GB2174852A

GB2174852A - Airfield lighting installations

Info

Publication number: GB2174852A
Application number: GB08610767A
Authority: GB
Inventors: Patrick H Tann
Original assignee: TANN ELECTRONICS Ltd
Current assignee: TANN ELECTRONICS Ltd
Priority date: 1985-05-02
Filing date: 1986-05-02
Publication date: 1986-11-12
Also published as: GB2174852B; GB8610767D0

Abstract

A switch circuit comprises a unit 40 connected between terminals of a lamp 52 of an airfield lighting installation and the terminals 44 of a secondary winding of a power transformer having a primary winding in a current loop. By providing the circuit as a separate unit, the unit can easily be replaced in a fault condition. The unit comprises a control transformer 60 connected to the power transformer, a rectifying arrangement 62 for deriving a dc power signal, and a control means 66, 72 responsive to input control signals from an airfield control centre for controlling a triac device 64 connected in parallel with the lamp 52. In another arrangement, the voltage developed across a current transformer (80), (Figure 4), in the lamp circuit is sensed, and in response to a query signal received from the airfield control centre, transmits an answer signal indicating lamp condition. <IMAGE>

Description

SPECIFICATION Airfield lighting installations The invention relates to airfield lighting installations and the selective switching of specific lamps in a lighting installation.

The usual arrangement in an airfield lighting installation is to have a large number of lights connected in a large current loop driven from a source of for example 1300 volts. Each lamp, which may require 6.6 amps, is driven by a transformer, and the primary winding of each transformer is connected in series in the current loop. In the particular case of aircraft taxi lights which are coloured green and mounted in the centre of the taxi way it is necessary frequently to selectively switch these lights so as to direct the aircraft on a particular path determined by the control centre. To this end each transformer has a switch mechanism for selectively switching the transformer in or out and these switch mechanisms are coupled to a central computer device at a control station.

A known mechanism for selective switching of the transformers is shown in figure 1 of the accompanying drawings wherein it may be seen the primary winding 12 of a transformer 10 is connected in a current loop Land the secondary winding 14 has a tapping 16 so that a portion 18 of the secondary winding provides power to an airfield lamp 20. A further portion of the secondary winding 20 provides powerto a switching installation 22 including a triac unit 24 connected between one end of the winding and earth. The gate of the triac is controlled by relay contacts Al of a relay 24A, the relay 24A being a reed relay and being controlled by control lines 26 from the central airfield control station.Thus when it is required to switch off lamp 20 a control signal is sent via lines 26 to energise relay 24A thereby closing contacts Al and switching on he triac unit 24 thereby to short circuit the secondary winding 14 to earth.

The disadvantages of this arrangement is that the switching arrangement is housed with the transformer in the same housing and potted therein with a suitable epoxy resin for effective protection against moisture. The reason for housing the switch unit together with the transformer is that the triac is connected to the tapped winding of the secondary and to a suitable earth point in the transformer.However this arrangement brings with it two disadvantages; firstly that the isolation of the control current to the relay 24A from external influences is not perfect and there may therefore be accidenta I switching of the lamp 20. In addition if the switch mechanism fails, for example the reed relay contacts are destroyed or become stuck then the whole unit including the transformer is not usable and must be replaced.

It is an object of the present invention to provide a switch mechanism for the selective switching of a lamp in an airfield installation which is of such a design that it is not necessary to mount the switch mechanism with the lamp transformer in a single housing.

The present invention consists of a switch mechanism comprising a unit which is adapted to be connected between the terminals of a lamp of an airfield lighting installation and the terminals of a secondary winding of a power transformer for driving the lamp wherein the switching unit comprises a switching device to be connected for controlling the switching of the lamp, means for deriving from the secondary winding of the power transformer a dc power signal for powering the switching mechanism, and a control means for controlling operation of the switching device and being adapted to receive input control signals from an airfield control centre for controlling the switching unit.

Preferably, the dc power signal deriving means comprises a control transformer whose primary winding is connected to the secondary winding of the power transformer, and a rectifying arrangement connected to the secondary winding of the control transformer.

Thus in accordance with the present invention by providing a dc power signal deriving means, preferably further control transformer it is possible to provide the switching mechanism for the power transformer as a separate unit not integral with the power transformer. Thus if the switching unit becomes faulty it may simply be replaced without replacing the power transformer. The switching unit may be potted in a suitable compound to ensure it is moisture proof.

