What's in self shunting Xmas light bulbs?

[Jeff Wisnia]

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:

http://home.howstuffworks.com/christmas-lights2.htm

I'm assuming that the little shunt pictured might 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 someone tell us what that material is and what actually occurs?

Thanks guys,

Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"

 

[Jim Thompson]

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:

http://home.howstuffworks.com/christmas-lights2.htm

I'm assuming that the little shunt pictured might 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 someone tell us what that material is and what actually occurs?

Thanks guys,

Jeff

I understand there's also a "tester" which forces shunts to close when
they didn't when the bulb failed. Anyone know about where to obtain
and how well it works? (I have a 12' pre-wired tree with one section
out.)

...Jim Thompson

 

[Guy Macon]

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:

http://home.howstuffworks.com/christmas-lights2.htm

I'm assuming that the little shunt pictured might 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.

I find it interesting that you are assuming voltage punch-through
when the howstuffworks.com web page claims heat punch-through,
especially considering that you are right and howstuffworks.com
is wrong!

"If you look closely at a bulb, you can see the shunt wire
wrapped around the two posts inside the bulb. The shunt wire
contains a coating that gives it fairly high resistance until
the filament fails. At that point, heat caused by current
flowing through the shunt burns off the coating and reduces
the shunt's resistance. (A typical bulb has a resistance of
7 to 8 ohms through the filament and 2 to 3 ohms through
the shunt once the coating burns off.)"
-howstuffworks.com


This is, of course, easy to test; break open a couple of bulbs
and measure the resistance with the filament broken, with the
shunt wire removed, and with both intact. Then measure the
resistance of a burnt-out bulb. Unless the design has changed
since the last time I designed a christmas bulb making machine,
you will find that the shunt starts with infinite resistance
and becomes a low resistance when it is doing its shunting job.

I don't have any mini christmas bulbs, so I would appreciate
it if someone would do the experiment I just described and
post the resistance readings. (If you want to do advanced
research, measure the voltage and current in circuit while
the string is operating; the resistance of the filament gets
lower when it is hot.)

Can someone tell us what that material is and what actually occurs?

The shunt is nothing more than aluminum wire. The christmas
bulb making machine heats it (around 400 or 500 degrees Celsius
if I remember correctly) in a steam chamber. This creates a
thin layer of aluminum oxide, which is a good insulator but
has a fairly low breakdown voltage if it is very thin. Some
christmas bulb making machines use aluminum foil instead of
wire, but I wouldn't sign off such a design - too hard to
control the tension of the shunt across the filament supports.

When the lamp fails, the shunt sees the full line voltage, which
punches through the oxide and welds the shunt to the filament
support wires. Older series streetlights have the same basic
setup, except the shunt is in the base and consists of metal
plates with thin silk between them. An open streetlight lamp
causes thousands of volts across the silk dielectric, which
punches through and welds the plates together. Look at
http://www.vintagestreetlights.com/history/series.html and
search for the word "cutout" for details.

 

[Spehro Pefhany]

It's an anodized aluminum foil wire wrapped around the filament
support pins. The very thin (something like 1 micron) oxide coating
typically breaks down (along with any oxide on the Dumet wires) when
subjected to full AC line voltage, but does not break down at the few
volts normally across the bulb (if it does, the bulb goes out just as
if the filament failed). It's similar to a low-voltage electrolytic
capacitor sans electrolyte. Heat is not involved except at the tiny
spots where the insulation breaks down and a little weld forms.

I understand there's also a "tester" which forces shunts to close when
they didn't when the bulb failed. Anyone know about where to obtain
and how well it works? (I have a 12' pre-wired tree with one section
out.)

...Jim Thompson

A small capacitor charged to a high voltage (and connected only to the
string with the bad bulb shunt) should work. Maybe 0.1J @500V or 1kV?
Removing single bulbs in other paralleled strings should isolate the
bad string. If the bulb has actually fallen out of its holder, then
none of this will work, of course.


Best regards,
Spehro Pefhany

 

[Guy Macon]

I understand there's also a "tester" which forces shunts to close when
they didn't when the bulb failed. Anyone know about where to obtain
and how well it works? (I have a 12' pre-wired tree with one section
out.)

High voltage across the entire string, current limited so that when
the shunt closes it doesn't supply enough current to burn out the
bulbs.

