On Dec 8, 4:15*pm, Jeff Wisnia wrote:
Just what is in the "shunt" built into series string decorative lamp
bulbs which takes over when the filament opens?

Is it something like a PTC thermistor which heats up and melts some
powdered metal into a permanent short?

My curious mind wants to know.

Thanks,

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.
The shunt is a loop of wire around the base of the bulb filament
supports with an insulation so thin that it will break down if the
voltage across the bulb goes above 10 - 20 volts. When the filament
opens as the bulb burns out, the full 120 V is across the open bulb
causing the insulation to fail and restoring continuity. Of course,
all the bulbs in the string now have a slightly higher voltage, and
they tend to burn out a little quicker. After a few more failures,
the voltage across the remaining bulbs is up by 10 -20%, and then it
is a runaway situation. That is why it is important to replce burned
out bulbs promptly.

Bob Hofmann