I happened to be reading the "How Stuff Works" article on Xmas lights
and was intrigued by the description of the self shunting feature of
today's series string bulbs as described on this page:
I'm assuming that the little shunt pictured may be just several wraps of
a fine wire with a coating on it which somehow changes state or "punches
through" when subjected to full line voltage.
Can you tell us what that material is and what actually occurs?
Sorry, I don't really know.
One possibility: Put a series zener diode pair in the socket. When
full line voltage appears, the zener diode pair conducts. Possibly
possibly the zeners overheat and fail short, or maybe just keep on ticking
and drop only a little more voltage than the bulb does.
But I don't really know what's in there.
I know there was something used back in the 1970's or early 1980's in
some strings and I doubt it had zener diodes. They apparently used
something that breaks down and/or heats up and then shorts but I don't
know what it was.
- Don Klipstein ( email@example.com)
| I know there was something used back in the 1970's or early 1980's in
| some strings and I doubt it had zener diodes. They apparently used
| something that breaks down and/or heats up and then shorts but I don't
| know what it was.
I remember one early version that used spring-loaded supports that were
held together by the filament. When the filament burned through the
supports moved away from each other and made contact with a surrounding
shorting band. Or at least that was the theory. I don't think I've ever
had any of these systems (including whatever they were selling as recently
as three years ago) work in practice.
The problem with a "series zener diode pair" is that it would prevent
the shunt in the light bulb from getting the full line voltage when a
filament actually does open. The shunt actually makes connection when
the filament opens, the voltage across one lamp goes to full line
voltage and the shunt "arcs" through. A zener would hold this voltage
to say, something just above a single lamp voltage, maybe 6 volts or so.
I haven't actually looked at one of these strings, so I really don't
know the actual mechanism either.
Don Klipstein wrote:
Series shunts are not necessarily an ideal solution. Even though they
keep the string of lamps burning, if too many bulbs burn out and
shunt, the voltage drop across the remaining bulbs is higher and
stressful. Eventually a cascade failure mode will occur.
I once lived in a community that had series streetlights (from 1923)
and got to talking with the electrician that maintained them. Across
each lamp was a removable shunt about the size of a dime. The disk
had two conductive metal faces with a thin insulator sandwiched
in-between. Nominal lamp voltage was 120 V. When a lamp burned
out, the full series voltage of 2300 V. was applied across the lamp
(and the shunt). This punctured the shunt and kept the current
flowing through the remaining series lamps.
The other part of the system was a constant current transformer that
kept the series current at a uniform level no matter how many lamps
were burned out. It was 100% mechanical adjustment, but completely
The Christmas tree lamp is somewhat similar.
The bulbs have a pair of oxidized aluminum wires wrapped around the wires
and twisted together. If the bulb burns out the full line voltage burns
through the oxide coating resulting in a short.
I tried the same question on:
And got a pretty good thread started there.
I never learned how to reference a thread on a newsgroup, so the best I
can do is say that my OP on that subjec in that newsgroup was dated
12/07/04 - 10:51 AM. (Would someone please tell me how to refer to a
newsgroup post in a message? - I use Netscape 7.2 for my news reader.)
The concensus was that the shunt is aluminum wire insulated by a very
thin layer of aluminum oxide which can withstand a few volts, but breaks
down at full line voltage and "punches through".
A "forcer" accessory device was also mentioned. It puts a high voltage
limited current whap across the string of bulbs to force that breakdown
in the bulb with an open filament in case it stubbornly didn't happen as
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