GB2026230A

GB2026230A - Double envelope discharge lamps

Info

Publication number: GB2026230A
Application number: GB7924870A
Authority: GB
Original assignee: Thorn Electrical Industries Ltd
Current assignee: Thorn Electrical Industries Ltd
Priority date: 1978-07-24
Filing date: 1979-07-17
Publication date: 1980-01-30

Abstract

High intensity discharge lamps which emit undesirable UV radiation normally blocked by an outer glass envelope 12 are provided with a safety switch which disconnects the inner envelope 1 from the supply if the outer envelope 12 is broken and comprises a resilient, compressible, electrically conducting member 15, such as a S-shaped strip of nickel-plated steel, attached to a lead wire 7 of the inner envelope 1 and held in compression between the lead wire 7 and the outer envelope 12, a portion of the resilient member being forced into contact with a conducting support frame 11 of the inner envelope 1 by the compression of the resilient member 15 so as to connect the inner envelope 1 to an external electrical contact. <IMAGE>

Description

SPECIFICATION Lamp safety switch The present invention relates to a safety switch for double envelope lamps and particularly for arc discharge lamps.

The discharge in certain high intensity arc discharge lamps produces ultraviolet (UV) radiation which is transmitted through the walls of the fused quartz arc tube. This radiation includes certain wavelengths which can cause discomfort to human skin and eyes. In normal use the arc tube is enclosed in a glass outer bulb which transmits visible radiation but cuts off the radiation at these undesirable wavelengths. If, however, the outer bulb is broken during use the arc tube may continue to operate and emit undesirable UV radiation which is no longer blocked by the outer envelope. It is thus desirable that some means be provided to rapidly extinguish the arc discharge when the outer envelope is so damaged that it no longer blocks the undesirable UV radiation.

It has been proposed to incorporate an oxidisable fuse element inside the outer envelope, which carries current to the arc tube and which will oxidise and break if air is admitted into the outer envelope.

This device suffers from the disadvantage that it must operate for many thousands of hours and any elements outgassed from the lamp components during life may react with the fuse element, causing premature breakage. Also the fuse element represents a power loss, reducing the overall lamp efficiency. These effects can be reduced by incorporating a bimetal switch to short-circuit the fuse element in normal operation, but this is a relatively complex device.

It has also been proposed to incorporate a mechanical switch which is held closed by the outer envelope and carries current to the arc tube, the switch opening if the outer envelope is broken. The switch devices suggested would require relatively expensive additions to the internal lamp construction.

According to the present invention there is provided a lamp safety swich for a lamp having an inner envelope with lead wires at its ends, an outer envelope surrounding the inner envelope and carrying external electrical contacts, and an electrically conducting frame assembly secured within the outer envelope which supports the inner envelope and is electrically connected to one of the external electrical contacts, the switch comprising a resilient, com pressible, electrically conducting member which is attached to a lead wire of the inner envelope and is held in compression between the lead wire and the outer envelope, a portion of the resilient member being forced into electrical contact with the frame assembly by the compression of the resilient member.

Preferably the resilient member is a strip of material formed into an S shape, a portion between the two bends making the electrical contact with the frame assembly.

A lamp incorporating a safety swich constructed in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a front elevation of the lamp; Figure 2 is a partial cross section of the lamp in Figure 1 taken on the line ll-ll and showing the safety switch in its closed position; and Figure 3 is a partial cross section corresponding to that in Figure 2 but with the safety switch in its open position.

Referring to the drawings, a typical high intensity arc dicharge lamp comprises an arc tube 1 made from fused quartz, which is supported in a rigid metal frame assembly 2 and is sealed into an outer glass bulb 3. The arc tube 1 contains a filling which when activated by a discharge between the electrodes 4, 5 emits radiation including untraviolet radiation. Typically the filling includes mercury, an inert gas and sometimes one or more metal halides.

Connections to the electrodes 4, 5 are made via lead-in wires 6,7 which are sealed through the ends of the arc tube.

The frame assembly 2 comprises an arc tube engaging portion 8 which supports one end of the arc tube 1, a first support portion 9 which is carried by the flare assembly 10, and a second support portion 11 which engages around an internal dimple 12 on the end of the outer bulb 3. Awire 13 sealed into the arc tube and secured to the second support portion 11 ensures that the frame assembly supports the arc tube at both ends.

Lead-in wire 6 is connected through the flare assembly 10 to a first contact on the cap 14. Lead-in wire 7 is welded to an 'S' shaped spring contact 15 which when the arc tube and frame assembly is sealed into the glass bulb 3 engages the dimple 12 and is compressed so that a portion of it between the two bends of the 'S' makes contact with the second support portion 11 of the frame assembly 2. The frame assembly is connected via a wire 16 which passes through the flare assembly 10 to a second contact on the cap 14. Thus in normal use electrical connection is made between the two contacts on the cap 14 and the respective two electrodes 4, 5.

The spring contact is shown in this normal position in Figure 2.

If the outer bulb 3 is broken this will allow the compressed spring contact 15 to expand and break contact with the second support portion 11 of the frame assembly 2. Thus the electrical connection to the electrode 5 will be broken and the discharge will extinguish. The spring contact is shown in its open position in Figure 3.

Even if the outer bulb is not completely broken the spring contact 15 will break the electrical connection provided the dimple is free to move by an amount sufficient to break the contact with the frame assembly 2.

This safety swich is of a very simple construction and does not require any special contact surfaces since it uses the frame assembly itself as one contact surface. However, the current carrying capacity of the swich is limited by the contact area between the spring contact 15 and the frame assembly. This can be increased if necessary by positioning a strip of metal along the second support portion 11 of the frame assembly and welding it at one end to form a contact surface, the strip being wider than the portion of the frame assembly to which it is secured.

Conveniently the strip is made of the same material as the spring contact 15. The wider contact surface of the strip enables a larger contact area to be achieved.

The spring contact 15 is nickel plated steel, the steel core being chosen to have the desired mecha nicai properties, and the plating to give an oxidation resistant surface to provide good electrical contact.

Other materials, such as the commercially available nickel chrome alloys, may also be used.

Claims

1. A lamp safety switch for a lamp having an inner envelope with lead wires at its ends, an outer envelope surrounding the inner envelope and carrying externalelectrical contacts, and an electriclly conducting frame assembly secured within the outer envelope which supports the inner envelope and is electrically connected to one of the external electrical contacts, the switch comprising a resilient, compressible, electrically conducting member which is attached to a lead wire of the inner envelope and is held in compression between the lead wire and the outer envelope, a portion of the resilient member being forced into electrical contact with the frame assembly by the compression of the resilient member.

2. A lamp safety switch according to Claim 1 wherein the resilient member is a strip of material formed into an S shape, a portion between the two bends making the electrical contact with the frame assembly.

3. A lamp safety switch according to Claim 1 or Claim 2 wherein the external electrical contacts are on a lamp cap at one end of the outer envelope and the resilient member abuts the outer envelope at its end opposite the lamp cap.

4. A lamp safety switch according to any preceding claim wherein the resilient member is made of nickel-plated steel.

5. A lamp safety switch according to any preceding claim wherein that part of the frame assembly which makes electrical contact with the resilient member has a contact surface formed by a strip of electrically conducting material.

6. A lamp safety switch substantially as herein described with reference to the accompanying drawings.

7. A lamp incorporating a lamp safety switch according to any preceding claim.