LED Christmas light problem / how are they wired?

[[email protected]]

I've worked on a lot of the old incandescent christmas light sets and
they are pretty straight forward in how they work, even if the series
wiring can be a big PITA to find the bad bulb.

In 2009 I bought a christmas display (after christmas), which was a
costly display, but they had it marked down after Christmas. I used
it in 2010, for a couple weeks. This year I put it on the lawn last
week and it worked fine.

Today I noticed that half the display is not lighting. Of course this
is after dark so it's kind of hard to see what is going on with the
wiring. However, I noticed that there are just two wires to each
socket, thus telling me they are wired in series.

I have a good understanding of electronics, but this is the first
string of LEDs that has failed on me. All of a sudden I'm looking at
this light string and wondering how they work.

I know LEDs are run on DC and each LED requires a resistor. Well,
this set is plugged into an AC 120v outlet. The plug has fuses, but
since half the string works, I know the fuses are ok. But this is
where I get confused. If this string is DC, where the heck are the
diodes to convert the AC to DC. I doubt they are in the small plug
which barely has the room for those fuses. Plus, a regular AC to DC
power supply has filter capacitors. Maybe these LEDs dont need the
capacitor?????

Then, where are the resistors for each bulb? Or are they built right
into the LEDs on these christmas lights? On top of that, I never knew
they were series wired, so one bad bulb knocks out half or the whole
string. [that sucks].
(I proved that on a spare string I have, by pulling out one LED, half
the string went off).

Also on this display I notice there are molded sections about 3/8"
thick and 2 inches long in a few places in the string. What are
these? Are they fusible links (which cant be replaced), are they the
location of the diodes to convert the AC to DC, or simply connections
to connect the different segments of the string?
A schematic sure would be nice right about now!!!

This display is a bit to large to bring in the house and I did not
feel like being outdoors in the dark and cold trying to trace the
wiring. I had no idea that they were wired so when one bulb died a
whole segment of the string went out. Nor did I expect that a string
of LEDs would burn out after only 3 weeks to total use, knowing LEDs
are supposed to last for many thousands of hours. (Of course anything
can fail, and christmas lights are mass produced as cheaply as
possible).

I'm suspecting there is one bad LED, but I have no spares, so I'll
have to buy some tomorrow. I could take a bulb out of my spare
string, but that brings up a whole other question.

My display has white LEDs, whereas my spare string is multicolor. I
know that white LEDs operate at a higher voltage than red yellow, etc.
and blue and green are slightly higher in voltage. Thus none of the
multicolor bulbs would be suitable as replacements for the white LEDs
on my display.

Yet, this still brings up more of a puzzle. My multicolor spare
string has red, yellow, green, and blue LEDs. Since these different
colors require different voltages, how can they all be run on the same
string?

A string of LEDs seems so simple, or so I thought until I began to try
to fix this one.....

Now I wish I could find one single socket to make a LED bulb tester to
hook to the ohm scale on my multimeter. I sure hate to cut up a whole
working set to get one socket.....


------

Totally off the topic, but worth a laugh.
I saw a set of christmas lights in a store that said on the box
"FOR INDOOR OR OUTDOOR USE ONLY"
Ummmmmmm, why is the word "ONLY" used? Is there a third option?

 

[Chiron]

I've worked on a lot of the old incandescent christmas light sets and
they are pretty straight forward in how they work, even if the series
wiring can be a big PITA to find the bad bulb.

In 2009 I bought a christmas display (after christmas), which was a
costly display, but they had it marked down after Christmas. I used it
in 2010, for a couple weeks. This year I put it on the lawn last week
and it worked fine.

Today I noticed that half the display is not lighting. Of course this
is after dark so it's kind of hard to see what is going on with the
wiring. However, I noticed that there are just two wires to each
socket, thus telling me they are wired in series.

I have a good understanding of electronics, but this is the first string
of LEDs that has failed on me. All of a sudden I'm looking at this
light string and wondering how they work.

