Wond＇rous Light Chain

Published:2011/8/17 3:03:00 Author:Amy From:SeekIC

With the Festive Season upon us, it seems fit­ting to devote this month’s Quizz’away instal­ment to the perennial problems associated with Christmas tree lighting. Conventionally, lamps are connected in series (Figure 1) and this immediately creates a problem: when one lamp fails, it breaks the series network and all other lamps will also go out. But that’s not all, because in the dark you will have hard time finding the defective lamp in the first place. A solution to this problem could be to simply wire up the lamps as shown in Figure 2.



Let’s assume each lamp is powered by way of a cable with a length L of 4.5 m. Twisted-pair wires from Ethernet cables are used because of their attractive colors. Close to the power supply, the cable ends are connected in series again. A photograph of the practical version is shown in Figure 3. Normal lamps rated at 12 V, 0.1 A are used.



So far so good; let’s connect the lot to a suit­able 36 VAC power supply. This time, much to everyone’s surprise, failure or removal of one lamp does not cause overall darkness — all other lamps in the Wond’rous Light Chain remain on and even light a little brighter! In this manner, the total brightness remains almost constant and the Christmas festivities are not spoilt, plus you will be able to easily spot the faulty lamp.

Can you explain what is happening?

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