I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.

On Dec 24, 1:54*am, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is. *Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.

THEY ARE LAZY... THE DESIGNERS USE THE WRONG SCR FOR WAVEFORM
RECTIFICATION, AT HIGH FREQUENCY IN WHICH CFL's WORK TRIMMIMG THE
INPUT WAVE IN BOTH VOLTAGE AND CURRENT RESULTS IN FLICKERING.
THE RIGHT ELECTRONIC DESIGN COMPONETS WOULD CORRECT AND PREVENT
THATNFLAW.
AS FOR THE LED's , THE SAME APPLIES IT IS NOT TOO DIFICULT TO REDUCE
THE AC INPUT WITHOUT CHANGING THE FREQUENCY....THIS CAN BE
ACCOMPLOSHED USING A CLAMPER IN THE IC INSTEAD OF A CLIPPER CIRCUIT.

EASY AS PUMKIN HEAD I MEAN, PIE.
THERE MUST BE GOLD IN THEM THERE HILLS (:/)
PATECUM

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.

On Dec 24, 7:28*am, Jeff Thies wrote:
On 12/24/2010 1:54 AM, Home Guy wrote:

I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

* I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.

YMMV



I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

* *Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

* *Jeff

* *Not being able to dim them makes them even



less desirable as a replacement for incandescent bulbs.

EVENTUALLY THEY SHOULD COMPLY WITH DEMAND.IN-THE-MEANTIME.THERE IS NO
FOUL IN KEEPING DIMMERS AND USING INCANDESCENT WHERE YOU WISH, THEY
STILL WARM THE HEART AND GIVE OFF SUITABLE ILLUMINATION.

PATECUM

Jeff Thies wrote in
:

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.



LEDs are CONSTANT CURRENT-driven,not voltage driven.
they have to have some sort of current regulator inside that keeps the
current constant regardless of input voltate swings.
To dim a LED,you would have to vary the input current,and the typical triac
lamp dimmers don't do that,they vary input voltage.

Plus,there's a fairly narrow range of current that produces light from the
LED chip,it's not linear.Most LED dimming schemes pulse modulate the
LED,changing the duty cycle of the current,and the eye averages the output.
you get better efficiency that way,but it's more complex electronically.

--
Jim Yanik
jyanik
at
localnet
dot com

On 12/24/2010 07:25 AM, G. Morgan wrote:
Home wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.


sure they are, I had a LED dash light kit in my old 944. It didn't dim
linearly like the incandescents though, so a slightly different dimmer
would be required. I thin kthe difficulty is with dimming with AC.

nate

--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel

EVENTUALLY THEY SHOULD COMPLY WITH DEMAND.IN-THE-MEANTIME.THERE IS NO
FOUL IN KEEPING DIMMERS AND USING INCANDESCENT WHERE YOU WISH, THEY
STILL WARM THE HEART AND GIVE OFF SUITABLE ILLUMINATION.

PATECUM


Yo.... Ghosty.... caps lock.
Don't need to shout here.

G. Morgan wrote in
:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.




I suppose you could simulate dimming by adjusting the pulse rate, the way
some automakers do it with their LED tail/brake lights. But that would
probably cause flickering at the lower rates.

Could you simply have a dimmer illuminate more or fewer LEDs as the dimmer
is turned up or down? Perhaps the voltage from the dimmer switch could be
used as a signal by the LED assembly, which would interpret it as a command
to turn-on or turn-off LEDs.

--
Tegger

On 12/24/2010 7:56 AM, Jim Yanik wrote:
Jeff wrote in
:

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.



LEDs are CONSTANT CURRENT-driven,not voltage driven.
they have to have some sort of current regulator inside that keeps the
current constant regardless of input voltate swings.
To dim a LED,you would have to vary the input current,and the typical triac
lamp dimmers don't do that,they vary input voltage.

Plus,there's a fairly narrow range of current that produces light from the
LED chip,it's not linear.Most LED dimming schemes pulse modulate the
LED,changing the duty cycle of the current,and the eye averages the output.
you get better efficiency that way,but it's more complex electronically.

One of the things that makes LEDs "harder" to dim is that they dim
linearly with current. Normally this would be a nice thing, however
present dimmers are made for traditional bulbs. Filament bulbs dim
very non linearly with voltage. LEDs typically will have some sort
of series limiting resistance, but the idea is to have as little
series resistance as possible, to cut down on wasted power in the
resistor. Pulse width dimming, as in car tail lights, is the
obvious answer. But, I think they should up the frequency of said
tail lights and future house lights dimming circuits, to prevent the
strobing effect you see when you turn your head rapidly. Some
people don't even notice it, but some do.

Nate Nagel wrote:

sure they are, I had a LED dash light kit in my old 944. It didn't dim
linearly like the incandescents though, so a slightly different dimmer
would be required. I thin kthe difficulty is with dimming with AC.

