GB2110486A - Automatic filament- changeover apparatus for multi- filament lamp installations - Google Patents

Abstract

Automatic filament-changeover apparatus for bringing into operation a standby filament 6 of a multi-filament lamp installation upon the failure of the main lamp filament 5 utilises a saturable core device (T2) which is connected into the auxiliary-filament energisation circuit and is arranged to selectively inhibit and enable auxiliary filament energisation in dependence on the d.c. energisation or non- energisation of a control winding 16 of the device T2. During normal functioning of the main lamp filament 5, the control winding 16 is fed with direct current derived, for example, from a current transformer T3 in series with the main filament 5. Failure of the main filament 5 cuts off current to the control winding 16 enabling auxiliary filament energisation. The saturable-core device is preferably a saturable transformer T2 supplying the auxiliary filament 6. The main filament 5 may also be supplied by a saturable transformer T1 having a control winding (21, Figure 5, not shown) energised by direct current derived from a current transformer (T4) in series with the auxiliary filament 6, such that when the auxiliary filament 6 is brought into operation the transformer T1 saturates to decrease its impedance and increase the current flow in transformer T2, the primary of transformer T1, T2 being connected in series. <IMAGE>

Description

SPECIFICATION Automatic filament-changeover apparatus for multi-filament lamp installations The present invention relates to automatic filament-changeover apparatus for bringing into operation a standby, filament or a multi-filament lamp installation upon the failure of the main lamp filament; in particular, but not exclusively, the invention relates to such apparatus, of failsafe form, for use with railway signal lamps.

It is common practice where filament lamps are used in critical signalling systems, for the integrity of the lamp filament itself to be electrically monitored, usually by monitoring the current flowing therethrough. Furthermore, in many systems (such as railway signalling) it is not sufficient to merely detect that a signal lamp filament has failed but it is also necessary to provide an auxiliary filament together with appropriate control means which can provide a "standby" illumination system for the period until the maintenance staff can replace the failed lamp.

A typical multi-filament lamp unit such as might be used for a trackside colour light railway signal is shown in Figure 1 of the accompanying drawings, this Figure being a diagrammatic section through the unit.

As can be seen, the lamp unit shown in Figure 1 comprises a lens system 1 and a filament lamp 2 placed at the focal point of the lens system. In the signal unit shown, the lens system is in fact constituted by an outer, clear Fresnel lens 3 and an inner, inverted toric lens 4 which is coloured, the combination of these lenses enabling the production of a long range, substantially parallel, beam of light from a very short focal length optical system. The lamp 2 itself has two filaments, namely a main filament 5 and a standby, auxiliary filament 6, these filaments typically being electrically connected both to a common output pin and to respective further output pins so as to form a tripole lamp unit.

During normal operation, the lamp is illuminated by energisation of the main filament 5 which is generally located at the focal point of the lens system. Upon failure of the main filament 5, the auxiliary filament 6 is brought into operation and in order to offset the slight de-focussing which occurs due to the off-focus positioning of the auxiliary filament, a higher rating of the latter may be adopted.

As previously mentioned, the bringing into operation of the auxiliary filament 6 upon failure of the main filament 5 is effected by appropriate control means and a typical known automatic filament-changeover arrangement is shown in Figure 2 of the accompanying drawings, this Figure being a circuit diagram of a lamp installation in which the filament-changeover arrangement is connected to control a lamp unit of the Figure 1 form.

As can be seen from Figure 2, the main filament 5 is arranged to be energised with alternating current from a remote energisation control unit (not shown), via a step-down transformer 1 3 inserted in circuit at the lamp unit to reduce cable losses from the remote control unit. The auxiliary filament 6 is also connected to the step-down transformer 1 3 for energisation therefrom.