I know LEDs are run on DC and each LED requires a resistor. Well, this
set is plugged into an AC 120v outlet. The plug has fuses, but since
half the string works, I know the fuses are ok. But this is where I get
confused. If this string is DC, where the heck are the diodes to
convert the AC to DC. I doubt they are in the small plug which barely
has the room for those fuses. Plus, a regular AC to DC power supply has
filter capacitors. Maybe these LEDs dont need the capacitor?????

Then, where are the resistors for each bulb? Or are they built right
into the LEDs on these christmas lights? On top of that, I never knew
they were series wired, so one bad bulb knocks out half or the whole
string. [that sucks].
(I proved that on a spare string I have, by pulling out one LED, half
the string went off).

Also on this display I notice there are molded sections about 3/8" thick
and 2 inches long in a few places in the string. What are these? Are
they fusible links (which cant be replaced), are they the location of
the diodes to convert the AC to DC, or simply connections to connect the
different segments of the string? A schematic sure would be nice right
about now!!!

This display is a bit to large to bring in the house and I did not feel
like being outdoors in the dark and cold trying to trace the wiring. I
had no idea that they were wired so when one bulb died a whole segment
of the string went out. Nor did I expect that a string of LEDs would
burn out after only 3 weeks to total use, knowing LEDs are supposed to
last for many thousands of hours. (Of course anything can fail, and
christmas lights are mass produced as cheaply as possible).

I'm suspecting there is one bad LED, but I have no spares, so I'll have
to buy some tomorrow. I could take a bulb out of my spare string, but
that brings up a whole other question.

My display has white LEDs, whereas my spare string is multicolor. I
know that white LEDs operate at a higher voltage than red yellow, etc.
and blue and green are slightly higher in voltage. Thus none of the
multicolor bulbs would be suitable as replacements for the white LEDs on
my display.

Yet, this still brings up more of a puzzle. My multicolor spare string
has red, yellow, green, and blue LEDs. Since these different colors
require different voltages, how can they all be run on the same string?

A string of LEDs seems so simple, or so I thought until I began to try
to fix this one.....

Now I wish I could find one single socket to make a LED bulb tester to
hook to the ohm scale on my multimeter. I sure hate to cut up a whole
working set to get one socket.....


------

Totally off the topic, but worth a laugh. I saw a set of christmas
lights in a store that said on the box "FOR INDOOR OR OUTDOOR USE ONLY"
Ummmmmmm, why is the word "ONLY" used? Is there a third option?

I've noticed that warning, too. Sad to say, until you pointed it out, I
didn't even think about how puzzling the warning was.

Reminds me of the product that offered you a chance to win a prize,
details inside the bag, no purchase necessary. I guess you could just
rip the bag open in the store to get the information.

 

[Martin Brown]

I've worked on a lot of the old incandescent christmas light sets and
they are pretty straight forward in how they work, even if the series
wiring can be a big PITA to find the bad bulb.

In 2009 I bought a christmas display (after christmas), which was a
costly display, but they had it marked down after Christmas. I used
it in 2010, for a couple weeks. This year I put it on the lawn last
week and it worked fine.

Today I noticed that half the display is not lighting. Of course this
is after dark so it's kind of hard to see what is going on with the
wiring. However, I noticed that there are just two wires to each
socket, thus telling me they are wired in series.

I have a good understanding of electronics, but this is the first
string of LEDs that has failed on me. All of a sudden I'm looking at
this light string and wondering how they work.

I know LEDs are run on DC and each LED requires a resistor. Well,
this set is plugged into an AC 120v outlet. The plug has fuses, but
since half the string works, I know the fuses are ok. But this is
where I get confused. If this string is DC, where the heck are the
diodes to convert the AC to DC. I doubt they are in the small plug
which barely has the room for those fuses. Plus, a regular AC to DC
power supply has filter capacitors. Maybe these LEDs dont need the
capacitor?????

Do they visibly flicker or leave a pattern of dots on your retina if you
move your head round quickly? If not then there is a rectifier and
capacitor somewhere. Some cheap skates just put one string on each half
phase and they flicker like hell. The circuit typically used for
domestic LED lights was discussed in an adjacent thread only recently.