Diodes have two states, off and on. I couldn't see a practical
way to dim them, without fooling the brain by frequency of the
light. That won't work well either, I may be able to see 3000
fps with my eyes, but you would need a high-speed camera and light
to control constantly changing lighting environments.

You probably had an adjustable current device potentiometer that
would limit DC current... That would explain the non-linear
aspect side of the theory. Perhaps there were more LED's that you
knew about. I could easily see Porsche make a multi-diode lamp.

"G. Morgan" wrote in message
...
Nate Nagel wrote:

sure they are, I had a LED dash light kit in my old 944. It didn't dim
linearly like the incandescents though, so a slightly different dimmer
would be required. I thin kthe difficulty is with dimming with AC.

Diodes have two states, off and on. I couldn't see a practical
way to dim them, without fooling the brain by frequency of the
light. That won't work well either, I may be able to see 3000
fps with my eyes, but you would need a high-speed camera and light
to control constantly changing lighting environments.

You probably had an adjustable current device potentiometer that
would limit DC current... That would explain the non-linear
aspect side of the theory. Perhaps there were more LED's that you
knew about. I could easily see Porsche make a multi-diode lamp.


You need to do more research on diodes, especially the LED types. They do
not have two states like you mentioned. They are non linear devices. Also
LED are not dropping .7 volts as you mentioned. The .7 volts is a nominal
voltage for most silicon diodes only. Most LEDs drop differant voltages.
They range from about 1.5 to 4.5 volts. LEDs are current dependant and
not so much voltage. The more current through them, the brighter they are.
The current must be limiated to prevent burn out.

If a resistor is placed in series with a led, the voltage can be raised and
lowered to change the brightness. This is because the resistor is in part
controlling the current.

They can be pulse controled also. The human eye is not fast enough to
respond to fast changing lights. It will tend to average the brightness.
That is the way the moving pictuers work.

On 12/24/2010 7:56 AM, Jim Yanik wrote:
Jeff wrote in
:

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.



LEDs are CONSTANT CURRENT-driven,not voltage driven.
they have to have some sort of current regulator inside that keeps the
current constant regardless of input voltate swings.
To dim a LED,you would have to vary the input current,and the typical triac
lamp dimmers don't do that,they vary input voltage.

I hadn't thought about this, but aren't light dimmers just varying
the fraction of a half wave (pulse width)? It would seem to me that it
should work with LEDs (although not particularly well) but I don't know
what circuitry is in an LED light.

I think the key is how the current source is constructed. Limiting
the power lost there is important.

Can you shed any light on just what is in an LED lamp? I'm curious
now. I figured a wee chopper with a bit of output filtering. Add a
resistor or a constant current configured transistor. Just speculating.


Plus,there's a fairly narrow range of current that produces light from the
LED chip,it's not linear.Most LED dimming schemes pulse modulate the
LED,

Is that a household type dimmer like on the wall?


changing the duty cycle of the current,and the eye averages the output.
you get better efficiency that way,but it's more complex electronically.

For the money they are, they can throw a little circuitry at it!
Chopping up DC is no big deal these days. With the low current drawn and
high voltage, I wouldn't think it would need a very large cap.

I have no practical knowledge of this, so I'll defer to those with
more experience.

Jeff

On Fri, 24 Dec 2010 12:11:20 -0500, Jeff Thies wrote:

On 12/24/2010 7:56 AM, Jim Yanik wrote:
Jeff wrote in
:

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.



LEDs are CONSTANT CURRENT-driven,not voltage driven.
they have to have some sort of current regulator inside that keeps the
current constant regardless of input voltate swings.
To dim a LED,you would have to vary the input current,and the typical triac
lamp dimmers don't do that,they vary input voltage.

I hadn't thought about this, but aren't light dimmers just varying
the fraction of a half wave (pulse width)? It would seem to me that it
should work with LEDs (although not particularly well) but I don't know
what circuitry is in an LED light.

Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. The range of adjustment will be very small. [*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear


I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

Can you shed any light on just what is in an LED lamp? I'm curious
now. I figured a wee chopper with a bit of output filtering. Add a
resistor or a constant current configured transistor. Just speculating.

You'd have to make a DC source, which won't be cheap because of the sine-wave
input. The source will have to store energy for the cycle.

Plus,there's a fairly narrow range of current that produces light from the
LED chip,it's not linear.Most LED dimming schemes pulse modulate the
LED,

Is that a household type dimmer like on the wall?

No, they adjust the (phase) angle of a triac firing. This controls the power
put into a resistive load just fine but doesn't work for a highly non-linear
load like a (long) string of diodes.

changing the duty cycle of the current,and the eye averages the output.
you get better efficiency that way,but it's more complex electronically.

