Is it possible to dim LED christmas lights?
DeanB
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
"DeanB" <deanb...@yahoo.com> wrote in message
news:1164861358....@16g2000cwy.googlegroups.com...
> 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
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
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 8-). 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
Bob Minchin
"Bob Engelhardt" <bobeng...@comcast.net> wrote in message
news:AdWdnS1-OZpxlvLY...@comcast.com...
quote 'LED's are binary devices - either ON or OFF'
This is rubbish.
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.
Another Bob
Pete Wilcox
On Thu, 30 Nov 2006, Bob Engelhardt wrote:
> 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
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
...
> 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
> Pete Wilcox (p...@st-andrews.ac.uk) writes:
>> On Thu, 30 Nov 2006, Bob Engelhardt wrote:
>> ... hobbyist, ...
> 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
"DeanB" <deanb...@yahoo.com> schreef in bericht
news:1164898733.8...@l39g2000cwd.googlegroups.com...
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
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
"Michael Black" <et...@FreeNet.Carleton.CA> schreef in bericht
news:eknfkb$hdd$1...@theodyn.ncf.ca...
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
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
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, 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
Bob Minchin
From other replies there are 900 LEDs. These must be connected in some form
of series/parallel network either connected to rectified mains or a lower
voltage transformer &rectifier arrangement - probably the latter if in UK to
meet safety regulations. Dunno about the other side of the pond with your
low voltage mains supplies.
Either way putting longer chains in series will drop the current nicely by
1/2 1/3,1/4 etc.
Bob In UK
Michael Black
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
Michael A. Terrell
>
> Either way putting longer chains in series will drop the current nicely by
> 1/2 1/3,1/4 etc.
> Bob In UK
No, it won't LEDs have a minimum operating voltage, and if the
available voltage drops below that you don't get any light. You either
run them from an adjustable constant current source, or Pulse Width
Modulate the supply to adjust the brightness.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
Bob Minchin
"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
news:457514E7...@earthlink.net...
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
psdayama
> > No, it won't LEDs have a minimum operating voltage, and if the
> > available voltage drops below that you don't get any light. You either
> > run them from an adjustable constant current source, or Pulse Width
> > Modulate the supply to adjust the brightness.
>
> 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.
John Fields
---
That's not true.
Assume an LED with 20 mA through it all the time. It will exhibit a
certain brightness. Now, still with 20 mA through it, PWM it so
it's only on for half the time but with a PRF high enough so that
you can't detect any flicker. It will appear to be half as bright
as before.
--
JF
Don Klipstein
psdayama wrote in part:
> 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.
The myth persists, and this is as bad as I ever heard it!
Meanwhile, I have more on:
http://ww.misty.com/~don/ledp.html about pulsing LEDs to increase ratio of
perceived output to average current (while appearing to glow
continuously), when this works, when this does not, and how little human
vision has to do with it when it does work!
>Only constant current regulation can do the job.
PWM is a common way to dim LEDs in RGB displays. Many manufacturers
recommend PWM because LED output and color as a function of varying
current can vary from one unit to another even when different units
perform the same at "characterization current", and some LEDs normally
have color varying with current.
- Don Klipstein (d...@misty.com)
psdayama
>
> Assume an LED with 20 mA through it all the time. It will exhibit a
> certain brightness. Now, still with 20 mA through it, PWM it so
> it's only on for half the time but with a PRF high enough so that
> you can't detect any flicker. It will appear to be half as bright
> as before.
YES IT IS TRUE!!!!!
First U dont know anything about LEDs. At 20mA normal LEDs will blow
up soon.
But U can get the same intensity of LED with oulsing 20mA current with
25% duty
cycle. Thats called PWM. It is not PWM of voltage which will not work.
But there
are ckts which can be current controllers. Now most of the LCD displays
have
also background LEDs pulsed current so they consume less power but
dispaly
is same. Above PRF of 100 persistance of vision works which shows LED
bright
as if consuming high current.