Then, where are the resistors for each bulb? Or are they built right
into the LEDs on these christmas lights? On top of that, I never knew
they were series wired, so one bad bulb knocks out half or the whole
string. [that sucks].
(I proved that on a spare string I have, by pulling out one LED, half
the string went off).

Also on this display I notice there are molded sections about 3/8"
thick and 2 inches long in a few places in the string. What are
these? Are they fusible links (which cant be replaced), are they the
location of the diodes to convert the AC to DC, or simply connections
to connect the different segments of the string?
A schematic sure would be nice right about now!!!

This display is a bit to large to bring in the house and I did not
feel like being outdoors in the dark and cold trying to trace the
wiring. I had no idea that they were wired so when one bulb died a
whole segment of the string went out. Nor did I expect that a string
of LEDs would burn out after only 3 weeks to total use, knowing LEDs
are supposed to last for many thousands of hours. (Of course anything
can fail, and christmas lights are mass produced as cheaply as
possible).

I'm suspecting there is one bad LED, but I have no spares, so I'll
have to buy some tomorrow. I could take a bulb out of my spare
string, but that brings up a whole other question.

It is much more likely to be a bad joint or a weak point breaking in the
cable that has failed in the wind and weather. Look for sharp bends or
even broken wires. LEDs are intrinsically very reliable provided that
you don't abuse them with over voltage or yanking their leads.

If they are series wired then remember that you are potentially dealing
with live mains and lethal voltages. Visual inspection of the entire
length of the failed string would be worthwhile.

My display has white LEDs, whereas my spare string is multicolor. I
know that white LEDs operate at a higher voltage than red yellow, etc.
and blue and green are slightly higher in voltage. Thus none of the
multicolor bulbs would be suitable as replacements for the white LEDs
on my display.

Yet, this still brings up more of a puzzle. My multicolor spare
string has red, yellow, green, and blue LEDs. Since these different
colors require different voltages, how can they all be run on the same
string?

A string of LEDs seems so simple, or so I thought until I began to try
to fix this one.....

Now I wish I could find one single socket to make a LED bulb tester to
hook to the ohm scale on my multimeter. I sure hate to cut up a whole
working set to get one socket.....

A 9v battery and a 10k resistor will do as an LED tester. One way round
the LED will light - no need for a multimeter.

------

Totally off the topic, but worth a laugh.
I saw a set of christmas lights in a store that said on the box
"FOR INDOOR OR OUTDOOR USE ONLY"
Ummmmmmm, why is the word "ONLY" used? Is there a third option?

OFF PLANET USE. (not space qualified)

Regards,
Martin Brown

 

[ehsjr]

I've worked on a lot of the old incandescent christmas light sets and
they are pretty straight forward in how they work, even if the series
wiring can be a big PITA to find the bad bulb.

In 2009 I bought a christmas display (after christmas), which was a
costly display, but they had it marked down after Christmas. I used
it in 2010, for a couple weeks. This year I put it on the lawn last
week and it worked fine.

Today I noticed that half the display is not lighting. Of course this
is after dark so it's kind of hard to see what is going on with the
wiring. However, I noticed that there are just two wires to each
socket, thus telling me they are wired in series.

I have a good understanding of electronics, but this is the first
string of LEDs that has failed on me. All of a sudden I'm looking at
this light string and wondering how they work.

I know LEDs are run on DC and each LED requires a resistor. Well,
this set is plugged into an AC 120v outlet. The plug has fuses, but
since half the string works, I know the fuses are ok. But this is
where I get confused. If this string is DC, where the heck are the
diodes to convert the AC to DC. I doubt they are in the small plug
which barely has the room for those fuses. Plus, a regular AC to DC
power supply has filter capacitors. Maybe these LEDs dont need the
capacitor?????

Then, where are the resistors for each bulb? Or are they built right
into the LEDs on these christmas lights? On top of that, I never knew
they were series wired, so one bad bulb knocks out half or the whole
string. [that sucks].
(I proved that on a spare string I have, by pulling out one LED, half
the string went off).

Also on this display I notice there are molded sections about 3/8"
thick and 2 inches long in a few places in the string. What are
these? Are they fusible links (which cant be replaced), are they the
location of the diodes to convert the AC to DC, or simply connections
to connect the different segments of the string?
A schematic sure would be nice right about now!!!