For the money they are, they can throw a little circuitry at it!
Chopping up DC is no big deal these days. With the low current drawn and
high voltage, I wouldn't think it would need a very large cap.

Right. But you don't have DC to chop. ;-) Making the DC isn't all that
cheap (or efficient).

I have no practical knowledge of this, so I'll defer to those with
more experience.

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

On Fri, 24 Dec 2010 12:11:20 -0500, Jeff Thies
wrote:


I hadn't thought about this, but aren't light dimmers just varying
the fraction of a half wave (pulse width)? It would seem to me that it
should work with LEDs (although not particularly well) but I don't know
what circuitry is in an LED light.

The problem is the clock rate. LEDs respond virtually instantly and at
60 or 50hz the flicker would be horrible.

As can be seen with LED Christmas lights. It's fairly cheap to double that
(full wave bridge or more LEDs), which would solve the problem.

You could either clock your chopper a lot faster or just vary the
current.

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

As soon as Hosfelt gets my box here I am going to start playing with
some high intensity LEDs to see what I can do with them,
My goal is dimmable under counter lights that run off a wall wart with
a very low profile. (perhaps even recessed into the cabinet bottom)

Good luck with that.

On 12/24/2010 12:50 PM, zzzzzzzzzz wrote:
On Fri, 24 Dec 2010 12:11:20 -0500, Jeff wrote:

On 12/24/2010 7:56 AM, Jim Yanik wrote:
Jeff wrote in
:

On 12/24/2010 1:54 AM, Home Guy wrote:
I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


Hey Don,

What do you know about what circuitry is in the commercial LED lamps
and dimming compatibility.

For those who don't know, Don Klipstein knows more about lighting
than anyone I know of:

http://members.misty.com/don/ledx.html

We'll see if he is around and interested.

Jeff




YMMV

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.

On Fri, 24 Dec 2010 18:41:14 +0000 (UTC), "A. Baum" wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, zzzzzzzzzz wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

On Fri, 24 Dec 2010 12:11:20 -0500, Jeff Thies
wrote:


I hadn't thought about this, but aren't light dimmers just varying
the fraction of a half wave (pulse width)? It would seem to me that it
should work with LEDs (although not particularly well) but I don't know
what circuitry is in an LED light.

The problem is the clock rate. LEDs respond virtually instantly and at
60 or 50hz the flicker would be horrible.

As can be seen with LED Christmas lights. It's fairly cheap to double
that (full wave bridge or more LEDs), which would solve the problem.

Uh the problem is an led fires at a certain voltage.

No, as has pointed out here by others, it doesn't "fire at a certain voltage".
It's light output is highly nonlinear WRT voltage, but there is no "on" or
"off", rather a continuum.

To dim the led you
need to apply that voltage via a PWM. Chop the firing voltage up into
pulses too fast for the eye to notice.

The output of an LED is a function of the average current through it. You can
either use PWM to change the average current or vary the current directly. The
light output is a pretty linear function of the current through the LED so
either method works.

The smaller the width of the pulses the dimmer the led.

No, the less the *ratio* of "on" to "off" the dimmer the LED. The important
variable is the *average* current.

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.

On Dec 24, 11:50*am, "
wrote:


Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.


Note: http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. I have one and it
works great. I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Red

On Fri, 24 Dec 2010 17:06:06 -0800 (PST), Red wrote:

On Dec 24, 11:50*am, "
wrote:


Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.


Note: http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. I have one and it
works great. I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Sure dimming LEDs is possible, and actually not all that hard if you start
with DC (the unit you linked is battery powered). Doing it from the AC line,
cheaply and efficiently, is more challenging. It's easy to do sloppily, but
then there is no gain over an incandescent bulb.

On Fri, 24 Dec 2010 17:35:37 -0600, "
wrote:

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.


As usual you are not "completely right". I won't argue and say you
are wrong in your assertions - but my UNDERSTANDING is that PEAK
current controlls the visible light output, and average current
affects lifespan (due to junction heating). It is not totally linear.

My experience is obviously different than yours.
As in many other cases, I need to say that just because you haven't
seen it, don't make it wrong or impossible. Just means your scope is
too narrow.

See: http://www.gardasoft.com/uploads/APP...ing%20LEDs.pdf
also http://www.lunaraccents.com/educational-LED-driver.html
and http://www.light-speed-tech.com/ltleds.htm
and
http://www.optoiq.com/index/machine-...intensity.html
and http://www.gardasoft.co.uk/
and http://www.smartvisionlights.com/pro...erdrive-series
and http://www.freepatentsonline.com/7639219.html
and
http://www.parameter.se/products/Def...D3=142&ID4=155

Are just a very few references for you to look at (commercial
applications of) pulsed overdrive applications for high luminence LED
applications.

On Dec 24, 9:01*am, Nate Nagel wrote:
On 12/24/2010 07:25 AM, G. Morgan wrote:





Home *wrote:

I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch..

