"petrus bitbyter" (pieterkraltlaatditweg@enditookhccnet.nl) writes:
"DeanB" <deanbrown3d@yahoo.com> schreef in bericht
news:1164898733.870033.297470@l39g2000cwd.googlegroups.com...

John wrote:
On 29 Nov 2006 20:35:58 -0800, "DeanB" <deanbrown3d@yahoo.com> wrote:

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

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.

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

DeanB wrote:
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

Pete Wilcox wrote:
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

Pete Wilcox wrote:
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

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

JeffM wrote:
DeanB wrote:
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

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:)

Either way putting longer chains in series will drop the current nicely by
1/2 1/3,1/4 etc.
Bob In UK

Bob Minchin wrote:

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

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

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.

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.

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!!!!!

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.

But U can get the same intensity of LED with oulsing 20mA current with
25% duty cycle.

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 !!!!

---
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
---

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.

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.