Filament Bypass Circuit

This invention relates to a filament bypass circuit and more particularly to a filament bypass circuit employing a voltage operated switch to maintain current flow when the filament fails.

In strip lighting configurations a plurality of lights are strung in series, for example, as in Christmas tree and other ornamental applications. When one bulb fails, the whole string of lamps goes dark. To replace the defective bulb it is necessary to inspect and check each bulb. In many of these arrangements, the bulbs are inexpensive and the inconvenience of locating the failed bulb may be acceptable. In certain other situations, this inconvenience may be unacceptable, with the result that a variety of arrangements for identifying the defective bulb are in existence, and in some configurations there is provision for maintaining the remaining bulbs lit until the defective bulb can be changed.

In some cases, it is necessary or desirable to use special light bulbs having characteristics which are required in particular applications, and an example of such bulbs is the halogen lamp which due to the presence of a halogen gas in the envelope permits higher intensity current flow through the filament with the result that much brighter light is obtainable in a very small bulb size as well as producting a color rendition which is highly desirable in certain applications. Examples of such specialized applications would be lighting of works of art in museums, lighting in discotheques, and stage lighting. The halogen lamp may be used alone or in strings. The halogen lamp requires a low voltage, such as 12 volts, for its operation, and when used alone or in parallel in order to avoid the problem of locating the failed bulb, a bulky and heavy transformer is required to reduce the voltage.

When the halogen lamp is employed in strings, the reduced voltage is obtainable by connecting them in series so that the drop across each lamp is at the rated value. However, in the case of the failure of a single lamp, the whole string goes dark without indicating which lamp has failed. When a bypass circuit, of design heretofore available, is employed to maintain the other lamps lit after a failure has occurred it may not even be noticeable that one lamp is dark, since there is no indication of such a failure other than one lamp being dark. In the environments where halogen lamps are or may be employed, it may not be immediately noticeable that a failure has occurred. In addition, in many of the bypass circuits known up to now, continuation of current flow results in an increased voltage across the remaining lamps with the result that the life of those lamps may be shortened by a very substantial amount. In the high intensity halogen lamp, small increases over rated voltage could cause a considerable shortening of the lives of the remaining lamps. Since the halogen as well as other specialized lamps may be quite expensive, present arrangements for maintaining current flow are not completely satisfactory.

In this invention, there is provided a filament bypass circuit for use with lamps which overcomes or reduces significantly the problems heretofore associated with arrangements designed to maintain current flow after a failure has occurred. In addition, where each bulb is supplied in a parallel arrangement to identify the failure of a single bulb, the present invention makes it possible to avoid the use of the transformer.

In accordance with a preferred embodiment of this invention, there is provided a string of lamps connected electrically in series in which each lamp is provided with a bypass circuit comprising a silicon bilateral voltage triggered switch in series with a diode. The bilateral voltage triggered switch is an element which closes only when a certain predetermined voltage is reached, which would occur with the failure of the bulb, and the voltage drop across the open filament reaches circuit voltage. The diode, in permitting flow in one direction only, reduces effective current flow so that all the remaining lamps will have diminished light output which will call attention immediately to the failed light as well as avoid the application of a greater voltage which would adversely affect the life of the remaining filaments. Interruption of the application of the voltage source to replace the defective lamp will cause the switch to open and resume normal operation once the lamp is replaced.

It is thus a principal object of this invention provide a filament bypass circuit of improved design and greater usefulness.

Other objects and advantages of this invention will hereinafter become obvious from the following detailed description of the preferred embodiments of this invention.

FIG. 1 is a circuit diagram showing ten incandescent lamps in series incorporating a preferred embodiment of this invention.

FIG. 2 is an exploded view of a lamp with a lamp socket in which is embedded a preferred embodiment of this invention.

Referring to FIG. 1, there is illustrated a circuit 10 consisting of ten filament operated lamps L.sub.1 through L.sub.10 connected in series across a voltage source of 115 volts A.C. through a plug 12. Each of the lamps thus is receiving approximately 12 volts, its rated requirement.

Across each of lamps L.sub.1 through L.sub.10, there is provided an identical bypass or shunt circuit. Referring to lamp L.sub.4, across its filament is connected in series a diode D.sub.4 and a silicon bilateral voltage triggered switch S.sub.4. Such switches are conventional and available commercially. For example, switches of this type are sold under the trademark SIDAC by Teccor Electronics, Inc. This switch turns on and conducts when a particular voltage drop across it appears, and remains in a low on-state voltage. That is, it will maintain current flow even through the voltage drop across it drops, as would be expected in this circuit.

Should lamp L.sub.4 fail in service by its filament opening with the result that current flow through all lamps will cease, the voltage drop across switch S.sub.4 will increase substantially, and switch S.sub.4 will begin to conduct, returning all other lamps to the on state. However, due to the presence of diode D.sub.4, which will almost halve the effective voltage due to its blocking flow in one direction, the voltage drop across each of the remaining lamps will be less, reducing the risk of filament failure in each of the others, and signalling by the diminished light output that there was a light failure requiring a bulb replacement. Without the presence of the diode in the bypass circuit, the voltage across each lamp would go from 12 volts to over 13 volts, an increase in excess of ten per cent which could result in additional bulb failures.

Plug 12 is then disconnected from its power source and bulb L.sub.4 replaced, with the result that circuit 10 is returned to normal operation.

The bypass circuit which has just been described may be incorporated as part of the socket into which each lamp is threaded or plugged. This arrangement is illustrated in FIG. 2 where is shown a light socket 14 consisting of a block of dielectric material in which is embedded a pair of hollow sleeve contacts 16a and 16b to receive the prongs 18a and 18b of a halogen lamp 22 having a filament 24. A pair of electrical conductors 26a and 26b making contact with contacts 16a and 16b extend down from socket 14. The bypass circuit consisting of diode D.sub.a and silicon bilateral voltage triggered switch S.sub.a are embedded in socket 14 connected by conductors as illustrated across contacts 16a and 16b.

It is thus seen that there has been provided a unique arrangement for maintaining current flow in a series connected circuit of lamps while at the same time preventing voltages from increasing on the remaining functioning lamps of the circuit and indicating that one lamp has failed.

While only certain preferred embodiments of this invention have been described, it is understood that many changes and variations thereof are possible without departing from the principles of this invention as defined in the claims which follow.