This display is a bit to large to bring in the house and I did not
feel like being outdoors in the dark and cold trying to trace the
wiring. I had no idea that they were wired so when one bulb died a
whole segment of the string went out. Nor did I expect that a string
of LEDs would burn out after only 3 weeks to total use, knowing LEDs
are supposed to last for many thousands of hours. (Of course anything
can fail, and christmas lights are mass produced as cheaply as
possible).

I'm suspecting there is one bad LED, but I have no spares, so I'll
have to buy some tomorrow. I could take a bulb out of my spare
string, but that brings up a whole other question.

My display has white LEDs, whereas my spare string is multicolor. I
know that white LEDs operate at a higher voltage than red yellow, etc.
and blue and green are slightly higher in voltage. Thus none of the
multicolor bulbs would be suitable as replacements for the white LEDs
on my display.

Yet, this still brings up more of a puzzle. My multicolor spare
string has red, yellow, green, and blue LEDs. Since these different
colors require different voltages, how can they all be run on the same
string?

A string of LEDs seems so simple, or so I thought until I began to try
to fix this one.....

Now I wish I could find one single socket to make a LED bulb tester to
hook to the ohm scale on my multimeter. I sure hate to cut up a whole
working set to get one socket.....


------

Totally off the topic, but worth a laugh.
I saw a set of christmas lights in a store that said on the box
"FOR INDOOR OR OUTDOOR USE ONLY"
Ummmmmmm, why is the word "ONLY" used? Is there a third option?

AC---+--->|--->|--- ... -->|---+
| |
+---|<---|<--- ... --|<---+
|
AC-----------------------------+

That is one scheme they use. One series string lights when
it "sees" + and the other lights when it "sees" - as the
current alternates. If one LED dies, then the string it is
in dies, leaving the other string working.

Each LED drops a certain voltage - roughly 1.8 for red, 2.1
for yellow, 2.2 for green, 4.5 for blue. So with the proper
number and combination of LEDs the strings can work without
individual resistors at each LED. For example, one of each
color above would have a total drop of 1.8 + 2.1 + 2.2 + 4.5
or about 10.6 volts. White LEDs have a drop of about 3.3
volts each, so you can approximate the total voltage drop
based on the number of LEDs in your white strings. Don't
forget to divide by two, as there are two series strings
in parallel with each other.

Ed

 

[[email protected]]

Do they visibly flicker or leave a pattern of dots on your retina if you
move your head round quickly? If not then there is a rectifier and
capacitor somewhere. Some cheap skates just put one string on each half
phase and they flicker like hell. The circuit typically used for
domestic LED lights was discussed in an adjacent thread only recently.

I have not noticed any flickering, but I never really stared at it up
close. The display looks best from a distance.

What is the name (subject line) of the recent thread you're referring
to. This is a pretty busy newsgroup so it might take awhile to find.

Thanks for the help.

 

[Martin Brown]

I have not noticed any flickering, but I never really stared at it up
close. The display looks best from a distance.

You don't need to be up close - just with it in your peripheral vision
and turn your head quickly to look at it. Multiplexed LED displays look
very funny when you do this as does anything that flickers.

What is the name (subject line) of the recent thread you're referring
to. This is a pretty busy newsgroup so it might take awhile to find.

Thanks for the help.

120v LED bulbs -- innards

About half a dozen threads further down the list. I'd check fuse(s) for
the strings first before doing anything else - and remember there are
potentially mains voltages involved. Series strings across mains voltage
are uncommon in the UK now - most lights for outside use is now on 24 or
less with a wall wart transformer.

Regards,
Martin Brown

 

[[email protected]]

You don't need to be up close - just with it in your peripheral vision
and turn your head quickly to look at it. Multiplexed LED displays look
very funny when you do this as does anything that flickers.

120v LED bulbs -- innards

About half a dozen threads further down the list. I'd check fuse(s) for
the strings first before doing anything else - and remember there are
potentially mains voltages involved. Series strings across mains voltage
are uncommon in the UK now - most lights for outside use is now on 24 or
less with a wall wart transformer.