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. *They had to develop a ballast that was
adjustable. *CFL's and LEDS are not a good comparison.

They're crazy-priced as is. *Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.

L.E.D.S. Are going to difficult (impossible) to dim. *Remember
they are DIODES that only need .7V *to illuminate. *AFAIK ---
LED's are not dim-able.

sure they are, I had a LED dash light kit in my old 944. *It didn't dim
linearly like the incandescents though, so a slightly different dimmer
would be required. *I thin kthe difficulty is with dimming with AC.

nate

--
replace "roosters" with "cox" to reply.http://members.cox.net/njnagel- Hide quoted text -

- Show quoted text -

LEDs arent dimable in the normal since of the word. The current to
them can be pulse width modulated to produce what we percive as a
dimming of the light. This is accomplished by switching the diode off
and on at a high enough rate that our eyes cant see the flicker.
Dimming is controled by changing the ratio of off to on time. You may
have noticed that LED Christmas tree light are a good bit dimmer than
the typical LED lamp found on an appliance like the computer you are
working at now. This is because the computer supplies a constant DC
voltage and the LED is on 100% of the time while the Christmas tree
lights are powered by a varying AC voltage that will only turn them on
for about 40% of the time.

On Sat, 25 Dec 2010 13:30:47 -0500, wrote:

On Fri, 24 Dec 2010 17:35:37 -0600, "
wrote:

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.


As usual you are not "completely right". I won't argue and say you
are wrong in your assertions - but my UNDERSTANDING is that PEAK
current controlls the visible light output, and average current
affects lifespan (due to junction heating). It is not totally linear.

Nope. Current controls the light output. Average current controls the
average light (which the eye detects, integrated over the "critical fusion
frequency"). Yes, peak current controls the peak light output, if you're
detecting peaks, this might be important. It is certainly *not* if you're
looking at it. Above the CFF, human eyes average the light intensity. Having
high peaks with long spaces does *nothing* to aid perception and in fact
reduces efficiency; LEDs are LESS efficient at high currents. Pulsing LEDs is
a lose-lose proposition.

My experience is obviously different than yours.
As in many other cases, I need to say that just because you haven't
seen it, don't make it wrong or impossible. Just means your scope is
too narrow.

No, you're just wrong. It's not the first time.

See: http://www.gardasoft.com/uploads/APP...ing%20LEDs.pdf

Did you actually *READ* that app note? An overdrive factor of *6* will
produce only 3 times the light (efficiency drops by half).

"The average current must be kept below the current rating for the LED."

IOW, you can't overdrive it for long.

The table "High Power LEDs" indicates that you can drive the LED up to 5x
current for 2ms, with a 10% duty cycle. A 5X current you get 2.5X the light
or ONE HALF the average light output as you would have gotten if you'd just
driven it at 100%, DC. IOW, a loser.

also http://www.lunaraccents.com/educational-LED-driver.html
and http://www.light-speed-tech.com/ltleds.htm
and
http://www.optoiq.com/index/machine-...intensity.html
and http://www.gardasoft.co.uk/
and http://www.smartvisionlights.com/pro...erdrive-series
and http://www.freepatentsonline.com/7639219.html
and
http://www.parameter.se/products/Def...D3=142&ID4=155

Are just a very few references for you to look at (commercial
applications of) pulsed overdrive applications for high luminence LED
applications.

As I've shown with the first article, pulsing LEDs is a loser. You're simply
*WRONG*.

On Sat, 25 Dec 2010 11:48:23 -0500, wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, "
wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

OK you have a string of LEDs dropping about 95% of the line voltage
and a resistor dropping the rest and limiting current now. How can
making that resistance more by adding a rheostat in series be more
inefficient?

Resistors heat == inefficient

On Sat, 25 Dec 2010 11:53:35 -0500, wrote:

On Fri, 24 Dec 2010 19:12:39 -0600, "
wrote:

On Fri, 24 Dec 2010 17:06:06 -0800 (PST), Red wrote:

On Dec 24, 11:50*am, "
wrote:


Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.


Note: http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. I have one and it
works great. I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Sure dimming LEDs is possible, and actually not all that hard if you start
with DC (the unit you linked is battery powered). Doing it from the AC line,
cheaply and efficiently, is more challenging. It's easy to do sloppily, but
then there is no gain over an incandescent bulb.

I still do not get how you arrive at that conclusion. A string of LEDS
will draw 15 - 20MA at full brightness and if you increase the size of
the current limiting resistor the current will drop from there in a
very linear manner.

Try it. You'll find that you are *very* wrong. LEDs are *not* in any way
linear. You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

If you use a current source it works, linearly, but is no more efficient
(think of the current source as a non-linear variable resistor).

