Is it possible to dim LED christmas lights?

[DeanB]

Because I have 900 of them on my tree and they are quite bright!

 

[John]

Because I have 900 of them on my tree and they are quite bright!

That depends on how they are wired.

If a transformer is used, then you could use an adjustable transformer
(Variac) to reduce the voltage to the transformer.

If a transformer is not used, you could use a standard light dimmer -
check the combined wattage of your lights to get the proper capacity
dimmer.

Note that standard light dimmers will NOT work with transformer
powered lights (or not for very long...

John

 

[Homer J Simpson]

Because I have 900 of them on my tree and they are quite bright!

You could cycle each set for lower power and a twinkle effect.

 

[DeanB]

That depends on how they are wired.

If a transformer is used, then you could use an adjustable transformer
(Variac) to reduce the voltage to the transformer.

If a transformer is not used, you could use a standard light dimmer -
check the combined wattage of your lights to get the proper capacity
dimmer.

Note that standard light dimmers will NOT work with transformer
powered lights (or not for very long...

John

I know they are 4V per bulb. If I take out one bulb, then all of them
go out so I assume they are in series and not in parallel. I can't see
any transformer.

 

[Bob Engelhardt]

LED's are binary devices - either ON or OFF. They cannot be dimmed in
the usual sense. Their average output can be changed by the duty cycle.
If they are switched on & off fast enough, the eye will not see them
flickering, but will see a lower average output. I.e., dimmer.

A 60 Hz cycle would be flicker-free and there is a convenient 60 Hz
signal source . There may or may not be a commercially available
black box to do this. It would be trivial for an electronics hobbiest,
but my guess is that you are not one.

HTH,
Bob

 

[Pete Wilcox]

It would be trivial for an electronics hobbiest, but my guess is that
you are not one.

Hobby, hobbier, hobbiest. ITYM hobbyist, but you may well be right, he
doesn't seem to be the hobbiest of them all!

Cheers,
Pete.

 

[Michael Black]

Hobby, hobbier, hobbiest. ITYM hobbyist, but you may well be right, he
doesn't seem to be the hobbiest of them all!

I'm not one to usually nitpick on that sort of thing, but the error
over "hobbyist" has always jumped out at me.

I suppose if I could figure out why it's a common error, I might
be more understanding. But I can't figure out why the mistake is
made, so it somehow seems worse.

Michael

 

[Bob Engelhardt]

Bob Minchin wrote:
....

The light output from a light emitting diode is a function of the current
passing through it. Most devices will show a wide variation of brightness
from say 1 to 20 mA.

OK, so how would the OP go about dimming his LEDs? Bob

 

[Bob Engelhardt]

I'm not one to usually nitpick on that sort of thing, but the error
over "hobbyist" has always jumped out at me.

I suppose if I could figure out why it's a common error, I might
be more understanding. But I can't figure out why the mistake is
made, so it somehow seems worse.

I don't know why, either. It just came out. If I HAD thought about it,
I would have used "hobbyist", probably. Maybe "hobbiest" is used
(incorrectly) so much that it has imprinted itself on me. Or maybe I
was just blindly converting the sound.

Sorry. It won't happen again. I am annoyed by that sort of thing, too.
But let's not turn this thread into "spelling errors that annoy me".

Bob

 

[petrus bitbyter]

I know they are 4V per bulb. If I take out one bulb, then all of them
go out so I assume they are in series and not in parallel. I can't see
any transformer.

Hmm... Nine hundred LEDs in series with 4V/LED will require 3.6kV. Unlikely.

How do you now they are LEDs anyway? If they are directly connected to the
mains - so no transformer or other black box in between - I assume them to
be incandescent bulbs.

petrus bitbyter

 

[Pete Wilcox]

Sorry. It won't happen again. I am annoyed by that sort of thing, too. But
let's not turn this thread into "spelling errors that annoy me".