 

[Guy Macon]

I don't have any mini christmas bulbs, so I would appreciate
it if someone would do the experiment I just described

I did some searching and found that someone already has:


From Jon Titus, Editor in Chief, Test & Measurement World:

"I ran a few experiments on series-string lights ... The
shunt wire may "spot weld" itself to form a shunt across a
bad bulb. Here's what I did:

"I carefully broke the glass on three replacement bulbs so as
to maintain the integrity of the filament. Prior to breaking
the glass, I checked each bulb to ensure it worked properly
in the string.

"1. First bulb inserted in the string with intact but exposed
filament. The string works fine. I expected the filament to
go "poof," and then I would observe the results. A closer
look showed that the filament had heated, but it was still
intact. Small traces of white (oxidation?), but mainly blue
discoloration in the middle section of the filament.

"2. Second bulb inserted with intact but exposed filament.
The string works fine again. The filament looks intact, but
discolored as above. Used a toothpick to break the filament
in the hot circuit while I observed any action on the shunt
wire. As I broke the filament, I saw small sparks at the
small shunt wire. Increased current through the shunt may
have caused more "welding" across resistive connections.

"3. Third bulb inserted with a purposely severed filament. As
soon as I inserted this bulb in the string I observed the
small sparks at the shunt wire. The bulb completed the
circuit, and the bulbs on the string all lit.

"So, I'd say the small wire does act like a shunt, getting
welded due to the initial high potential. That's neat. It
must take some interesting QC to ensure a uniform coating on
the wire. Or perhaps the surface is just oxidized slightly.

"An interesting effect was noticed in bulb #1. Even though
exposed to air, the filament didn't heat up and break.
Perhaps the filament started to overheat and oxidize, thus
raising its resistance. As the resistance increases, the
potential across the bulb increased enough to cause the
shunt to weld itself in place. Thus the filament fails but
does not necessarily break. In years past, I have looked at
several series of dead lights with a magnifying glass to try
to find a broken filament. In most cases, I could not find
such a bulb. This effect may be the reason. As the bulb
starts to fail, the shunt kicks in so quickly that the
filament never has to break. Some clever engineering in a $3
string of cheap lights."

 

[Arie de Muynck]

From Jon Titus, Editor in Chief, Test & Measurement World:
"An interesting effect was noticed in bulb #1. Even though
exposed to air, the filament didn't heat up and break.

My guess:
a) almost constant current through the rest of the filaments
b) this open air filament is cooled, stays low-ohmic and low-voltage.

Ever tried to start a halogen bulb on a constant voltage / constant current
source where the current limit is just above the normal operating current?

I did.
It doesn't.

Regards,
Arie de Muynck.

 

[Carl D. Smith]

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:

There can be one impressive side effect of these self shunting
bulbs. If you don't replace the bad bulbs the whole string can
go out in a successive failure as the voltage increases on the
remaining bulbs.

This was demonstrated once while I was at my parents place for
Christmas a few years back. I had noticed one string of lights
on the tree was a bit brighter than the others, and that it had
several bulbs out.

Later, another bulb or two went out right when I was looking at
the tree. Then a few seconds later several more went, and the
remaining bulbs got much brighter. But that didn't last long.
Just as I was thinking it was time to go unplug the tree, the
remaining bulbs went off like old fashioned camera flash bulbs in
a series of flashes.

Now I kind of want to go get a string of lights, and short out a
few bulbs and see if I can make it happen again.

 

[Arie de Muynck]

"Arie de Muynck" ...

Ever tried to start a halogen bulb on a constant voltage / constant current
source where the current limit is just above the normal operating current?

I did.
It doesn't.

That was of course one with a current foldback, not constant current or it
would have worked. I did change the supply to constant current.
Typically R_cold = 0.1 R_hot

Arie.

 

[John Woodgate]

I read in sci.electronics.design that Guy Macon <_see.web.page_@_www.guy

If you want to do advanced research, measure the voltage and
current in circuit while the string is operating; the resistance of the
filament gets lower when it is hot.

That'll be the day.

 

[Nicholas O. Lindan]

Later, another bulb or two went out right when I was looking at
the tree. Then a few seconds later several more went, and the
remaining bulbs got much brighter. But that didn't last long.
Just as I was thinking it was time to go unplug the tree, the
remaining bulbs went off like old fashioned camera flash bulbs in
a series of flashes.

