Dimming LEDs (in bulbs that are designed to be dimmed) - why the minimum brightness?

[NY]

I've noticed that various LEDs which are designed to be dimmed have a
minimum brightness - if you dim them from bright, they go gradually dimmer
and then turn off while still partly lit; if you increase the brightness
from zero, they come on suddenly at partly lit, and then increase from
there.

This applies to Philips Hue bulbs and to various makes of Christmas tree
lights. Is there some inherent design problem with a minimum brightness
below which the LED will not produce any light? Given that the LEDs produce
high-frequency flashing light (you can see separate images of your finger if
you move it quickly across the light) I'd have thought that the dimming is
achieved by varying the mark:space ratio. Will LEDs not turn on if the M:S
ratio is below a certain level? I could understand it if the LEDs were fed
with DC from a constant-voltage/variable-current supply, but that would not
produce the multiple images when you move your eyes, so evidently that's not
how these lights dim the LEDs.

[Rod Speed]

NY <m...@privacy.invalid> wrote

> I've noticed that various LEDs which are designed to be dimmed have a
> minimum brightness - if you dim them from bright, they go gradually dimmer
> and then turn off while still partly lit; if you increase the brightness
> from zero, they come on suddenly at partly lit, and then increase from
> there.

> This applies to Philips Hue bulbs and to various makes of Christmas tree
> lights. Is there some inherent design problem with a minimum brightness
> below which the LED will not produce any light?

Nope, you can get very dim leds with individual
leds with very high resistances that limit the current.

Likely the current regulator is the problem with those that do it that way.

> Given that the LEDs produce high-frequency flashing light

Not all of them do.

> (you can see separate images of your finger if you move it quickly across
> the light) I'd have thought that the dimming is achieved by varying the
> mark:space ratio.

Yes.

> Will LEDs not turn on if the M:S ratio is below a certain level?

Nope.

> I could understand it if the LEDs were fed with DC from a
> constant-voltage/variable-current supply,

Depends on the current regulator.

> but that would not produce the multiple images when you move your eyes,

Your finger.

> so evidently that's not how these lights dim the LEDs.

Some do.

[Dave Plowman (News)]

In article <sol1d7$fm0$2...@dont-email.me>,

NY <m...@privacy.invalid> wrote:
> I've noticed that various LEDs which are designed to be dimmed have a
> minimum brightness - if you dim them from bright, they go gradually dimmer
> and then turn off while still partly lit; if you increase the brightness
> from zero, they come on suddenly at partly lit, and then increase from
> there.

That seems to be a function of LEDs. I converted the interior lighting in
the old car to LED. Nice to have lighting bright enough to read by without
burning the lamp housings of the old 6 watt units.
And I added a fade in, fade out device. Which just ramps up and ramps down
the current flowing through basic LEDs. They fade out nicely - but come on
with a jump.

--
*Indian Driver - Smoke signals only*

Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.

[Martin Brown]

On 06/12/2021 12:57, NY wrote:
> I've noticed that various LEDs which are designed to be dimmed have a
> minimum brightness - if you dim them from bright, they go gradually
> dimmer and then turn off while still partly lit; if you increase the
> brightness from zero, they come on suddenly at partly lit, and then
> increase from there.

The problem is with the control circuitry not the LED itself.

Making dimmable LEDs on mains is somewhat tricky since the most obvious
design will simply draw ever more current to provide the required
constant output if you modulate the incoming waveform by chopping part
of it off. My friend with dimmable LEDs and officially compatible
controllers seems to blow them up with monotonous regularity.

The work around to make is somewhat imperfect. The small load LEDs
present can also cause problems for the controller as there is a strict
minimum power that it is able to control reliably. I suspect that is why
it snaps on at a particular brightness setting.

> This applies to Philips Hue bulbs and to various makes of Christmas tree
> lights. Is there some inherent design problem with a minimum brightness
> below which the LED will not produce any light? Given that the LEDs
> produce high-frequency flashing light (you can see separate images of
> your finger if you move it quickly across the light) I'd have thought
> that the dimming is achieved by varying the mark:space ratio. Will LEDs
> not turn on if the M:S ratio is below a certain level? I could
> understand it if the LEDs were fed with DC from a
> constant-voltage/variable-current supply, but that would not produce the
> multiple images when you move your eyes, so evidently that's not how
> these lights dim the LEDs.