The automatic filament-changeover arrangement is built around an a.c. relay 10 (typically a shaded-pole octal base relay) provided both with a very low impedance coil 11 connected in series with the main filament 5, and with a normally-closed contact set 12 connected in series with the auxiliary filament 6. Upon a.c.

energisation of the main filament 5, current flows through the coil 11 causing the relay to operate and open the contact set 1 2 so as to prevent energisation of the auxiliary filament 6. Upon failure of the main filament, current ceases to flow through the coil 11 so that the relay 10 drops out and the contact set 12 closes to energise the auxiliary filament 6.

In order to generate a warning signal upon main filament failure, the relay 10 is provided with a normally-open, second contact set 12 connected into a warning circuit 1 5. Failure of the main filament will result in the contact set 12 changing from a closed to an open state thereby opening the warning circuit 1 5 to cause the generation of a warning signal.

Although the filament changeover arrangement of Figure 2 has the advantage of being fairly simple, it does, however, suffer from a number of drawbacks, namely: 1) When the lamp unit is first energised, there is a small delay before the relay 10 opens the contact set 12 and during this time the current in the auxiliary filament 6 will rise. As soon as the relay 10 starts to energise, its contact set 12 is thus required to break the "cold" lamp current of the auxiliary filament (which can be as much as 10 amps compared to a normal current of typically 2 amps). On a busy railway where energisation of the lamp unit could happen as often as ten times an hour, the repeated breaking of the "cold" lamp current will fairly rapidly lead to deterioration of the relay contact set 12.

2) Commercial AC shaded pole octal base relays such as are generally used for the relay 10, are inherently rather poor performers in applications such as railway signalling where the environmental conditions can be severe in the extreme.

3) With rising train speeds there are demands for additional indications from the signalling system to the train driver. These demands are frequently being met by causing flashing of a lamp; in order to enhance lamp life, the flashing is often realised by merely "dimming"-i.e.

reducing the lamp current to a value where the lamp lens system appears to the external viewer to be extinguished. Under this reduced current condition it is required that the relay 10 should remain energised (provided of course, that the main filament has not failed); for various technical reasons, this requirement is difficult to achieve with the relays presently used.

It is an object of the present invention to provide a filament change-over arrangement for a multi-filament lamp installation which avoids at least some of the disadvantages cited above.

According to one aspect of the present invention, there is provided, in a multi-filament lamp installation, filament changeover apparatus for automatically bringing into operation a standby auxiliary lamp filament upon failure of a main lamp filament of the installation, said apparatus comprising:: a saturable-core device connected in a circuit which includes both an a.c. energisation source of the installation and the said auxiliary filament, the saturable-core device including a control winding for controlling the saturation of at least part of said core whereby to vary the electrical characteristics of the device in such a manner that when the control winding is fed with direct current having at least a predetermined minimum value, energiation of the auxiliary filament is inhibited, whereas in the absence of said direct current, the auxiliary filament can be energised from said source, and control-winding energisation means operative during normal functioning of the main lamp filament to supply the control winding with a direct current having a magnitude at least equal to said predetermined maximum value, failure of the main filament causing the control-winding energisation means to cut off said direct current to the control winding.

In one embodiment of the invention, the saturable-core means comprises a saturable auxiliary-filament transformer having primary and secondary windings wound on said core and respectively connected to said a.c. source and to the auxiliary filament, energisation of said control winding by direct current of a magnitude at least equal to said predetermined minimum value being effective to saturate the said core whereby to substantially collapse the transformer action which otherwise exists between said primary and secondary windings.

Generally, the main filament of the lamp installation will also be connected for energisation from the said a.c. source, in which case the said control-winding energisation means can conveniently take the form of a rectifier circuit connected on its output side to the said control winding and on its input side to a current transformer inserted in series in a supply line feeding the main lamp filament from the said a.c.

energisation source. By suitable choice of circuit parameters, energising current flow through the main lamp filament can be arranged to produce a d.c. current in the control winding of a magnitude at least equal to said predetermined minimum value sufficient to saturate the core of the auxiliary-filament transformer.