Regards,
Martin Brown

Thanks for the link and the help. I'll check those fuses. I'm not
too worried about the mains, because this is on a GFCI outlet. Of
course I still practice safety, which is the reason for the GFCI. All
outdoor stuff should be on GFCIs, except for the occasional use stuff
such as a power tool, which these days are all plastic anyhow.

 

[[email protected]]

I've worked on a lot of the old incandescent christmas light sets and
they are pretty straight forward in how they work, even if the series
wiring can be a big PITA to find the bad bulb.

In 2009 I bought a christmas display (after christmas), which was a
costly display, but they had it marked down after Christmas.  I used
it in 2010, for a couple weeks.  This year I put it on the lawn last
week and it worked fine.

Today I noticed that half the display is not lighting.  Of course this
is after dark so it's kind of hard to see what is going on with the
wiring.  However, I noticed that there are just two wires to each
socket, thus telling me they are wired in series.

I have a good understanding of electronics, but this is the first
string of LEDs that has failed on me.  All of a sudden I'm looking at
this light string and wondering how they work.

I know LEDs are run on DC and each LED requires a resistor.  Well,
this set is plugged into an AC 120v outlet.  The plug has fuses, but
since half the string works, I know the fuses are ok.  But this is
where I get confused.  If this string is DC, where the heck are the
diodes to convert the AC to DC.  I doubt they are in the small plug
which barely has the room for those fuses.  Plus, a regular AC to DC
power supply has filter capacitors.  Maybe these LEDs dont need the
capacitor?????

Then, where are the resistors for each bulb?  Or are they built right
into the LEDs on these christmas lights?  On top of that, I never knew
they were series wired, so one bad bulb knocks out half or the whole
string.  [that sucks].
(I proved that on a spare string I have, by pulling out one LED, half
the string went off).

Also on this display I notice there are molded sections about 3/8"
thick and 2 inches long in a few places in the string.  What are
these?  Are they fusible links (which cant be replaced), are they the
location of the diodes to convert the AC to DC, or simply connections
to connect the different segments of the string?
A schematic sure would be nice right about now!!!

This display is a bit to large to bring in the house and I did not
feel like being outdoors in the dark and cold trying to trace the
wiring.  I had no idea that they were wired so when one bulb died a
whole segment of the string went out.  Nor did I expect that a string
of LEDs would burn out after only 3 weeks to total use, knowing LEDs
are supposed to last for many thousands of hours.  (Of course anything
can fail, and christmas lights are mass produced as cheaply as
possible).

I'm suspecting there is one bad LED, but I have no spares, so I'll
have to buy some tomorrow.  I could take a bulb out of my spare
string, but that brings up a whole other question.

My display has white LEDs, whereas my spare string is multicolor.  I
know that white LEDs operate at a higher voltage than red yellow, etc.
and blue and green are slightly higher in voltage.  Thus none of the
multicolor bulbs would be suitable as replacements for the white LEDs
on my display.

Yet, this still brings up more of a puzzle.  My multicolor spare
string has red, yellow, green, and blue LEDs.  Since these different
colors require different voltages, how can they all be run on the same
string?

A string of LEDs seems so simple, or so I thought until I began to try
to fix this one.....

Now I wish I could find one single socket to make a LED bulb tester to
hook to the ohm scale on my multimeter.  I sure hate to cut up a whole
working set to get one socket.....

------

Totally off the topic, but worth a laugh.
I saw a set of christmas lights in a store that said on the box
"FOR INDOOR OR OUTDOOR USE ONLY"
Ummmmmmm, why is the word "ONLY" used?  Is there a third option?

Not familiar with these lights myself, my understanding (via a recent
conversation with another engineer who claims to understand these
things) is that: 100-bulb strings are wired in series-parallel.
Each bulb socket has some sort of internal collapsible coil the makes
the circuit (keeps the circuit) if the bulb burns out. The coil
somehow manages to shunt the bulb.
On some other lights, this functionality is built into the bulb, not
the socket... or so I am told.

Thus, a single bulb burnout would disable the whole string.