Pls go to some wikipedia or some sites of LED manufacturers and U will
know.
Ignorance is greatest bliss !!!!
John Fields
>> That's not true.
>>
>> Assume an LED with 20 mA through it all the time. It will exhibit a
>> certain brightness. Now, still with 20 mA through it, PWM it so
>> it's only on for half the time but with a PRF high enough so that
>> you can't detect any flicker. It will appear to be half as bright
>> as before.
>> JF
> YES IT IS TRUE!!!!!
---
No, it isn't.
---
> First U dont know anything about LEDs.
---
Yes, I do.
---
>At 20mA normal LEDs will blow up soon.
---
No, they won't. "Normal/Standard" LEDs are rated for 20mA Forward
current, and have been for many, many years. Go to any LED
manufacturer's website and see for yourself.
---
>But U can get the same intensity of LED with oulsing 20mA current with
>25% duty cycle.
---
No, you can't. If you have 20mA through the LED continuously and
then you change that to 20mA through the LED for 1/4 of the time,
what makes you think the LED is going to appear to be as bright as
it was with four times the photons coming out of it?
---
That's called PWM. It is not PWM of voltage which will not work.
---
Sure it will. If I know the precise relationship between LED forward
voltage and LED forward current I can PWM the input voltage and
easily control the charge and time through the LED.
---
>But there are ckts which can be current controllers.
>Now most of the LCD displays have also background LEDs pulsed
>current so they consume less power but dispaly is same.
---
Sorry, but no. Watts is watts no matter how you slice it up.
For example, let's say you've got an LED backlight which is on all
the time and consumes 4 coulombs per second for, say, 100 lumens
out.
Now, if for some reason you wanted to PWM it at a 25% duty cycle and
maintain the same apparent brightness you'd have to pump 16 coulombs
through it for 1/4 of the total time, which is still 4 coulombs per
second or, 4 watts
---
>Above PRF of 100 persistance of vision works which shows LED
>bright as if consuming high current.
---
No. When you PWM an LED the peak current has to increase in order
to make the apparent brightness of the LED the same as when driving
the LED with CWDC.
> Pls go to some wikipedia or some sites of LED manufacturers and U will
>know.
---
Been there, done that, and also designed lots of circuits to PWM
LEDs and EL lamps.
---
>Ignorance is greatest bliss !!!!
---
Then you must be posting from Nirvana.
--
JF
John Fields
<jfi...@austininstruments.com> wrote:
>---
>Sorry, but no. Watts is watts no matter how you slice it up.
>
>For example, let's say you've got an LED backlight which is on all
>the time and consumes 4 coulombs per second for, say, 100 lumens
>out.
>
>Now, if for some reason you wanted to PWM it at a 25% duty cycle and
>maintain the same apparent brightness you'd have to pump 16 coulombs
>through it for 1/4 of the total time, which is still 4 coulombs per
>second or, 4 watts
>---
Oops...
Joules per second.
--
JF
jasen
>> That's not true.
>>
>> Assume an LED with 20 mA through it all the time. It will exhibit a
>> certain brightness. Now, still with 20 mA through it, PWM it so
>> it's only on for half the time but with a PRF high enough so that
>> you can't detect any flicker. It will appear to be half as bright
>> as before.
>> JF
> YES IT IS TRUE!!!!!
> First U dont know anything about LEDs. At 20mA normal LEDs will blow
> up soon.
20mA is fine for normal leds.
> But U can get the same intensity of LED with oulsing 20mA current with
> 25% duty cycle.
BULLSHIT! try it.
> Thats called PWM. It is not PWM of voltage which will not work.
that's just nonsense.
> But there are ckts which can be current controllers.
resistors work great for LEDs.
Bye.
Jasen
psdayama
John Fields wrote:
It is the paradox of vision.
Ur eye cant detect fast changing intensities and due to persistance of
vision
it appears as bright as if 100% lumens out.