On Sat, 25 Dec 2010 22:26:43 -0500, wrote:

On Sat, 25 Dec 2010 19:56:44 -0600, "
wrote:

On Sat, 25 Dec 2010 11:48:23 -0500, wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, "
wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

OK you have a string of LEDs dropping about 95% of the line voltage
and a resistor dropping the rest and limiting current now. How can
making that resistance more by adding a rheostat in series be more
inefficient?

Resistors heat == inefficient

But the resistor will always be there. You are just making a bigger
resistor, the current will drop and the light will dim in a vary
linear way.
The voltage you drop across your resistor will be the same no matter
how big it is. That is not like a rheostat on an incandescent where
you are changing the voltage applied todrop the filament.

The voltage across the resistor *does* change. Also, P=I^2R.

On Sat, 25 Dec 2010 22:29:03 -0500, wrote:

On Sat, 25 Dec 2010 20:04:00 -0600, "
wrote:

Try it. You'll find that you are *very* wrong. LEDs are *not* in any way
linear. You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

Why do you think the voltage across the resistor changes?

Because it does?

You have a
certain voltage dropped across each junction and the resistor takes
the rest.


The resistor is a current regulator not a voltage regulator.

On Dec 25, 10:04*pm, "
wrote:
On Sat, 25 Dec 2010 11:53:35 -0500, wrote:
On Fri, 24 Dec 2010 19:12:39 -0600, "
wrote:

On Fri, 24 Dec 2010 17:06:06 -0800 (PST), Red wrote:

On Dec 24, 11:50*am, "
wrote:

Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

Note:http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. *The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. *I have one and it
works great. *I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Sure dimming LEDs is possible, and actually not all that hard if you start
with DC (the unit you linked is battery powered). *Doing it from the AC line,
cheaply and efficiently, is more challenging. *It's easy to do sloppily, but
then there is no gain over an incandescent bulb.

I still do not get how you arrive at that conclusion. A string of LEDS
will draw 15 - 20MA at full brightness and if you increase the size of
the current limiting resistor the current will drop from there in a
very linear manner.

Try it. *You'll find that you are *very* wrong. *LEDs are *not* in any way
linear. *You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

If you use a current source it works, linearly, but is no more efficient
(think of the current source as a non-linear variable resistor).- Hide quoted text -

- Show quoted text -

Nothing linear about a diode, he should actually try ploting a curve
and see how much the resistance of a diode changes as you change the
current, even much more so on a series string of the things like a
lamp cluster.. Also no big deal to convert to DC to run an LED they
are after all diodes they will do it for you. Thats the really cheap
way out . Slightly more expesive and only arguably of better design is
using a match head size bridge rectifier, cost about as much as a
match too.

Jimmie

Jimmie.

On Sat, 25 Dec 2010 21:59:15 -0600, z wrote:

On Sat, 25 Dec 2010 22:29:03 -0500, wrote:

On Sat, 25 Dec 2010 20:04:00 -0600, "
wrote:

Try it. You'll find that you are *very* wrong. LEDs are *not* in any way
linear. You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

Why do you think the voltage across the resistor changes?

Because it does?

You have a
certain voltage dropped across each junction and the resistor takes
the rest.

sorry, new netbook and keyboard/trackpad is driving me nuts

No, that would assume an ideal diode. They are not ideal, the I-V
curve is not a vertical line. The more diodes you put in series and
the lower the voltage across the resistor, the more pronounced this
becomes. From some recent work, a blue LED at about 5mA drops about
3V. At 20mA the drop is closer to 3.3V. Now, put thirty of these in a
string and the difference is 10V. You only have 20V across the
resistor - it's changed 50%. ...and this is quite nonlinear.

The resistor is a current regulator not a voltage regulator.

A resistor regulates nothing. R==V/I. If you say that I is constant
because V is constant and V is constant because I is constant, you're
getting nowhere. ;-).

On Sat, 25 Dec 2010 20:14:45 -0800 (PST), JIMMIE
wrote:

On Dec 25, 10:04*pm, "
wrote:
On Sat, 25 Dec 2010 11:53:35 -0500, wrote:
On Fri, 24 Dec 2010 19:12:39 -0600, "
wrote:

On Fri, 24 Dec 2010 17:06:06 -0800 (PST), Red wrote:

On Dec 24, 11:50*am, "
wrote:

Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

Note:http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. *The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. *I have one and it
works great. *I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Sure dimming LEDs is possible, and actually not all that hard if you start
with DC (the unit you linked is battery powered). *Doing it from the AC line,
cheaply and efficiently, is more challenging. *It's easy to do sloppily, but
then there is no gain over an incandescent bulb.

I still do not get how you arrive at that conclusion. A string of LEDS
will draw 15 - 20MA at full brightness and if you increase the size of
the current limiting resistor the current will drop from there in a
very linear manner.

