
peak occurs on ignition that can reach 10 to
15 times the nominal current for a few
milliseconds or even several milliseconds.
This constraint affects both ordinary lamps and
halogen lamps: it imposes a reduction in the
maximum number of lamps that can be powered
by a device such as remote-control switch,
modular contactor or relay for busbar trunking.
Varying the brightness
This can be obtained by varying the voltage
applied to the lamp.
This voltage variation is usually performed by a
device such as a triac dimmer switch, by varying
its firing angle in the line voltage period.  . This technique known as
“cut-on control” is suitable for supplying power
to resistive or inductive circuits. Another
technique suitable for supplying power to
capacitive circuits has been developed with
MOS or IGBT electronic components. This
technique varies the voltage by blocking the
current before the end of the half-period and is known as “cut-off control”.
The latest devices use both these techniques
while adapting automatically to the nature of
their load.
Switching on the lamp gradually can also reduce,
or even eliminate, the current peak on ignition.
Note that light dimming:
c is accompanied by a modification in the color
temperature;
c has an adverse effect on the service life of
halogen lamps when a low voltage is maintained
for a long time. Indeed, the filament regeneration
phenomenon is less effective with a lower
filament temperature.
Another technique is used for timer switch-off
warnings. These devices warn that the lighting
will shortly be switched off by reducing the
luminous intensity by 50% for a several seconds.
This reduced brightness is obtained by applying
a voltage half-wave, positive or negative, to the
lamps at intervals of one second, using a triac
device.
Extra low voltage halogen lamps
Constraints
Some low-power halogen lamps are supplied
with ELV 12 or 24 V, via a transformer or an
electronic converter.
c With a transformer, the magnetization
phenomenon combines with the filament
resistance variation phenomenon at switch-on.
The inrush current can reach 50 to 75 times the
nominal current for a few milliseconds.
The use of dimmer switches placed upstream
significantly reduces this constraint.
c Electronic converters, with the same power
rating, are more expensive than solutions with a
transformer. This commercial handicap is
compensated by a greater ease of installation
since their low heat dissipation means they can
be fixed on a flammable support. Moreover, they
usually have built-in thermal protection.
These devices can therefore be marked
(IEC 60417 – 1st October 2000):
Varying the brightness
There are a variety of possible technical
solutions:
c dimmer switch and transformer,
c electronic converter controlled by a 0-10 V
external signal,
c dimmer switch and converter; this solution is
used to control the brightness of several lamps
with a single dimmer switch, but it is important to
check carefully that the dimmer switch and
converters are compatible.
Developments
New ELV halogen lamps are now available with
a transformer integrated in their base. They can
be supplied directly from the LV line supply and
can replace normal incandescent lamps without
any special adaptation.