In signal lamp installations, where the multifilament lamp is only intermittently energised, it will of course be appreciated that the said a.c.

energisation source can be selectively controlled to provide the required signalling function. Where flashing of the lamp is required and is effected by reducing, but not eliminating, the lamp current during the dark periods of the flash-cycle, then the control-winding energisation means is so designed that the direct current fed to the control winding is still adequate to produce core saturation even during periods of reduced current flow through the main filament.

During saturation of the core of the saturable auxiliary-filament transformer, the primary winding of the transformer will generally present a very low impedance. If this primary winding is directly connected across the a.c. source, the magnitude of the current drawn from the a.c.

source will be large which is generally undesirable. One possible way of overcoming this problem where the main lamp filament is arranged to be energised from the said a.c. source via a main-filament transformer, is to connect the primary windings of the saturable auxiliaryfilament transformer and main-filament transformer in series across the a.c. source. In this case, the impedance of the main-filament transformer serves to limit the current through the primary of the saturable transformer during saturation of the latter. However, this arrangement suffers from the drawback that upon failure of the main filament, the primary winding of the main-filament transformer presents a high impedance which considerably restricts the power flow to the auxiliary filament through the now unsaturated auxiliary-filament transformer.

In a preferred embodiment of the invention, therefore, current flow through the primary winding of the auxiliary-filament transformer during saturation of the latter, is restricted by connecting in series with the primary winding, a second saturable-core device which presents a substantially higher impedance when unsaturated than when saturated, saturation of this device being effected by the d.c. energisation of a control winding of the device in response to current flow through the secondary of the auxiliary-filament transformer. Advantageously, the control winding of the second saturable-core device is arranged to be d.c. energised by a circuit comprising a rectifier bridge fed on its input side from a current transformer inserted in the secondary winding circuit of the auxiliary-filament transformer. With this arrangement, a regenerative action occurs upon the main filament failing and the consequential desaturation of the saturable transformer. Thus initially upon desaturation, a small but significant a.c. current will flow in the secondary circuit of the auxiliary-filament transformer, this a.c. current in turn resulting in direct current flow in the control winding of the second saturable-core device thereby bringing about partial saturation of the latter and causing its impedance to fall and allow a larger current to flow in the primary winding of the auxiliaryfilament transformer: this larger primary current in turn results in a larger secondary current which in turn results in the second saturable-core device being driven further into saturation and so on.

Advantageously, the second saturable-core device is in fact constituted by a second saturable transformer used to energise the main lamp filament, the primary windings of the two transformers being connected in series. The various circuit impedances are so arranged that with normal functioning of the main filament, current builds up more rapidly in the secondary circuit of the main-filament transformer upon a.c.

energisation of both primaries, whereby the auxiliary filament transformer saturates first, cutting off current both to the auxiliary filament and to the control winding of the main filament transformer. Upon failure of the main filament, the auxiliary-filament transformer comes out of saturation thereby energising the auxiliary filament and saturating the main-filament transformer.

Remote warning of the failure of the main filament can be readily achieved with the apparatus of the present invention by connecting a d.c. relay in series of parallel with the control winding of the first-mentioned saturable-core device, a normally-open contact set of the relay serving to open a warning signal circuit upon deenergisation of the relay following main filament failure.

The automatic filament changeover apparatus of the present invention overcomes the disadvantages, discussed above, of the prior art control arrangements inasmuch as in the apparatus of the invention, control of the auxiliary filament current is no longer effected by a relay so that deterioration of relay contacts, poor relay performance, and relay insensitivity to low mainfilament energising currents are no longer problems.