Try it. *You'll find that you are *very* wrong. *LEDs are *not* in any way
linear. *You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

If you use a current source it works, linearly, but is no more efficient
(think of the current source as a non-linear variable resistor).- Hide quoted text -

- Show quoted text -

Nothing linear about a diode, he should actually try ploting a curve
and see how much the resistance of a diode changes as you change the
current, even much more so on a series string of the things like a
lamp cluster.. Also no big deal to convert to DC to run an LED they
are after all diodes they will do it for you. Thats the really cheap
way out . Slightly more expesive and only arguably of better design is
using a match head size bridge rectifier, cost about as much as a
match too.

No, they are diodes, but it's not a good idea to use them as
rectifiers. The reverse breakdown of LEDs is pretty poor. As you
note, a bridge rectifier is cheap enough. The probelm with running
them on DC is storing the energy over the cycle (filtering the DC).
That isn't cheap or particularly efficient.

On Sat, 25 Dec 2010 23:25:40 -0500, wrote:

On Sat, 25 Dec 2010 22:15:14 -0600, z wrote:

On Sat, 25 Dec 2010 21:59:15 -0600, z wrote:

On Sat, 25 Dec 2010 22:29:03 -0500, wrote:

On Sat, 25 Dec 2010 20:04:00 -0600, "
wrote:

Try it. You'll find that you are *very* wrong. LEDs are *not* in any way
linear. You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

Why do you think the voltage across the resistor changes?

Because it does?

You have a
certain voltage dropped across each junction and the resistor takes
the rest.

sorry, new netbook and keyboard/trackpad is driving me nuts

No, that would assume an ideal diode. They are not ideal, the I-V
curve is not a vertical line. The more diodes you put in series and
the lower the voltage across the resistor, the more pronounced this
becomes. From some recent work, a blue LED at about 5mA drops about
3V. At 20mA the drop is closer to 3.3V. Now, put thirty of these in a
string and the difference is 10V. You only have 20V across the
resistor - it's changed 50%. ...and this is quite nonlinear.

The resistor is a current regulator not a voltage regulator.

A resistor regulates nothing. R==V/I. If you say that I is constant
because V is constant and V is constant because I is constant, you're
getting nowhere. ;-).


Lets get to the bottom line. Are you trying to say a dimmed LED (by
increasing the size of the resistor) draws more current than one at
full brightness?

Of course not. I'm saying that it is NOT LINEAR and NOT EFFICIENT.

You may have a more efficient way of dimming a LED lamp but how many
decades would it take to get your money back over just using a
rheostat? Sometimes KISS is the best rule.

A reostat is a BAD idea. The worst, in fact. It's easy enough to
perform this electronically, but the results aren't good. Doing the
job right is somewhat more difficult.

I like doing experiments like this and I will be back as soon as my
high intensity LEDs arrive because this is my plan. We will see.
I have no problem admitting when I am wrong but I want to see it.

Go for it. I do this stuff all the time, though with indicators, not
for illumination.

I just did one where I used current sources instead of resistors to
eek out the last couple of tenths of a volt on the supply. It was an
application where we were using red and green LEDs but the owner like
blue but the product is LiIon powered so we have to operate from 3.6V
to 5V (when charging we run off the input). The current source
allowed a constant intensity over this range (3.6V is pushing it)
while having suffiient drive at the low end.

On Dec 26, 12:27*am, wrote:
On Sat, 25 Dec 2010 20:14:45 -0800 (PST), JIMMIE





wrote:
On Dec 25, 10:04*pm, "
wrote:
On Sat, 25 Dec 2010 11:53:35 -0500, wrote:
On Fri, 24 Dec 2010 19:12:39 -0600, "
wrote:

On Fri, 24 Dec 2010 17:06:06 -0800 (PST), Red wrote:

On Dec 24, 11:50*am, "
wrote:

Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. *If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. *The range of adjustment will be very small.
[*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear

* I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

Note:http://joby.com/store/gorillatorch/switchback

Joby makes several models of quality dimmable LED lights, but as you
said they are not cheap. *The one in the link will dim thru a range of
5 - 130 lumens, mostly linear and without flicker. *I have one and it
works great. *I have no idea of the circuitry they use to dim the LED
and I'm not going to take it apart to find out.
This response is just to let the doubters know that the technology to
dim LED's is real and available.

Sure dimming LEDs is possible, and actually not all that hard if you start
with DC (the unit you linked is battery powered). *Doing it from the AC line,
cheaply and efficiently, is more challenging. *It's easy to do sloppily, but
then there is no gain over an incandescent bulb.

I still do not get how you arrive at that conclusion. A string of LEDS
will draw 15 - 20MA at full brightness and if you increase the size of
the current limiting resistor the current will drop from there in a
very linear manner.