As preferred, the switching unit comprises a triac connected across the lamp and a transistor for driving the gate of the triac. The transistor is preferably driven by a light sensitive transistor which is coupled to the control signal from the airfield control station by a light emitting diode, thereby providing perfect isolation between the control signal and extraneous factors such as interference from the transformer unit. Further since the airfield control station need only provide power to illuminate the light emitting diode, the switching unit can operate at very low current levels, typically 5.5 mA at 50 volts dc. This compares with 18mA in the case of triac controlled transformers as described in Figure 1.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of a known switching mechanism for the transformer of a lamp on an airfieid lighting installation; Figure 2 is a circuit diagram of the switching unit according to the invention; and Figure 3 is a schematic view of the switching unit according to the invention connected between a transformer and a lamp for an aircraft lighting installation.

Referring to Figures 2 and 3 there is shown a switching unit 40 which is encapsulated in resin in a 160 mm long by 60 mm diameter cylinder with four twin cables protruding from one end of the cylinder.

One cable 42 is connected to the secondarytermin- als 44 of a power transformer 46 whose primary winding has terminals 48 which are connected in a current loop which is powered from the central station. Afurther cable 50 is connected to a lamp 52 forming part of an aircraft lighting installation and may be for example coloured green and mounted in a position to provide a taxi light indication for aircraft. Two cables 54, 56 are connected to the switch unit 40 and provide control signals from a central control station of the airport.

The switching unit 40 comprises a control transformer 60 whose primary winding is connected to the secondary transformer of power transformer 46. The secondary winding of transformer 60 is connected to a diode bridge arrangement followed by a zener diode and capacitor, all indicated by reference numeral 62 for providing a smooth dc power level for operating the switching circuit. The switching circuit comprises a triac 64 connected in parallel across lamp 52 the gate of which is connected in the collector circuit of a drive transistor 66 and a light responsive transistor 68. Transistors, 66, 68 are connected in a Darlington pair configuration and transistor 68 is optically coupled to a light emitting diode 70 so as to be responsive to control signals emitted therefrom. Light emitting diode 70 is connected across control cables 54, 56 which emanate from the central control station.These control cables provide a control signal which is smoothed by a zener diode-capacitance-resistance arrangement indicated generally at 72. Thus in operation of the circuit, power to drive the circuit is provided from the secondary winding of power transformer 46 via rectifying arrangement 62. A control signal is provided from controlled cables 54, 56 via smoothing arrangement 72 to light emitting diode 72 to light emitting diode 70. When it is required to switch off the lamp installation 52, the control signal energises diode 70 thereby providing light to the base of transistor 68 to switch on transistor 68, thereby switching on transistor 66 and providing a signal to the gate of triac 64 to switch on triac 64 thereby shorting the power supply to lamp installation 52.

It will therefore be seen there is provided a switching circuit which, being separate from the main transformer for the lamp installation, can simply be replaced if it is faulty. This physical separation is enabled bythe provision of a separate transformer connected in the secondary transformer circuit of the main transformer for providing power to the switching circuit. In addition the provision of an optically coupled diode transistor arrangement increases the security of the switching arrangement and enables far less power to be drawn from the control lines for controlling the switching operation than heretofore.

In a modification of the invention the switching circuit may be modified so as to provide an indication of the condition of the lamp. In view of the large number of lamps in an airfield lighting installation, it is desirable to have some automatic means of detecting whether a lamp is operative or has failed.

The present invention can provide such an indication by monitoring the current through the lamp when the lamp is switched on. If there is substantial current flow detected, then the lamp will be operative, but if there is no current flow and yet the central station of the airfield has issued an instruction that the lamp is to be switched on, then the lamp has failed.

The present invention therefore provides in a further aspect ofthe invention an indicating mechanism for indicating the condition of the lamp of an airfield lighting installation, the mechanism comprising a unit adapted to be connected between the terminals of the lamp and the terminals of a power transformer for driving the lamp, the unit including detecting means connected to the secondary wind- ing of the powertransformerfordetecting current or voltage across the lamp, the unit including means for signalling the condition of the lamp to a central station based upon a signal provided by the detection means.

Preferably the detecting means comprises receiver means for receiving an instruction from a central station querying the condition of the lamp, and transmitter means coupled to said detection means for transmitting to the central station an indication of the condition of the lamp based upon a signal provided by said detection means.

Preferably the detecting means includes a control transformer whose primary is connected to the secondary winding of the power transformer and whose secondary winding has connected thereto a means for detecting current or voltage in or across the secondary winding of the control transformer.