According to another aspect of the present invention, there is provided apparatus for automatically bringing into operation a standby auxiliary filament of a multi-filament lamp unit upon the failure of the main lamp-unit filament, said apparatus comprising: a saturable main-filament transformer arranged for connection on its secondary side with said main filament, a saturable auxiliary-filament transformer arranged for connection on its secondary side with said auxiliary filament, the primaries of the main-filament and auxiliary-filament transformers being arranged in series for connection across an a.c. filament energisation source, first saturation control means responsive to current flow through the main filament to supply a direct current to a control winding of the auxiliaryfilament transformer in order to bring about saturation of the latter, and second saturation control means responsive to current flow through the auxiliary filament, to supply a direct current to a control winding of the main-filament transformer in order to bring about saturation of the latter, the arrangement being such that upon initial energisation of the transformer primaries from said source, then provided that the main filament has not failed, the auxiiiary-filament transformer will saturate while the main-filament transformer effects energisation of the main filament, failure of the main filament resulting in the auxiliaryfilament transformer coming out of saturation and causing energisation of the auxiliary filament with consequent saturation of the main-filament transformer.

Filament changeover apparatus embodying the invention will now be particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is, as already described, a diagrammatic section through a multi-filament lamp unit; Figure 2 is, also as already described, a circuit diagram of a multi-filament lamp installation in which a prior-art control arrangement is connected to control the auxiliary filament of a lamp unit of the Figure 1 form; Figure 3 is a circuit diagram of a multi-filament lamp installation in which a first form of filament changeover apparatus embodying the invention is connected to control the auxiliary filament of a lamp unit of the Figure 1 form; Figure 4 is a diagram of a saturable transformer forming part of the filamentchangeover apparatus shown in Figure 3; and Figure 5 is a circuit diagram similar to Figure 3 but showing a second form of filament changeover apparatus.

As with the Figure 2 installation described hereinbefore, in the Figure 3 multi-filament installation, the main filament of the lamp unit is arranged to be energised from a remote energisation control unit (not shown) via a stepdown main-filament transformer (referenced T1 in Figure 3). However, in contrast to the Figure 2 arrangement, in the Figure 3 installation the auxiliary lamp filament 6 is not arranged to be energised via the same transformer as the main filament 5 but via a separate auxiliary-filament transformer T2, although the primaries of the transformers T1 and T2 are, in fact, connected in series to the energisation unit.

The transformer T2, which constitutes part of the filament changeover apparatus, is a saturable transformer which, as shown in Figure 4, has a saturable core made up of a central limb 30, two parallel outer limbs 31, and two transverse yokes 32 connecting together the corresponding ends of the limbs 30, 31. A primary winding of the transformer T2 is wound in two parts 33A, 33B mounted on respective ones of the outer limbs 31; similarly a secondary winding of the transformer T2 is wound in two parts 34A, 34B mounted on respective ones of the limbs 31. A control winding 16 is wound on the central limb 30 and, optionally, a copper slug 35 encompasses this latter limb to assist in keeping a.c. flux out of the central limb 30.Upon a.c. energisation of the primary winding parts 33A, B and with the contrc winding 1 6 de-energised, a.c. fluxes will flow around the limbs 31 and yokes 32 inducing voltages in the secondary winding parts 33A, B.

D.C. energisation of the control winding 16 will produce a d.c. flux passing out of the central limb 30 through both of the limbs 31. With a sufficiently large d.c. energising current flowing in the control winding 16, the core of the transformer T2 will saturate and destroy the transformer action between the primary and secondary windings.

In addition to the transformer T2, the filament changeover apparatus also comprises a current transformer T3 the primary of which is connected in series with the main lamp filament 5, a fullwave rectifier bridge DB1 connected across the secondary of the current transformer T3, and a smoothing capacitor C1 which together with the control winding 16 is connected across the d.c.

side of the rectifier bridge DB1.

During operation of the installation, as soon as the remote energisation control unit causes a.c.

energisation of the primaries of the transformers T1 and T2, the power supplied to the transformers is initially shared between the main and auxiliary filaments 5, 6 in a ratio defined by the transformer characteristics. As current begins to flow in the main filament circuit, a voltage appears across the secondary of the current transformer T3 resulting in a d.c. potential across capacitor C1. This d.c. potential is applied across the control winding 1 6 of the transformer D2 and results in a d.c. current flowing through this winding. The effect of this d.c. current is to begin to saturate the core of the transformer T2 so that the proportion of the input power transferred by the transformer T2 decreases while that transferrred by the transformer T1 increases.As a consequence, the current in the main filament 5 rises resulting in an increased d.c. current through the control winding 1 6 and thus an enhanced saturation of the core of the transformer T2. By this process, very shortly after energisation of the primaries of the transformers T1 and T2, virtually all of the input power is transferred to the main filament circuit and the auxiliary filament is effectively in an un-energised condition.