Great product design: If the customer is too cheap to buy a box of
replacement bulbs he gets to buy a whole new string ...

 

[Guy Macon]

There can be one impressive side effect of these self shunting
bulbs. If you don't replace the bad bulbs the whole string can
go out in a successive failure as the voltage increases on the
remaining bulbs.

This was demonstrated once while I was at my parents place for
Christmas a few years back. I had noticed one string of lights
on the tree was a bit brighter than the others, and that it had
several bulbs out.

Later, another bulb or two went out right when I was looking at
the tree. Then a few seconds later several more went, and the
remaining bulbs got much brighter. But that didn't last long.
Just as I was thinking it was time to go unplug the tree, the
remaining bulbs went off like old fashioned camera flash bulbs in
a series of flashes.

Now I kind of want to go get a string of lights, and short out a
few bulbs and see if I can make it happen again.

A Variac should do the trick.

 

[Guy Macon]

That'll be the day.

Note to self: next time, smoke the crack *after* posting...

Seriously, I had trouble sleeping last night so I got up and
read a few newsgroups, and I manages to get several things
exactly backwards. D'oh! The resistance of the filament
gets higher when it is hot. The current through the filament
gets lower when it is hot.

I blame Bush for any errors I may have made.

 

[Genome]

Guy Macon Says.


"Unless the design has changed since the last time I designed a christmas
bulb making machine."

"The shunt is nothing more than aluminum wire. The christmas bulb making
machine heats it (around 400 or 500 degrees Celsius if I remember correctly)
in a steam chamber. This creates a thin layer of aluminum oxide, which is a
good insulator but has a fairly low breakdown voltage if it is very thin.
Some christmas bulb making machines use aluminum foil instead of wire."

"BUT I WOULDN'T SIGN OFF ON SUCH A DESIGN."

"Too hard to control the tension of the shunt across the filament supports."

Then Carl says

Later, another bulb or two went out right when I was looking at
the tree. Then a few seconds later several more went, and the
remaining bulbs got much brighter. But that didn't last long.
Just as I was thinking it was time to go unplug the tree, the
remaining bulbs went off like old fashioned camera flash bulbs in
a series of flashes.

[email protected]

This idiot claims management experience of product development.

I suppose someone else has to do the FMEA.

DNA

He would request an FMEA

 

[Richard H.]

I understand there's also a "tester" which forces
shunts to close when they didn't when the bulb
failed. Anyone know about where to obtain and how
well it works?

Sounds clever. I haven't seen that, but this one works 100% of the
time. Home Depot sells them.

http://www.amazon.com/exec/obidos/tg/detail/-/B00004WLKP/

It's dead simple - wand it over the dark bulbs to find the last one with
live AC. That's the dead one.

Pre-wired trees are a little harder to trace the string, but it still
works great.

Cheers,
Richard

 

[Richard H.]

Great product design: If the customer is too cheap
to buy a box of replacement bulbs he gets to buy a
whole new string ...

Except... a whole new string is cheaper than replacement bulbs!

 

[Nicholas O. Lindan]

Except... a whole new string is cheaper than replacement bulbs!

You know that, I know that, but "No one has ever gone broke
by underestimating man's intelligence".

 

[Jeff Wisnia]

You know that, I know that, but "No one has ever gone broke
by underestimating man's intelligence".

Or by underestimating american taste.

See this page I couldn't resist putting up:

http://home.comcast.net/~jwisnia18/jeff/mickeys.html

Happy Holidays and thanks for answering my question guys,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"

 

[Nicholas O. Lindan]

Or by underestimating American taste.

America has no special lock on bad taste.

http://home.comcast.net/~jwisnia18/jeff/mickeys.html

Would this be from EuroDisney?

 

[Jeff Wisnia]

America has no special lock on bad taste.




Would this be from EuroDisney?

I wouldn't know, but it looks to me like a mother-daughter pair.

But, I just noticed that the buildings behind them have *functional*
shutters, not like the ubiquitous fake shutters I see most everywhere
nowadays which are usually the wrong width to "fit" if they *could*
hinge closed, and are invariably mounted with the louvers pointing the
opposite of where they should.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"