Re: Christmas Light Puzzler - HELP

"James Sweet" <jamessweet@xxxxxxxxxxx> hath wroth:

A better question would be why are your bulbs burning out? It might
be that the safety features is actually doing its job and preventing a
meltdown. 2500 bulbs belches quite a bit of heat. Before you disarm
the safety features, do some calculating:

Each bulb burns about 1/2 watt. 2500 lights burn about 1250 watts,
most of which goes up in heat, not light. That's quite a bit of heat
that has to go somewhere. My guess(tm) is that your tree is
overloaded with lamps and they are blowing because they're getting too
hot.

No way, incandescent lamps will work fine in environments hot enough to burn
the insulation off the wires. Unless the tree is on fire, heat will
absolutely not cause these lamps to fail.

True. Heat does not cause the lamp to fail, but does aggrivate the
problem. Although the author does not specifically mention the
obvious, the hotter the bulb, the faster the evaporation, and
therefore, the faster the failure.

<http://members.misty.com/don/bulb1.html>
How light bulbs burn out.

Due to the high temperature that a tungsten filament is operated
at, some of the tungsten evaporates during use. Furthermore,
since no light bulb is perfect, the filament does not evaporate
evenly. Some spots will suffer greater evaporation and become
thinner than the rest of the filament.

These thin spots cause problems. Their electrical resistance is
greater than that of average parts of the filament. Since the
current is equal in all parts of the filament, more heat is
generated where the filament is thinner. The thin parts also have
less surface area to radiate heat away with. This "double whammy"
causes the thin spots to have a higher temperature. Now that the
thin spots are hotter, they evaporate more quickly.

It becomes apparent that as soon as a part of the filament becomes
significantly thinner than the rest of it, this situation compounds
itself at increasing speed until a thin part of the filament either
melts or becomes weak and breaks.

This is not rocket science, bulbs
burn out, connections in sockets get bad, series wired lights are notorious
for problems like this. Additionally, as each lamp fails and shunts, the
voltage across the remaining lamps increases and if left unchecked they'll
start to burn out too. More than once I've had this cascade in smaller
strings to the point that every lamp burned out until the fuse in the plug
opened.

I haven't. There are usually 50 bulbs in series. When the Noma
"Stay-Lit" bulbs blow up, a roughly 5 ohm shunt ends up across the
bulb. With 50 bulbs, it would take quite a few bulbs to blow, before
the current would creep up to the point where the others might blow.
<http://christmas.howstuffworks.com/christmas-lights2.htm>
<http://www.planetchristmas.com/Minis.htm>
However, if your lamp string use some other method of maintaining
operation, such as shorting instead of introducing a 5 ohm shunt, then
it might blow the other bulbs.


--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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