From the foregoing it can be seen that the current transformer T3, the diode bridge DB1, and the capacitor C1, together form control-winding energisation means for inhibiting operation of the transformer T2, and thus energisation of the auxiliary filament 6, during normal functioning of the main filament 5.

Upon failure of the main filament 5, the d.c.

potential on the capacitor C1 rapidly disappears and d.c. current ceases to flow through the control winding 1 6 of the transformer T2. As a result, the core of transformer T2 comes out of saturation so that the transformer starts to act in a conventional manner. The auxiliary filament 6 now becomes energised and serves as a stand-in for the failed main filament.

The characteristics of the current transformer T3 (and, in particular, its turns ratio) are chosen in conjunction with those of the control winding of transformer T2 such as to ensure that during normal functioning of the main filament 6, the core of the transformer T2 stays saturated even when the lamp is "dimmed" as part of a flashing signal function.

In the Figure 3 installation, warning is given of main filament failure by means of a d.c. relay the excitation winding 1 7 of which is connected across the output of the rectifier bridge DB 1. A normally open set of relay contacts 1 8 is interposed in a warning circuit 1 5. With this arrangement, upon energisation of the main filament 5, the d.c. relay operates and the contact set 1 8 closes. Should the main filament 5 fail, then the d.c. relay drops out and the contact set 18 opens to provide a remote indication of the main filament failure via the warning circuit.

As an alternative to using the d.c. relay 17, 18 to provide a warning of main filament failure, a warning circuit using no moving parts can be employed as is indicated in dashed lines in Figure 3. This warning circuit comprises an additional winding 20 on the current transformer T3, and a second rectifier bridge DB2 which is connected on its a.c. side to the winding 20. During normal operation of the main filament 5, a d.c. signal is provided across the d.c. side of the bridge DB2.

This d.c. signal is used to provide a remote indication of the state of the main filament 5; failure of the main filament 5 being indicated by cessation of the d.c. signal.

It will be noted that the auxiliary-filament controlled apparatus illustrated in Figure 3 is of a fail-safe design inasmuch as any failure in the inhibit means constituted by the transformer T3, the diode bridge DB1, and the capacitor C1, or in the control winding of the transformer T2 will prevent saturation of the core of the transformer T2, and, at the same time, will result in the relay contact set 1 8 opening.

One drawback of the Figure 3 arrangement is that upon failure of the main filament 5, the inductance of the primary winding of the mainfilament transformer T1 increases significantly and restricts the power flow to the auxiliary filament 6 through the auxiliary-filament transformer T2. Although suitable transformer design can lessen this problem, it nevertheless makes it difficult to achieve high power output from the auxiliary filament. Furthermore, while the problem could be overcome by connecting the primary of the auxiliary-filament transformer T2 in parallel with the primary of the transformer T1 rather than in series therewith, this arrangement also presents difficulties since the very low impedance of the primary of the transformer T2 when saturated will result in very large current flows.

To overcome these difficulties, in the preferred embodiment of the invention illustrated in Figure 5, the main-filament transformer T1 is constituted by a saturable transformer of the Figure 4 form. In this case, the primary of the main filament transformer T1 (which is connected in series with the primary of the transformer (T2) is controlled to present a low impedance after failure of the main filament 5 by arranging for the core of this transformer to progressively pass into saturation as current flow builds up through the auxiliary filament 6, the action being a regenerative one.

Saturation of the core of the main-filament transformer T1 is effected by the d.c. energisation of a control winding 21 of this transormer, energisation of the winding 21 being effected through a circuit comprising a rectifier bridge DB3, a capacitor C2, and a current transformer T4 connected in series with the auxiliary filament 6.

