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Properties of Tungsten Filament Lamps
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Mechanical
Properties
A tungsten filament is more fragile
at room temperature than at elevated
temperatures, particularly when there
is some embrittlement due to crystalization
of the aged wire. In cooling from an
elevated temperature, the filament passes
through the “brittle-ductile transition
region” (250° to 350°C
for tungsten filaments) and is much
more liable to rupture below this region.
Addition of small amounts of such elements
as thorium, potassium, or rhenium have
been found to alter ductility, tensile
strength, and shock resistance, thereby
directly or indirectly increasing lamp
life in certain applications. A simple
method to increase service life is to
apply a small amount of power (usually
less than 1% of operating power) to
keep the filament warmed in the OFF
mode. This “keep-alive”
technique greatly increases the reliability
of standard lamp types at small cost.
Electrical
Properties
Resistance
Tungsten filaments have a resistivity
approximately three times that of copper
at room temperature. As shown in Figure
3, the resistivity increases several
thousand percent as a filament is heated
to incandescence. Since the temperature
coefficient of resistivity for pure
tungsten is relatively constant over
the range of lamp temperatures, it provides
a convenient means for measuring the
average temperature of a filament. |
Fig. 1 click to enlarge chart |
Effects
of operating voltage variations
When incandescent lamps are operated
at other than the nominal or rated voltage,
the color temperature of the filament
varies. The resistance, current, wattage,
lumen output, and efficacy all vary
as shown in Figure 4. The curves shown
apply to both conventional non-halogen
and halogen incandescent lamps and represent
the average of many lamps. The characteristics
of individual lamps may deviate increasingly
from these curves for variations greater
than +/- 10%. The characteristic curves
may be represented near rated voltage
by the equations given in Figure 5 where
the values at rated voltage are in capital
letters and the values at other than
rated voltage are in lower case letters.
The exponents given were empirically
determined from many lamp types and
are applicable to lamps having efficacies
of about 10 to 25 lumens per watt. Actual
exponents may vary from those given
for specific lamp types. |
| Inrush Current
Since the cold resistance of tungsten
filaments is about 1/12 to 1/16 the
hot resistance, it might be expected
that the initial inrush current when
rated voltage is applied to a cold
filament would be 12 to 16 times the
rated current which flows when the
filament heats up to rated temperature.
The actual initial inrush current
is generally limited to some smaller
value by circuit reactance and is
a function of the position on the
ac wave at which the voltage is applied.
The time for the initial inrush current
to decay to the rated current is determined
almost entirely by the thermal mass
of the filament, and ranges from about
0.05 seconds in 15W lamps to about
0.4 seconds in 1500W lamps.
The inrush current and decay time
of incandescent lamp loads is important
in the design, selection, and use
of circuit breakers, switches, fuses,
and other control devices |
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