Try it. *You'll find that you are *very* wrong. *LEDs are *not* in any way
linear. *You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

If you use a current source it works, linearly, but is no more efficient
(think of the current source as a non-linear variable resistor).- Hide quoted text -

- Show quoted text -

Nothing linear about a diode, he should actually try ploting a curve
and see how much the resistance of a diode changes as you change the
current, even much more so on a series string of the things like a
lamp cluster.. Also no big deal to convert to DC to run an LED they
are after all diodes they will do it for you. Thats the really cheap
way out . Slightly more expesive and only arguably of better design is
using a match head size bridge rectifier, cost about as much as a
match too.

No, they are diodes, but it's not a good idea to use them as
rectifiers. *The reverse breakdown of LEDs is pretty poor. *As you
note, a bridge rectifier is cheap enough. *The probelm with running
them on DC is storing the energy over the cycle (filtering the DC).
That isn't cheap or particularly efficient. *- Hide quoted text -

- Show quoted text -

The string itself is a rectifier. The current through them will be DC,
no reason to further rectify it.. In the case of a string of xmas
lights they have nearly the full line voltage across them. Works fine.

Jimmie

On Sat, 25 Dec 2010 19:54:58 -0600, "
wrote:

On Sat, 25 Dec 2010 13:30:47 -0500, wrote:

On Fri, 24 Dec 2010 17:35:37 -0600, "
wrote:

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.


As usual you are not "completely right". I won't argue and say you
are wrong in your assertions - but my UNDERSTANDING is that PEAK
current controlls the visible light output, and average current
affects lifespan (due to junction heating). It is not totally linear.

Nope. Current controls the light output. Average current controls the
average light (which the eye detects, integrated over the "critical fusion
frequency"). Yes, peak current controls the peak light output, if you're
detecting peaks, this might be important. It is certainly *not* if you're
looking at it. Above the CFF, human eyes average the light intensity. Having
high peaks with long spaces does *nothing* to aid perception and in fact
reduces efficiency; LEDs are LESS efficient at high currents. Pulsing LEDs is
a lose-lose proposition.

My experience is obviously different than yours.
As in many other cases, I need to say that just because you haven't
seen it, don't make it wrong or impossible. Just means your scope is
too narrow.

No, you're just wrong. It's not the first time.

See: http://www.gardasoft.com/uploads/APP...ing%20LEDs.pdf

Did you actually *READ* that app note? An overdrive factor of *6* will
produce only 3 times the light (efficiency drops by half).

"The average current must be kept below the current rating for the LED."

IOW, you can't overdrive it for long.

The table "High Power LEDs" indicates that you can drive the LED up to 5x
current for 2ms, with a 10% duty cycle. A 5X current you get 2.5X the light
or ONE HALF the average light output as you would have gotten if you'd just
driven it at 100%, DC. IOW, a loser.

also http://www.lunaraccents.com/educational-LED-driver.html
and http://www.light-speed-tech.com/ltleds.htm
and
http://www.optoiq.com/index/machine-...intensity.html
and http://www.gardasoft.co.uk/
and http://www.smartvisionlights.com/pro...erdrive-series
and http://www.freepatentsonline.com/7639219.html
and
http://www.parameter.se/products/Def...D3=142&ID4=155

Are just a very few references for you to look at (commercial
applications of) pulsed overdrive applications for high luminence LED
applications.

As I've shown with the first article, pulsing LEDs is a loser. You're simply
*WRONG*.


I refuse to argue with an idiot

On Sat, 25 Dec 2010 21:56:12 -0600, z wrote:

On Sat, 25 Dec 2010 22:26:43 -0500, wrote:

On Sat, 25 Dec 2010 19:56:44 -0600, "
wrote:

On Sat, 25 Dec 2010 11:48:23 -0500, wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, "
wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

OK you have a string of LEDs dropping about 95% of the line voltage
and a resistor dropping the rest and limiting current now. How can
making that resistance more by adding a rheostat in series be more
inefficient?

Resistors heat == inefficient

But the resistor will always be there. You are just making a bigger
resistor, the current will drop and the light will dim in a vary
linear way.
The voltage you drop across your resistor will be the same no matter
how big it is. That is not like a rheostat on an incandescent where
you are changing the voltage applied todrop the filament.

The voltage across the resistor *does* change. Also, P=I^2R.
The voltage drop across a diode is relatively constant, so the
voltage across the resistor has to also be relatively constant, with
only the current being changed by the change of resistance. This is
not 100% accurate, but for this discussion I believe it is close
enough.

Not agueing with krw - just agreeing (to a point) with gfretwell. I
say "to a point" because it is not totally linear. Much more linear
than some would have you believe in a DC circuit.