Considering the action of the Figure 5 embodiment in more detail, upon failure of the main filament 5, the auxiliary-filament transformer T2 comes out of saturation and its transformer action is restored thereby causing current flow through the auxiliary filament 6; the magnitude of this current is, however, initially restricted by the impedance of the primary of the transformer T1. Current flow in the secondary circuit of the transformer T2 causes d.c.

energisation of the control winding 21 , via the transformer T4 and bridge DB3, and, as a result, the core of the transformer T1 begins to saturate reducing the impedance of the primary of this transformer. This decreased impedance results in increased current flow in the secondary of the transformer T2 which in turn results in the core of transformerT1 being driven further into saturation, and so on until the core of transformer T1 is fully saturated and the auxiliary filament 5 is fully energised.

Of course, the impedances of the various components of the Figure 5 embodiment, must be such that, with the filament 5 operative, energisation of the primaries of the transformers T1 and T2 will result in the transformer T2 saturating and the main filament 5 being fully energised through the transformer T1. This can be achieved, for example, by arranging for the energisation circuit of the control winding 21 to have a time constant much greater than that associated with the energisation circuit of the winding 16; with such an arrangement, the current initially flowing in the main filament circuit will start to cause saturation of the transformer T2 in advance of any corresponding effect on the transformer T1 resulting from the initial auxiliary-filament current.As the transformer T2 starts to saturate, the auxiliary filament current reduces while the main filament current increases, the net result being that the auxiliary-filament transformer T2 rapidly becomes fully saturated and the main filament 6 fully energised.

In a variant of the Figure 5 embodiment, the primary of the auxiliary-filament transformer T2 is connected in series with a saturable-core device separate from the main-filament transformer T1 (the latter being of ordinary construction and having its primary connected in parallel with the series combination formed by the primary of transformerT2 and the saturable-core device).

This saturable-core device has a control winding connected for d.c. energisation in the same manner as the control winding 21 in Figure 5. The general operation of this variant is similar to the Figure 5 embodiment except that it is the impedance of the said saturable-core device, rather than that of the primary of main-filament transformer, which is decreased upon current flow in the auxiliary filament circuit.

Various other forms of filament-changeover apparatus incorporating saturable-core devices can be used to bring into operation an auxiliary filament upon failure of a main lamp filament, energisation of the saturation control windings of the saturable-core devices being effected in dependence on current flow in the main filament circuit. Thus, magnetic amplifiers can be used to effect the desired filament changeover and suitable circuit configurations for such amplifiers will be apparent to persons skilled in the art.

Furthermore, it is to be noted that the d.c.

energisation of the control winding of a saturablecore device could be arranged to cause the device to come out of saturation, rather than to saturate, this being achieved by providing a suitable d.c.

biasing current flowing through a biasing winding and operative, in isolation, to cause saturation of the device. In one implementation of the present invention using such a device, the device is connected in series with the primary of a normal auxiliary-filament transformer and is arranged to have a sufficiently high impedance when saturated as to virtually extinguish secondary current flow in the auxiliary filament transformer, the impedance of the saturated device being small enough to permit substantial power flow through the auxiliary-filament transformer to the auxiliary filament. In this arrangement, energisation of the control winding of the saturable-core device is effected in response to current flow through the main filament.

Claims (9)

Claims

1. In a multi-filament lamp installation, filament-changeover apparatus for automatically bringing into operation a standby auxiliary lamp filament upon failure of a main lamp filament of the installation, said apparatus comprising: a saturable-core device connected in a circuit which includes both an a.c. energisation source of the installation and of the said auxiliary filament, the saturable-core device including a control winding for controlling the saturation of at least part of said core whereby to vary the electrical characteristics of the device in such a manner that when the control winding is fed with direct current having at least a predetermined minimum value, energisation of the auxiliary filament is inhibited, whereas in the absence of said direct current, the auxiliary filament can be energised from said source, and control-winding energisation means operative during normal functioning of the main lamp filament to supply the control winding with a direct current having a magnitude at least equal to said predetermined minimum value, failure of the main filament causing the control-winding energisation means to cut off said direct current to the control winding.