The requirement is for the control electronics to figure out what
brightness you actually want and then they add some hysteresis so that
it doesn't hunt. The result is that you have a dead band.

A bare modern high efficiency LED with 10uA flowing through it is bright
enough to see by once you are dark adapted. Some designs of domestic
mains LED bulbs can pick up enough capacitively coupled current to glow
very faintly even when nominally off (particularly when on 2 way
switches with a nice long run of cable between them).

--
Regards,
Martin Brown

[Peeler]

On Tue, 7 Dec 2021 01:30:33 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

<FLUSH the abnormal trolling senile cretin's latest trollshit unread>

WTF are you doing in European ngs, you deranged senile troll from Oz?

--
Bod addressing senile Rodent:
"Rod, you have a sick twisted mind. I suggest you stop your mindless
and totally irresponsible talk. Your mouth could get you into a lot of
trouble."
Message-ID: <gfbb94...@mid.individual.net>

[Pamela]

On 12:57 6 Dec 2021, NY said:
>
> This applies to Philips Hue bulbs and to various makes of Christmas
> tree lights. Is there some inherent design problem with a minimum
> brightness below which the LED will not produce any light?

I recently got a bundled bargain from Amazon which included a white
Philips Hue bulb for £2. I was surprised to find they nomally cost £15
or more as it doesn't seem all that special.

It's only real use seems as a ceiling pendant bulb. To get a
wifi-switched bulb in a standard lamp, you may as well use a smartplug
-- which can be transferred to other standard lamps as needed.

Am I overlooking something?

[Andy Burns]

Pamela wrote:

> I recently got a bundled bargain from Amazon which included a white
> Philips Hue bulb for �2. I was surprised to find they nomally cost �15
> or more as it doesn't seem all that special.

<paulhibbert>corporate greed</ph>

[Peeler]

On Tue, 7 Dec 2021 08:26:12 +1100, cantankerous trolling geezer Rodent

Speed, the auto-contradicting senile sociopath, blabbered, again:

<FLUSH the abnormal trolling senile cretin's latest trollshit unread>

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asshole.
MID: <ev1p6ml7ywd5$.d...@sqwertz.com>

[Rod Speed]

Pamela <pamela.priv...@gmail.com> wrote
> NY wrote

>>
>> This applies to Philips Hue bulbs and to various makes of Christmas
>> tree lights. Is there some inherent design problem with a minimum
>> brightness below which the LED will not produce any light?
>
> I recently got a bundled bargain from Amazon which included a white
> Philips Hue bulb for £2. I was surprised to find they nomally cost £15
> or more as it doesn't seem all that special.

The color bulbs are.

> It's only real use seems as a ceiling pendant bulb.

That’s bullshit, you can use them in any bulb holder.

> To get a wifi-switched bulb in a standard lamp, you may as well use a
> smartplug -- which can be transferred to other standard lamps as needed.

That’s nothing like as convenient and you don’t need a dimmer.

And you can use switches that have no wires and no batterys
which be trivially placed anywhere you like too.

> Am I overlooking something?

Yep, as always.

[Brian Gaff (Sofa)]

Hysteresis perhaps? I don't know much about modern leds, but I was under the
impression that the white ones actually contain more than one and the
linearity of light is very poor at below certain currents, but if as you say
its done by duty cycle, then maybe there is more to it than just non
linearity, perhaps actual switch on voltage varies and although it could be
a square wave its more likely to be a ramp.
Brian

--

This newsgroup posting comes to you directly from...
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Note this Signature is meaningless.!
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news:sol1d7$fm0$2...@dont-email.me...

[Dave Plowman (News)]

In article <son8b8$sun$1...@dont-email.me>,

Brian Gaff \(Sofa\) <bri...@blueyonder.co.uk> wrote:
> Hysteresis perhaps? I don't know much about modern leds, but I was under the
> impression that the white ones actually contain more than one and the
> linearity of light is very poor at below certain currents,

It's true that once LEDs used a few of different colours to get white. And
this might still be the case with cheap ones.

