Simple scheme saves string-light situation

-- EDN, 12/23/1999

As you enter the holiday season with strings of ac-line-powered lights hanging everywhere, do you ever wonder how bulb vendors overcame the frustrating problem of having an entire string go dark when just one of the series-wired bulbs failed? The solution does not involve software, microcontrollers, or complex hardware-just a clever application of materials science and understanding of the situation.

Here's how vendors eliminated the extreme frustration of an entire string's going dark, leaving you to frantically search for that one bulb that's the culprit. As in a conventional bulb, the filament of each bulb attaches to the two support wires that carry the current through the bulb. Just below the active filament is a specially coated, conductive shunt wire, which also attaches to the supports. Normally, the resistance of the shunt is much higher than the resistance of the filament, so most of the current flows through the filament, and the bulb glows. When the filament fails and opens, more current goes through the shunt, which heats it. This heating melts the shunt's high-resistance coating, thus transforming the shunt into a low-resistance path. Current now can flow through this bulb without undue IR drop, and the rest of the bulbs in the string get the current they need. As an added feature, you can quickly troubleshoot the string by seeing which bulb is dark because of the apparent contradiction of a dark bulb that still conducts current.

Note that some special-purpose industrial bulbs use the same technique but have a coated backup filament in place of the nonglowing shunt; this spare filament automatically comes into operation when the primary filament fails.

-by Bill Schweber