As per Ur theory the whole new led lamps will go out of business as 3W
high
efficiency LED shines as if 40W GSL lamp. It doesnt consume that much
power
to give that Lumens.
Shallow waters make biggest noise!
John Fields
wrote:
>
>John Fields wrote:
>
>> On Sat, 09 Dec 2006 10:10:20 -0600, John Fields
>> <jfi...@austininstruments.com> wrote:
>>
>> >---
>> >Sorry, but no. Watts is watts no matter how you slice it up.
>> >
>> >For example, let's say you've got an LED backlight which is on all
>> >the time and consumes 4 coulombs per second for, say, 100 lumens
>> >out.
>> >
>> >Now, if for some reason you wanted to PWM it at a 25% duty cycle and
>> >maintain the same apparent brightness you'd have to pump 16 coulombs
>> >through it for 1/4 of the total time, which is still 4 coulombs per
>> >second or, 4 watts
>> >---
>>
>> Oops...
>>
>> Joules per second.
>>
>>
>> --
>> JF
> It is the paradox of vision.
>Ur eye cant detect fast changing intensities and due to persistance of
>vision
>it appears as bright as if 100% lumens out.
---
Nope. Your eye integrates the light energy that comes into it, so
if you see four times as many photons in 1/4 of the time that the
same number of photons entered continuously your eye would see both
sources as equally bright.
Don't believe me? Try it.
Get two LEDs that are equally bright with, say, 2mA going through
them from a DC source. Now, disconnect one of them and assemble
some equipment, like this: (View in Courier)
+------+ 2mA-->
| DC +|--[1000]---+
|SUPPLY| |
| -|--[<LED]---+
+------+
+-----+ +-----+
|PULSE|---[LED>]---+---|SCOPE|
| GEN | | +--+--+
+--+--+ [1000] |
| | |
+---------------+------+
|
GND
Adjust the pulse generator's output amplitude so that it's below the
illumination threshold of the LED, its waveform for 25% duty cycle,
and its rep rate high enough so that you can't see flicker. (I used
100Hz and 1000 Hz...)
Now, adjust the generator's output amplitude, watching both LEDs
until they appear to be the same brightness, then read the peak
amplitude of the waveform on the scope and multiply it by the
resistance of R2. You'll find it's four times the current in the
other LED.
I chose to use high-efficiency red LEDs (HLMP4700) but you should
get the same results, should you choose to run the experiment, with
whatever LEDs you choose.
---
>As per Ur theory the whole new led lamps will go out of business as 3W
>high
>efficiency LED shines as if 40W GSL lamp. It doesnt consume that much
>power
>to give that Lumens.
---
That's efficiency, and has nothing to do with what we've been
talking about at the level at which we've been talking.
---
> Shallow waters make biggest noise!
---
Then you must be very nearly dry...
--
JF
Frithiof Andreas Jensen
> LED's are binary devices - either ON or OFF. They cannot be dimmed in
> the usual sense.
Sure they can: The light output is proportional to the current through them
edegner
that is turned OFF. When an X10 outlet is turned off it is still
putting out about 60 volts (I checked over a dozen different types of
X10 outlet, in-wall type as well as plug-in type, and they all put out
around 60 volts). Might be a little TOO dim though.
I'm trying to talk my friend into converting his display to all LED's,
but he doesn't like the way they look. Check out his display at
http://www.thatchristmashouse.com and tell me what you think.
On Nov 30 2006, 2:50 pm, "JeffM" <jef...@email.com> wrote:
> DeanB wrote:
> >Because I have 900 of them on my tree and they are quite bright!Calculate the power consumed by yourlightsand,
> 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=...
JeffM
>When an X10 outlet is turned off it is still putting out about 60 volts
>(I checked over a dozen different types of X10 outlet,
>in-wall type as well as plug-in type,
>and they all put out around 60 volts).
Try it with a load that is more meaningful than just a 10Mohm
voltmeter.