This modification of the invention is shown in more detail in Figure 4 which is a block diagram of this modified arrangement.

Referring to Figure 4 there is shown a control current transformer 80 whose primary winding 82 is connected in series with a lamp (not shown) and the secondary winding of a power transformer (not shown). The secondary winding 86 of the transformer 80 is connected to a detection means comprising a decision logic array 88. The described logic is arranged to receive instruction signals from a receiver and instruction decoder 90 and the decision logic is arranged to provide answer signals to an instruction encoder and transmitter 92. Both receiver 90 and transmitter 92 are connected to a communication link 94 which is connected to a central signalling station of the airfield. A power supply 96 is also provided which may be derived from the control currenttransfomer 80 or may be separately provided.

In operation, the central station (not shown) may issue an instruction across the communication link 94 to receiver 90. Receiver 90 receives the signal from the communication link and by means of a decoding arrangement of known type decodes the signal from the central station and issues to the decision logic 88 a signal querying the condition of the lamp. The voltage developed across the secondary winding of transformer 86 is monitored in the decision logic detection means 88 for example by means of a comparator amplifier having one input connected to a threshold voltage reference means and the other input connected to the secondary winding. If the voltage from the transformer is above the threshold then a signal is provided to the instruction encoder and transmitter 92 indicating that the lamp is operative. If however no voltage is developed across the secondary, indicating thatthe lamp is open circuit and therefore has failed, a zero signal is sent to the encoder thereby indicating that the lamp has failed. In either event the instruction encoder provides a signal with requisite information and this is transmitted along the communication link to the central station.

It will be understood the arrangement shown in Figure 4 may be incorporated in the arrangement shown in Figure 2.

Claims

1. A switch mechanism comprising a unit which is adapted to be connected between the terminals of a lamp of an airfield lighting installation and the terminals of a secondary winding of a powertransformer for driving the lamp wherein the switching unit comprises a switching device to be connected for controlling the switching of the lamp, means for deriving from the secondary winding of the power transformer a dc power signal for powering the switching mechanism, and a control means for controlling operation of the switching device and being adapted to receive input control signals from an airfield control centre for controlling the switching unit.

2. A switch mechanism as claimed in claim 1, wherein the switching device comprises a silicon controlled rectifier.

3. A switch mechanism as claimed in claim 1 or 2, wherein the control means includes a light radiating element connected to input terminals for receiving said input control signals, and a light responsive device arranged to receive light from the light radiating element to provide an electrical signal in response thereto.

4. A switch mechanism as claimed in claim 3 wherein the light responsive device is connected to a transistor amplifier for providing a switching signal to the switching device.

5. A switch mechanism as claimed in claim 1 wherein the switching device has a main current path to be connected in parallel with the lamp.

6. An indicating mechanism for indicating the condition of the lamp of an airfield lighting installation, the mechanism comprising a unit adapted to be connected between the terminals of the lamp and the terminals of a power transformer for driving the lamp, the unit including detecting means connected to the secondary winding of the power transformer for detecting current or voltage across the lamp, the unit including means for signalling the condition of the lamp to a central station based upon a signal provided by the detection means.

7. A mechanism as claimed in claim 6 wherein the signalling means includes receiver means for receiving an instruction from a central station querying the condition of the lamp, and transmitter means for transmitting to the central station an indication of the condition of the lamp based upon a signal provided by the detection means.

8. A mechanism as claimed in claim 6 or 7 wherein the detection means comprises a decision logic array responsive to the voltage developed across the secondary winding of the control transformer.

9. A mechanism as claimed in claim 8 as dependent on claim 7 wherein the receiver means is arranged to provide a query signal to the decision logic array, and the decision logic array is arranged to provide an answer signal to the transmitter means.

10. An indicating mechanism as claimed in claim 6 wherein the detecting means comprises a transformer whose primary winding is connected to the secondary winding of the power transformer and a decision means connected to the secondary winding of the control transformer.

11. An indicating mechanism substantially as described with reference to Figure 4 of the accompanying drawings.

12. A switching mechanism as claimed in claim 1 wherein the dc power signal deriving means comprises a control transformer whose primary winding is connected to the secondary winding of the power transformer, and a rectifying arrangement connected to the secondary winding of the control transformer.

13. A switching mechanism substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

14. An indicating mechanism as claimed in any of claims 6 to 11 incorporated in a switching mechanism as claimed in any of claims 1 to 5 or 12 or 13.