[0001] The invention relates to an electric lamp provided with a lamp envelope which is
sealed in a vacuum-tight manner and which is secured at one end to a lamp cap, current-supply
conductors extending from contacts on the lamp cap through the wall of the lamp envelope
to a light source arranged inside the lamp envelope and a fuse being included in at
least one of the current-supply conductors inside the space limited by the wall of
the lamp envelope and the lamp cap, which fuse is located inside a sealed tube of
electrically insulating material filled with an electrically insulating material.
[0002] Such an electric lamp is known from the British Patent Specification 830,360. In
this known lamp, the fuse is included in a glass tube which is filled with glass pellets
or sand and which is fused at its end with the current-supply conductor. The fuse
is reliable and safe, but is expensive due to the two fusions.
[0003] The invention has for its object to provide a lamp which incorporates a reliable
and safe fuse of a simple and inexpensive construction.
[0004] According to the invention, this is achieved in a lamp of the kind mentioned in the
preamble in that the tube is sealed at its ends onto the current-supply conductor
by means of a coherent mass.
[0005] The lamp according to the invention can be manufactured in a simpler and cheaper
manner in that, before the lamp cap is mounted on the lamp envelope, a tube, for example,
of glass or ceramic material, is slid over the current-supply conductor until it surrounds
the fuse and the tube is filled with electrically insulating material and is sealed
at its end onto the current-supply conductor by providing a coherent mass.
[0006] The coherent mass used can consist of cement which is utilized for securing lamp
caps to lamp envelopes. Examples that can be mentioned are mixtures of talcum powder,
zinc oxide and potassium silicate as well as mixtures of quartz powder, sodium silicoftuoride
and sodium silicate. Instead, a lead borate glass may alternatively be used, which
is filled, for example, with sand, hard glass powder, quartz powder, aluminium oxide
powder, or calcium oxide powder, An example thereof is 3 parts by weight of glass
(3 % by weight of SiO
2, 18 % by weight of B
2O
3, 68 % by weight of PbO, 8 % by weight of ZnO, 3 % by weight of Al
2O
3) and 1 part by weight of sand.
[0007] The tube can be filled between its ends with glass powder, glass pellets, sand and
the like. In a favourable embodiment, the tube is filled with the same cement or glass
composition as that with which it is closed at its ends.
[0008] Numerous constructions of fuses in lamps have been described in literature. Some
of these are expensive, whilst others are unreliable or unsafe. Thus, it has been
proposed to fill the space limited by the lamp cap and the wall of the lamp envelope
for a large part with cement so that the fuse is embedded therein. However, gaps can
be formed in the mass of cement through which a discharge arc which has formed during
melting of the fuse flashes over to the other current-supply conductor or to metal
parts of the lamp cap. Due to the fact that the mass of cement assumes its ultimate
form after the lamp cap has been arranged, these gaps are not perceptible in the finished
lamp.
[0009] In the lamp according to the invention, however, the insulating mass need be provided
only very locally to enclose the fuse on all sides. The mass can be provided before
the lamp cap is fitted so that the fuse can be enclosed completely in a controlled
manner.
Due to the fact that the fuse is embedded in an insulating mass, generally no discharge
arc is formed during melting of the fuse. In cases in which an arc is formed as yet,
it remains inside the tube and extinguishes rapidly.
[0010] The construction of the lamp according to the invention is particularly suitable
for heavily loaded lamps, such as halogen mains voltage lamps of high power, but the
construction may also be used in other halogen lamps and in lamps which have as a
light source both a filament and a discharge arc, for example, a high-pressure mercury
vapour discharge arc, the filament acting at the same time as a current limiter.
[0011] An embodiment of a lamp according to the invention is shown in the drawing, the lamp
envelope being shown in side elevation and the lamp cap in longitudinal sectional
view.
[0012] The lamp envelope 1 is closed at one end by a pinch 5 and is secured by means of
cement 6 to a lamp cap 7. Current-supply conductors 2 and 3 extend from contacts 11
on the lamp cap 7 through the wall of the lamp envelope 1 to a filament 4 arranged
as a light source inside this envelope. The lamp cap 7 has a ceramic moulding 8 which
is fixed in a metal sheath 9. In the space 12 limited by the pinch 5 and the lamp
cap 7, a fuse wire 10 is included in the current-supply conductor 3, which fuse wire
is located inside a tube 13 of electrically insulating material (in the drawing, glass),
which tube 13 is sealed at its ends onto the current-supply conductor 3 and is filled
between its ends with a coherent electrically insulating mass 14. In the lamp shown,
the mass 14 is identical with the cement 6 and consists mainly of quartz powder, zinc
oxide and sodium silicate.
[0013] The lamp shown is a halogen incandescent lamp with a gas filling consisting of krypton
and methylene bromide.
[0014] When it is operated at 225 V, the lamp consumes a power of 150 W and is intended
to be used as a projection lamp.
[0015] The lamp was operated at normal voltage and was then loaded with an overvoltage of
60 %. The fuse interrupted the current through the lamp 5 msec after the overvoltage
was applied. Apart from the fuse wire 10 and the filament 4, the lamp was still completely
intact.
1. An electric lamp provided with a lamp envelope which is sealed in a vacuum-tight
manner and which is secured at one end to a lamp cap, current-supply conductors extending
from contacts on the lamp cap through the wall of the lamp envelope to a light source
arranged inside the lamp envelope, a fuse being included in at least one of the current-supply
conductors inside the space limited by the wall of the lamp envelope and the lamp
cap, which fuse is located inside a sealed tube of electrically insulating material
filled with an electrically insulating material, characterized in that the tube is
sealed at its ends onto the current-supply conductor by means of a coherent mass.
2. An electric lamp as claimed in Claim 1, characterized in that the coherent mass
fills the whole tube.
