Philips Entertainment Lamps catalogue

The halogen cycle
Halogen lamps on different supply voltages
Since the chemical processes which take place only function within
certain temperature limits, a halogen lamp cannot be used on any
desired voltage. Over- and under-rating can only be carried out within
the limitations discussed below.
a.Under-voltage
Under-voltage of a lamp will decrease the filament temperature and
therefore the rate of evaporation. For example, 5% under-voltage
will result in 200% life expectancy. However, reduced evaporation
of tungsten means more free halogens in the lamp. These free
halogens will attack cold parts in the lamp; the coldest parts are
the lead-in wires. Now, dissipation of tungsten takes place from the
lead-in wires to the hot filament. The lead-in wires get thinner and
thinner and after some time the filament collapses. To prevent this, a
minimum bulb temperature of 250 °C must be maintained.
b.Over-voltage
Over-voltage will increase the filament temperature, causing a higher
rate of evaporation. For example, 5% over-voltage results in 50%
life expectancy. Moreover, zone “B” will be further away from the
filament. At a certain moment zone “B” will be partly inside the bulb
and partly outside.This means that evaporated particles of tungsten
will blacken the inner side of the bulb wall. A black body absorbs
more infrared, this infrared heats the glass, causing more blackening,
etc., etc. At a particular instant the glass melts, the bulb will start to
bulge and, some time later, the lamp will leak. Oxygen will enter and
the filament will burn itself out soon after. To prevent the start of
this disastrous sequence, a maximum bulb temperature of 900 °C is
recommended.
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Philips Entertainment Lamps catalogue
c.Short switching
The percentage of added halides in a lamp is determined by the
application of that lamp. Three main groups of lamps can be stated:
a)Continuous use - studio lamps - car lamps, etc.
b)Continuous or switching - slide projector lamps
c)Very short switching - lamps for copying machines
Lamps cannot be used for an application other than that for which
they are made without serious effects on the life of the lamp occuring.
Summary
It is not possible to quote minimum or maximum voltages on
which halogen lamps can be used: the processes are governed by
temperature. If a lamp is cooled very well, it can be satisfactorily
operated on over-voltage, but it should not be under-run. When a lamp
is under-run, the cooling must be cut down in order to maintain the
250 °C bulb temperature.
filament 3000˚C
1200˚C
600˚C
min 200˚C
Fig. 2. Cross- section through a halogen lamp
A
B
C
filamentW
gas nX
W + nX
W + nX
bulb wall
WXn
WXn