He showed me some tool you had to squeeze the trigger on up to 30
times to repair a string of lights.
I confess I have no idea how it works (doesn't interest me anyway),
but I could post a link to it if enough want to know, or someone else
here doesn't beat me to the punch.

-mpm

Yes, please post the link.....

I have to admit that I never thought these LED strings could be as
complicated as they are. In fact I thought they were each wired
individually, and those old days of fighting with series wired bulbs
was gone. Boy was I wrong!

Wiring one LED in a circuit is very simple, but these strings are hard
to understand because the resistors and any other parts apparently
hidden in the molded plastic globs at certain points along the string.
(and of course no schematic).

The good news is that I found the problem on my display. It turns out
there are THREE sections that light individually on this display. One
of the three was dead. I began to substitute bulbs from a working
section, and it ended up there were 3 dead LED bulbs and all of them
were next to each other (I wonder why that is?). The funny thing is
that after I replaced the first bad one, the other bulbs got a very
dim glow. All I had to do was look for others that had no glow and I
found the other two dead ones. (I dont understand that dim glow).

Oddly enough, I have had a few LEDs burn out on this string, and the
rest of the string still worked. (Apparently this must be that
"shunt" which I guess dont always work). Anyhow, the whole display
has white LEDs, but I did not have any spare white ones so I replaced
those initial two with yellow ones. This is the section that now
burned out. I now wonder if the reason for the recent burnout was
because of incorrect voltages in the string due to using those 2
yellow bulbs ???????

Although I got it working, I unplugged it. I only have colored
replacement bulbs. I'll have to see if I can track down some white
ones tomorrow.

Actually, I wish I could leave the colored ones. I put them in
specific places on the display and like the colored ones in those
places. But I guess this will likely cause more failures so I better
stick to the white ones.

Actually, the thought of rewiring the whole display with each bulb
having a separate resistor, and using a 6vdc wall wart seems
appealing, but this seems like a huge job for little gain. Anyhow,
now that I worked on this thing, I want to learn more about these
light strings.

 

[ehsjr]

and there needs to be some current limiting resistor or capacitor...

Not necessarily. With the right number and combination of LEDs, no
LED gets enough voltage to drive excessive current. It's like those
cheap flashlights with white LEDs that run off 2 AA's in series.
They don't use a current limiting component because the voltage
isn't high enough to drive excessive current. Of course, the better
flashlights use a driver circuit - and the better xmas lights use
a driver (smps) too.

if one string opens, not only will those not be lit, the string that
still works will be exposed to excessive reverse voltage since the
other string is no longer conducting.

Excessive reverse voltage would depend on what's in the string.
LEDs typically have a reverse voltage rating of 5 volts, so if
there are enough of them in the string that shouldn't be a problem.
That said, it seems to me that the manufacturers should include a
regular diode in each string anyway, even in junk xmas lights.
Not sure how they overcome flicker in minimal component junk -
maybe they don't?

Ed

 

[[email protected]]

Not necessarily. With the right number and combination of LEDs, no
LED gets enough voltage to drive excessive current. It's like those
cheap flashlights with white LEDs that run off 2 AA's in series.
They don't use a current limiting component because the voltage
isn't high enough to drive excessive current. Of course, the better
flashlights use a driver circuit - and the better xmas lights use
a driver (smps) too.

Some of them use the internal resistance of the battery, too. Substitute an
alkaline cell in for the supplied zinc and *PHUT*. This is a really bad plan
for line connected LEDs, too. Line variation is just too much.

Excessive reverse voltage would depend on what's in the string.
LEDs typically have a reverse voltage rating of 5 volts, so if
there are enough of them in the string that shouldn't be a problem.
That said, it seems to me that the manufacturers should include a
regular diode in each string anyway, even in junk xmas lights.
Not sure how they overcome flicker in minimal component junk -
maybe they don't?

Some don't. One set (ten strings) I bought a few years ago flickered so bad I
threw the junk away when I took it all down - went back to incandescents.

Not sure how all do it, but anti-parallel LEDs (should be cheap on the same
die) would solve both problems. A Zener (or two) across each LED will "solve"
the "one light out" problem, too.