GB2144000A - Apparatus for varying the intensity of illumination of a light source - Google Patents

Abstract

The apparatus includes first and second switching means for connecting one or other of two supply voltages to the light source as required. The first switch means includes first and second switches RLA1 and RLA2 connected one to each supply voltage. The second switch means includes a switch RLB1 arranged to connect one or other of the switches RLA1 and RLB2 to a light source. The first switch means is arranged such that only one of the two switches RLA1 and RLA2 may connect its supply voltage to the switch RLB1 at any instant. Control means RLA, RLB, SW are provided and arranged so that the condition of switch RLB1 is always changed before that of RLA1 and RLA2. Semiconductor switches and a time delay circuit may be used instead of the illustrated relays. The apparatus may be used to decrease the intensity of traffic lights when the ambient light level is low. <IMAGE>

Description

SPECIFICATION Apparatus for controlling the intensity of illumination of a light source This invention relates to apparatus for controlling the intensity of illumination of a light source. It is particularly, though not exclusively, concerned with controlling the intensity of traffic control signals. It is current practice to enable the intensity of the light sources used in traffic lights to be varied between two values, the lower level being used at night or at other times when the ambient light level is low. Since changes in intensity are relatively infrequent it is usual to provide two voltage supply levels to the lamps. All the lamps are supplied from the normal mains supply voltage either directly or by way of a transformer, and it is simple to vary the supply voltage.

This is usually done by providing an autotransformer fed from the 240 volt main supply and providing both 240 volts and, say, 1 60 volts from a suitable tapping. Changing from one supply voltage to another is a question of switching the appropriate voltage to the lamps, but problems arise when using small electromagnetic relays with changeover contact sets to perform the switching. The autotransformer is a low impedance source, and it is possible that when the moving relay contact moves away from the higher voltage fixed contact an arc may be formed between the two contacts. The plasma from this arc may exist for as long as it takes the moving contact to reach the other fixed contact, in which case a low impedance short circuit exists across the 80 volt supply from part of the transformer winding.This can cause the the moving contact and the low voltage fixed contact to become welded together. The problem rarely affects the high voltage fixed contact since an arc is less likely to form when breaking the lower voltage. Whilst the problem is relatively rare, occurring perhaps only once every few months, it still requires attention and hence causes expense.

It is an object of the invention to provide apparatus for controlling the intensity of illumination of a light source which overcomes the above-mentioned problems.

According to the present invention there is provided apparatus for varying the intensity of illumination of a light source by selecting either of two different supply voltages for connection to the light source, which includes first switch means comprising two switches connected one to each supply voltage, and second switch means arranged to connect one ar other of the switches of the first switch means to the light source, the first switch means being arranged such that only one of the two switches may connect its supply voltage to the second switch means at any instant, and control means for changing the condition of the second switch means before that of the first switch means.

An embodiment of the invention will now be described with reference to the accompanying drawing. This shows a schematic circuit diagram of one form of the apparatus.

Referring now to the drawing, an autotransformer AT is supplied with standard mains voltage of 240 volts. The transformer supplies 240 volts and 1 60 volts to the changeover switching arrangement. As will be seen from the drawing this comprises a first switch having contacts sets RLA1 and RLA2, and a second switch having contact sets RLB1 and RLB2. The 240 volt supply from the autotransformer AT is connected to one fixed contact of contact set RLA1 whilst the 1 60 volt supply is connected to a fixed contact of contact set RLA2. The two contact sets are operated together by a relay coil RLA, and are arranged such that one of the two contact sets is closed whilst the other is open. The moving contacts of contact Sets RLA1 and RLA2 are connected to separate fixed contacts of changeover contact set RLB1.This contact set is operated by a relay coil RLB, which also operates a contact RLB2 which connects an operating voltage V to the relay coil RLA. The relay coil RLB is connected through a switch SW to the operating voltage V.

The moving contact of contact set RLB1 supplies the individual lamps through lamp switches LS. As already stated, the lamps may be supplied with the supply voltage selected by the switches, or may be fed from individual transformers at a lower voltage.

In operation, the switching arrangement, when in the condition shown in the drawing, supplies the normal 240 volt supply from the autotransformer AT to the individual lamps.

When it is required to reduce the intensity of illumination of the lamps the switch SW is operated. This energises relay RLB and hence operates contact sets RLB1 and RLB2. The former disconnects the 240 volts supply as the moving contact moves over to the other fixed contact which is, at this time, disconnected. An arc may be drawn across the contacts of RLB1 at this stage. The operation of contact RLB2 energises the other relay RLA, which causes both contact sets RLA1 and RLA2 to operate. Contact sets RLA1 provides a further disconnection of the 240 volt supply and causes the extinction of any arc which may have formed across contact set RLB1. When contact sets RLA1 and RLA2 have completed their movement, the 1 60 volt supply is connected to the lamps by way of contact sets RLA2 and RLB1.

When switch SW is opened the reverse procedure is followed, with relay RLB being de-energised and returning contact sets RLB1 and RLB2 to their original position. Contact set RLB2 causes the de-energisation of relay RLA, and contact sets RLA1 and RLA2 are subsequently restored to their original positions.

Despite requiring a second relay, the switching apparatus described above has solved a problem which has caused considerable concern in the past. In particular, the use of relays means that the necessary time delay between the operation of the first and second relays is inherent in the construction of the relays. Hence the arrangement is fail-safe.

It will be appreciated that variations may be applied to the switching arrangement showing in the drawing. For example, the connections to the fixed and moving contacts of contact sets RLA1 and RLA2 may be reversed. Similarly, relay coil; RLA and RLB may be in the energised condition when the contacts sets are in the positions shown in the drawing.

Furthermore, it is not necessary to use relays for the switches. Other switching devices may be used, such as semiconductor switches, with the necessary control means including a time delay circuit.

Whilst the above description refers to the switching apparatus of the invention as applied to traffic control signals, it will be appreciated that it may be applied to light sources used for any other purpose where changes in the intensity of illumination are required.

Claims (5)

1. Apparatus for varying the intensity of illumination of a light source by selecting either of two different supply voltages for connection to the light source, which includes first switch means comprising two switches connected one to each supply voltage, and second switch means arranged to connect one or other of the switches of the first switch means to the light source, the first switch means being arranged such that only one of the two switches may connect its supply voltage to the second switch means at any instant,and control means for changing the condition of the second switch means before that of the first switch means.

2. Apparatus as claimed in Claim 1 in which. the first switch means and the second switch means each comprise a separate electromagnetic relay having a relay coil and a number of contact sets.

3. Apparatus as claimed in Claim 2 in which the relay coil of the first switch means is energised and de-energised by a contact set of the relay forming the second switch means.

4. Apparatus as claimed in any one of Claims 1 to 3 in which the light source forms part of a traffic control system.

5. Apparatus for varying the intensity of illumination of a light source substantially as herein described with reference to the accompanying drawing.