2. Apparatus according to Claim 1, wherein said saturable-core device comprises a saturable auxiliary-filament transformer having primary and secondary windings wound on said core and respectively connected to said a.c. source and to the auxiliary filament, energisation of said control winding by direct current of a magnitude equal to said predetermined minimum value being effective to saturate the said at least part of the core whereby to substantially collapse the transformer action which otherwise exists between said primary and secondary windings.

3. Apparatus according to Claim 1 or Claim 2, wherein the said control-winding energisation means comprises a rectifier circuit connected on its output side to the said control winding, and on its input side to a current transformer connected in series with the main lamp filament for joint energisation therewith from said a.c. source.

4. Apparatus according to Claim 2 or Claim 3, further comprising a second saturable-core device connected in series with the primary winding of the auxiliary-filament transformer across the said a.c. source, said second device presenting a substantially higher impedance when unsaturated than when saturated and including a control winding which can be d.c. energised to bring about saturation of said second saturable-core device, said apparatus also comprising second control-winding energisation means arranged to effect d.c. energisation of the control winding of the second saturable core device upon current flow through the secondary winding circuit of the auxiliary filament transformer.

5. Apparatus according to Claim 4, wherein said second control-winding energisation means comprises a rectifier circuit connected on its output side to the control winding of the second saturable-core device, and on its input side to a current transformer connected in series with the auxiliary filament across the secondary winding of the auxiliary-filament transformer.

6. Apparatus according to Claim 4 or Claim 5, wherein said second saturable-core device comprises a second saturable transformer used to energise the main lamp filament, the primary winding of the two saturable transformers being connected in series across said a.c. source and the secondary winding of the second, mainfilament, transformer being connected in circuit with said main filament, the arrangement being such that upon initial energisation of the transformers from said a.c. source, and provided that the main filament has not failed, the auxiliary-filament transformer will saturate while the main-filament transformer will operate to energise the main filament, failure of the main filament resulting in the auxiliary-filament transformer coming out of saturation to energise the auxiliary filament and cause saturation of the main-filament transformer.

7. Apparatus according to any one of Claims 2 to 6, wherein the or each of said saturable transformer has a core formed by a central limb, two parallel side limbs, and two transverse yokes connecting the corresponding ends of the three said limbs, the central limb being wound with the control winding of the transformer while the primary and secondary windings are each wound in two parts, one part on one outer limb and the other part on the other outer limb.

8. Apparatus according to Claim 7, wherein a copper slug is provided around the central limb of the transformer.

9. Apparatus for automatically bringing into operation a standby auxiliary filament of a multifilament lamp unit upon the failure of the main lamp-unit filament, said apparatus comprisingt a saturable main-filament transformer arranged for connection on its secondary side with said main filament, a saturable auxiliary-filament transformer arranged for connection on its secondary side with said auxiliary filament, the primaries of the main-filament and auxiliary-filament transformers being arranged in series for connection across an a.c. filament energisation source, first saturation control means responsive to current flow through the main filament to supply a direct current to a control winding of the auxiliaryfilament transformer in order to bring about saturation of the latter, and second saturation control means responsive to current flow through the auxiliary filament, to supply a direct current to a control winding of the main-filament transformer in order to bring about saturation of the latter, the arrangement being such that upon initial energisation of the transformer primaries from said source, then provided that the main filament has not failed, the auxiliary-filament transformer will saturate while the main-filament transformer effects energisation of the main filament, failure of the main filament resulting in the auxiliaryfilament transformer coming out of saturation and causing energisation of the auxiliary filament with consequent saturation of the main-filament transformer.

1 0. Automatic filament-changeover apparatus substantially as hereinbefore described with reference to Figure 3 or Figure 5 of the accompanying drawings.