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Jeff Wisnia

hr(bob) wrote:

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

Thanks, Bob...

I wouldn't have guessed anything as simple as that, but I suppose in
that application it doesn't need to be 100% reliable, so if an
occasional shunt failed closed prematurely it'd be no big whoop.

I guess the reason for TWO fuses in the plug must be to protect against
the wire loop getting shorted to ground somewhere along it's
length...two are needed because the plug isn't polarized. Cause, one
fuse would be enough to prevent a wire burning up in those runaway
situations you described.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.