Shouldn't have sent the correction, sorry. Thing is, it didn't annoy me,
I just thought it was amusing and I was in a silly mood.
Note to self: Do NOT post while drunk... Do NOT post while drunk...

Cheers,
Pete.

 

[JeffM]

Because I have 900 of them on my tree and they are quite bright!

Calculate the power consumed by your lights and,
being careful not to exceed the rating of the device, get one of these.
http://froogle.google.com/froogle?scoring=p&price=between&price1=1&q=tabletop-lamp-dimmer

 

[Michael Black]

Hmm... Nine hundred LEDs in series with 4V/LED will require 3.6kV. Unlikely.

How do you now they are LEDs anyway? If they are directly connected to the
mains - so no transformer or other black box in between - I assume them to
be incandescent bulbs.

For what it's worth, I bought some LED Christmas lights (they certainly
don't get warm) two years ago, and there's no transformer. I assume
they use some sort of series and parallel combination. Though I've
not wanted to sacrifice the string to figure out what's going on.

I suspect it's not a single string of 900 lights, but a number of separate
strings, coupled from one to the other in the same way that traditional
incandescent Christmas tree lights went, a plug at one end and an outlet
at the other to plug in the next string. That way you don't have to
bring each string down to the outlet, but each individual string is
seeing 120VAC.

Michael

 

[Bob Engelhardt]

Shouldn't have sent the correction, sorry. Thing is, it didn't annoy
me, I just thought it was amusing and I was in a silly mood.
Note to self: Do NOT post while drunk... Do NOT post while drunk...

No no no. I was replying to Michael Black, who was annoyed. Your post
was clever and amusing itself and not offending (oh,no is that the right
word?). I would try drinking while posting, but I think that my posts
would become more error-filled rather than more clever.

Bob

 

[petrus bitbyter]

For what it's worth, I bought some LED Christmas lights (they certainly
don't get warm) two years ago, and there's no transformer. I assume
they use some sort of series and parallel combination. Though I've
not wanted to sacrifice the string to figure out what's going on.

I suspect it's not a single string of 900 lights, but a number of separate
strings, coupled from one to the other in the same way that traditional
incandescent Christmas tree lights went, a plug at one end and an outlet
at the other to plug in the next string. That way you don't have to
bring each string down to the outlet, but each individual string is
seeing 120VAC.

Michael

So you may have thirty LEDs in a string and thirty strings in parallel.
Which seems to make sense but how to explain that when you remove one bulb
all of them go out? Besides, when they are real LEDs, there should be some
rectifiing mechanism as a LED - one LED - cannot handle AC.

*If* they are real LEDs *and* there are no coils *and* no transformers *and*
no electronics but rectifiers *then* you may use an ordinary dimmer. But as
long as you are not sure about the components and the circuit it's gambling
with 900 small lights at risk.

petrus bitbyter

 

[DeanB]

Calculate the power consumed by your lights and,
being careful not to exceed the rating of the device, get one of these.
http://froogle.google.com/froogle?scoring=p&price=between&price1=1&q=tabletop-lamp-dimmer

They are 60 bulbs on an individual string. I guess in the wire there
are 2 pairs of wires, giving 110 V over a set of 30 bulbs, or just
under 4V each in series, which is why they all go out (actually only
half of the string goes out).

I will try a dimmer (I have one like that) and give and update tonight.
Oooh, the tension is killing you all, I know! I'll be sure to post
before midnight, then you will be able to get some sleep without being
concerned

 

[Radiosrfun]

No no no. I was replying to Michael Black, who was annoyed. Your post
was clever and amusing itself and not offending (oh,no is that the right
word?). I would try drinking while posting, but I think that my posts
would become more error-filled rather than more clever.

Bob

Hell, I've sent messages when drinking - no big deal. "My" problem came when
I sent a message and was so damned tired, even the toothpicks were snapping
under pressure to hold my eyes open. I was replying to one person - kept
dozing off and when "trying" to wake up - in my dreamy state - thought I was
writing to a girl friend and well - sent some stuff (words) to someone -
they "shouldn't" have seen. That was downright embarassing. I bet they got
one hell of a good laugh!