On Sun, 26 Dec 2010 00:38:39 -0500, wrote:

On Sat, 25 Dec 2010 19:54:58 -0600, "
wrote:

On Sat, 25 Dec 2010 13:30:47 -0500, wrote:

On Fri, 24 Dec 2010 17:35:37 -0600, "
wrote:

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.


As usual you are not "completely right". I won't argue and say you
are wrong in your assertions - but my UNDERSTANDING is that PEAK
current controlls the visible light output, and average current
affects lifespan (due to junction heating). It is not totally linear.

Nope. Current controls the light output. Average current controls the
average light (which the eye detects, integrated over the "critical fusion
frequency"). Yes, peak current controls the peak light output, if you're
detecting peaks, this might be important. It is certainly *not* if you're
looking at it. Above the CFF, human eyes average the light intensity. Having
high peaks with long spaces does *nothing* to aid perception and in fact
reduces efficiency; LEDs are LESS efficient at high currents. Pulsing LEDs is
a lose-lose proposition.

My experience is obviously different than yours.
As in many other cases, I need to say that just because you haven't
seen it, don't make it wrong or impossible. Just means your scope is
too narrow.

No, you're just wrong. It's not the first time.

See: http://www.gardasoft.com/uploads/APP...ing%20LEDs.pdf

Did you actually *READ* that app note? An overdrive factor of *6* will
produce only 3 times the light (efficiency drops by half).

"The average current must be kept below the current rating for the LED."

IOW, you can't overdrive it for long.

The table "High Power LEDs" indicates that you can drive the LED up to 5x
current for 2ms, with a 10% duty cycle. A 5X current you get 2.5X the light
or ONE HALF the average light output as you would have gotten if you'd just
driven it at 100%, DC. IOW, a loser.

also http://www.lunaraccents.com/educational-LED-driver.html
and http://www.light-speed-tech.com/ltleds.htm
and
http://www.optoiq.com/index/machine-...intensity.html
and http://www.gardasoft.co.uk/
and http://www.smartvisionlights.com/pro...erdrive-series
and http://www.freepatentsonline.com/7639219.html
and
http://www.parameter.se/products/Def...D3=142&ID4=155

Are just a very few references for you to look at (commercial
applications of) pulsed overdrive applications for high luminence LED
applications.

As I've shown with the first article, pulsing LEDs is a loser. You're simply
*WRONG*.


I refuse to argue with an idiot

As usual, when you're shown to be wrong you run away with fingers in
your ears rather than admitting it and learning something. You *are*
an idiot.

On Sun, 26 Dec 2010 00:45:29 -0500, wrote:

On Sat, 25 Dec 2010 21:56:12 -0600, z wrote:

On Sat, 25 Dec 2010 22:26:43 -0500, wrote:

On Sat, 25 Dec 2010 19:56:44 -0600, "
wrote:

On Sat, 25 Dec 2010 11:48:23 -0500, wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, "
wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

OK you have a string of LEDs dropping about 95% of the line voltage
and a resistor dropping the rest and limiting current now. How can
making that resistance more by adding a rheostat in series be more
inefficient?

Resistors heat == inefficient

But the resistor will always be there. You are just making a bigger
resistor, the current will drop and the light will dim in a vary
linear way.
The voltage you drop across your resistor will be the same no matter
how big it is. That is not like a rheostat on an incandescent where
you are changing the voltage applied todrop the filament.

The voltage across the resistor *does* change. Also, P=I^2R.
The voltage drop across a diode is relatively constant, so the
voltage across the resistor has to also be relatively constant, with
only the current being changed by the change of resistance. This is
not 100% accurate, but for this discussion I believe it is close
enough.

It is *NOT* "relatively constant" when you multiply the change times
the number of LEDs in a 120V string and compare that to the voltage
across the balast resistor. If you string a few together and have a
large balast resistor it matters less but you're simply wasting that
much more power, losing gains you made by using LEDs in the forst
place.

Not agueing with krw

Of course not. You could *NEVER* admit that you're wrong.

- just agreeing (to a point) with gfretwell. I
say "to a point" because it is not totally linear. Much more linear
than some would have you believe in a DC circuit.

Of course you would be WRONG, as usual.

On Sun, 26 Dec 2010 00:52:51 -0500, wrote:

On Sat, 25 Dec 2010 22:36:45 -0600, z wrote:



Lets get to the bottom line. Are you trying to say a dimmed LED (by
increasing the size of the resistor) draws more current than one at
full brightness?

Of course not. I'm saying that it is NOT LINEAR and NOT EFFICIENT.


I guess the point is LEDs are so efficient to start with and dimming
them will cut the draw even more, why make things hard on yourself to
squeeze out an extra few pennies a year.

I could say the same thing about replacing incandescents in the first
place. Why, for a few mennies a year. In fact, that's what I do.

If you are replacing an incandescent any thing you do has to be
better.

Not necessarily true. You still haven't come up with a good scheme to
dim them.