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Science Forum Index » Electronics - Misc Forum » Is it possible to dim LED christmas lights?
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| psdayama |
 Posted: Sun Dec 10, 2006 8:38 am |
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John Fields wrote:
Quote: On Sat, 09 Dec 2006 10:10:20 -0600, John Fields
jfields@austininstruments.com> wrote:
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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.
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! |
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| John Fields |
| Posted: Sun Dec 10, 2006 11:53 am |
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On 10 Dec 2006 04:38:48 -0800, "psdayama" <psdayama@gmail.com>
wrote:
Quote:
John Fields wrote:
On Sat, 09 Dec 2006 10:10:20 -0600, John Fields
jfields@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.
---
Quote: 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.
---
Quote: Shallow waters make biggest noise!
---
Then you must be very nearly dry...
--
JF |
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| Frithiof Andreas Jensen |
| Posted: Thu Dec 14, 2006 8:53 am |
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"Bob Engelhardt" <bobengelhardt@comcast.net> wrote in message
news:AdWdnS1-OZpxlvLYnZ2dnUVZ_tOdnZ2d@comcast.com...
Quote: 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 |
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| edegner |
| Posted: Mon Jan 29, 2007 4:46 am |
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Easy way to dim LED christmas lights... Plug them into an X10 outlet
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:
Quote: 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=... |
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| JeffM |
| Posted: Mon Jan 29, 2007 12:32 pm |
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edegner wrote:
Quote: 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. |
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