 

[Michael Black]

No no no. I was replying to Michael Black, who was annoyed. Your post
was clever and amusing itself and not offending (oh,no is that the right
word?). I would try drinking while posting, but I think that my posts
would become more error-filled rather than more clever.

Bob

No, I'm not annoyed.

It is just something that I can remember being misspelled thirty years
ago when I first came across the term, and I do wonder why it so often
is spelled the wrong way.

Michael

 

[Michael Black]

They are 60 bulbs on an individual string. I guess in the wire there
are 2 pairs of wires, giving 110 V over a set of 30 bulbs, or just
under 4V each in series, which is why they all go out (actually only
half of the string goes out).

I will try a dimmer (I have one like that) and give and update tonight.
Oooh, the tension is killing you all, I know! I'll be sure to post
before midnight, then you will be able to get some sleep without being
concerned

Like I said, I'm not willing to open mine up to figure out how
they've got things arranged. Wait a few years, when LED lights
are the norm, and they get cheaper, and maybe when they go on
sale after Christmas one year, I'd be willing to dissect a set.

YOu could get less light with less density. Drop back from 900 lights,
and you will get less light.

What I did do was get out my one set of LED Christmas lights, since
they'll be coming out soon anyway. Made by Sylvania, and there's
35 LEDs n the string. No transformer, and no obvous lumps (unless
that AC plug has something hiding in it, not transformer size but
maybe a few components), and the end away from the plug has the
usual feedthrough AC outlet so you can connect another string but
it will be 120VAC there.

I have a little box with an outlet and a common light dimmer in it,
for controlling my soldering iron. So I unplugged the iron, and plugged
in the LED lights. With the control at full, light output seems
the same as when I plug the string right into an outlet.

It doesn't matter which way I plug in the lights.

There is control of the brightness over a segment of the dimmer travel,
maybe about a quarter? The LEDs never go completely off, even with the
dimmer at it's lowest. At a point right before the dimmer control reaches
the point where the brightness is at the lowest it gets, the LEDs flicker.
The flicker range occurs over more of the dimmer control range when
the LED string is plugged in one way than the other. And this flicker
is mostly on, with just a small off period, and the rate must be either
60 or 120Hz, I have no reference flash.

You most definitley do not get a lot of control over the brightness, but
it might be suitable if it's just a matter of trying to reduce overall
brightness of all the LED lights. If you wanted to create "mood" by
adjusting the light level, you wouldn't be happy.

I have a switch on this box that flips out the dimmer, but puts
a single diode in series with the outlet. At least, I think that's
the way I wired it. With the LED lights plugged in one way, the diode
position just puts the LEDs at full brightness, but with them plugged
in the other way, the LED lights don't go on.

The diode causing no light requires the string to be plugged in
the same way as that which gives the most flickering.

THus ends the "black box" discussion of LED Christmas lights. I
don't have a second string to see if a different brand or a string
with a different number of LEDs would provide different results.

Michael

 

[psdayama]

Fair point but I assume there must be some form of current limiting in the
exisiting strings. Surely these are not just connected across the rectified
mains supply?
If the current is limited by a resistor then the brightness could be
controlled by increasing its value.

Bob in UK

Whats problem with dimming? Simply get rheostat and use in serries.
There has to be even nos of rows as LEDs are connected reverse in one
row and forward
in another. So assuming a small resistor in each row, the rheostat will
control current
till it becomes very small and LEDs are off. We can asume that each row
has 60LEDs and
current is 5mA(10 mA halfwave) so resistor in each row is 1200 ohms. &
toatal 16 rows.
If one connects 500 ohm 10W rheostat or potentiometer then intensity
can be controlled.

It is not posible to dim LEDs with PWM as peak current in them
remains same and
due to persistance of vision intensity appears same. Though current may
be less.
Only constant current regulation can do the job.