This invention relates to a lamplight circuit system in which an alternating current (AC) power source is connected to a plurality of lamps, and more particularly to a lamplight failure detection system used for the lamplight circuit system.
Serial lamplight circuits are often used for the many lamps used as landing lights for airport runways. In such serial lamplight circuits, there are detectors which detect when any of these lamps experiences a lamplight failure.
FIG. 1 is a circuit diagram showing a conventional lamplight failure detector.
In FIG. 1, a constant current power source unit 12 supplies electricity to a serial lamplight circuits unit 66 on the basis of a power supply from an alternating current (AC) power source 11. Serial lamplight circuits unit 66 has insulated current transformers CT.sub.1, CT.sub.2 . . . CT.sub.n connected in series on the primary side circuit and lamps L.sub.1, L.sub.2 . . . L.sub.n connected to the secondary side circuit of, respectively, insulated transformers CT.sub.1, CT.sub.2 . . . CT.sub.n. The brightness of lamps L.sub.1, L.sub.2 . . . L.sub.n is maintained at a constant level by the current output by constant-current power source unit 12 and supplied to them via insulated transformers CT.sub.1, CT.sub.2 . . . CT.sub.n.
Lamplight failure detector 65 detects lamplight failure in L.sub.1, L.sub.2 . . . L.sub.n from changes in signals input via a current transformer 13 and a potential transformer 14.
The process by which lamplight failure detector 65 detects lamplight failure in a lamp is as follows. If any of lamps L.sub.1, L.sub.2 . . . L.sub.n experiences a lamplight failure, the secondary side circuit of the insulated current transformer connected to this lamp becomes open. When the secondary side circuit of the insulated current transformer becomes open, there is change in the load impedance as seen from the constant current power source 12 which supplies current to the lamplight circuit whose lamplight has failed. The output voltage wave form and output current wave-form of the constant current power source unit 12 produced due to the changes in the load impedance are as shown in FIG. 2. The theory of detecting lamplight failures of lamps by this method is described, for example, in JP 61-15556 (B). When the secondary side circuit of the insulated current transformer becomes open due to lamplight failure, there is a consequent magnetic saturation phenomenon and the rise of output current of constant current power source unit 12 is shallow until the insulated current transformer becomes magnetically saturated. The wave form of the output current thus shows a rise which is later than when there is no lamplight failure in the lamp. On the other hand, in the case of the output voltage of the constant current power source unit 12, the rise of the output voltage is steep during the delay in the start of the output current (the saturation time .alpha., .alpha. is the phase control angle). The time integral values m.sub.1, m.sub.2 . . . m.sub.n, equivalent to the parts shown as hatched in FIG. 3, are proportional to the number of lamps with failed lamplights. Thus, if the time integral value when one lamplight has failed is expressed as m.sub.1, if the time integral value found from lamplight failure detector is m.sub.3, then the number of lamps with failed lamplight is 3.
However, although the conventional lamplight failure detectors are capable of detecting the number of lamplights which have failed they are not capable of detecting which of the series of lamps L.sub.1, L.sub.2 . . . L.sub.n has failed.
Because of this, when the lamplight failure detector 65 detects that there has been a lamplight failure in one of L.sub.1, L.sub.2 . . . L.sub.n, an inspector must conduct checks on the lamplights on the runway until the failed lamplight is found. The efficiency of the maintenance and inspection works is thus poor.
If the replacement of the lamp in which the lamplight has failed is delayed, the insulated current transformer connected to the failed lamp is left for a long time with the secondary side circuit in an open state. Consequently, there is a possibility of short circuits in the coil and heat damage caused to the coil by raised temperatures.