Decent ones these days can give an approximation of daylight or warm white
in a single LED. For ordinary domestic use.

Think if you want a decent continuous spectrum luminair - like for film
etc work, you may well use a mixture of white and spot colour LEDs. Like
found in operating theatres.

--
*Beauty is in the eye of the beer holder *

[Paul]

On 12/7/2021 4:08 AM, Brian Gaff (Sofa) wrote:
> Hysteresis perhaps? I don't know much about modern leds, but I was under the
> impression that the white ones actually contain more than one and the
> linearity of light is very poor at below certain currents, but if as you say
> its done by duty cycle, then maybe there is more to it than just non
> linearity, perhaps actual switch on voltage varies and although it could be
> a square wave its more likely to be a ramp.
> Brian
>

White LEDs are a blue LED with a phosphor coating.
Blue light is energetic, and makes good "excitation"
for the phosphor.

It's a single LED diode, as an electrical device.
The Vf and If are coupled, so if the If went from
20mA to 1000mA, you would expect the Vf to go above
the 2.5 volt value, to maybe 3V or 3.5V .

The linearity of light, is much better using the
PWM method, than using the "resistive limiter" method.
To make the "last step" in the PWM method seem
less abrupt, you simply need to run the circuit with
a faster clock. (Like on for 1 cycle, off for 9999 cycles,
repeat at least 200 times a second, so 2MHz.)

Large LEDs can be switched at 10MHz or so. This was
a relatively popular project to set up free space optical
networks. They were switching LEDs at that speed.

http://ronja.twibright.com/

But if much smaller LEDs are used, they can be switched
at 1GHz to 3GHz, for in-room communications. The 10MHz project
is switching higher power devices.

The most current I've ever heard of, going through a LED,
was around 17 amps. It was done, by sweat soldering a power
LED to a copper anvil, so that the copper anvil could
act as a heatsink while the LED was tested for a few seconds.
The LED doesn't actually put out that much extra light when
abused like that. If doesn't put out 17x as much light as it
would at 1 ampere. Maybe the light is only 3x as great.

Many of the light bulbs you see, use array LEDs, which raises
the forward voltage up to levels better suited to mains powering
solutions. You can still PWM such things.

I would think, using a DC supply and your own extended-count PWM
design, you could make the last vestige of light very close to zero.
So the last step didn't seem as abrupt.

Modern power supplies, some of them "cheat" to remain stable. A
coveted rating, would be for an ATX computer power supply, to run
with no load on it at all. But inside the supply, sometimes
there is a 5W to 10W load, which ensures stability when the
open circuit test is done. So they don't really run at zero load.
The SMPS PSU inside a LED lightbulb, might have similar issues,
where its behavior when PWMing to extremes, uncovers a non-linear
part of the control loop. Simply squelching the operation of
the circuit, is "safer" than using a resistive load to bodge
the thing to look like it runs down to zero.

You'd need Big Clive to tear one of these apart, to see
what circuit choices they've made. I actually avoid dimmable
bulbs, because in non-dimming applications, they fret a bit
and "wobble". I want a nice light that puts out a constant
level, all the time it is operating, with no signs of
jitter in the output light level.

I've even had bulbs here exhibiting "coil noise". And those
eventually, I got sick of that, and replaced them with
a similar bulb not making that noise.

Paul

[Rod Speed]

Paul <nos...@needed.invalid> wrote

> Brian Gaff (Sofa) wrote

>> Hysteresis perhaps? I don't know much about modern leds, but I was under
>> the
>> impression that the white ones actually contain more than one and the
>> linearity of light is very poor at below certain currents, but if as you
>> say
>> its done by duty cycle, then maybe there is more to it than just non
>> linearity, perhaps actual switch on voltage varies and although it could
>> be
>> a square wave its more likely to be a ramp.

Don’t get anything like that with the Philips Hue leds.

[Peeler]

On Wed, 8 Dec 2021 06:16:07 +1100, cantankerous trolling geezer Rodent

[Unknown]

NY submitted this idea :

It can happen with some dimmers and lamp combination, it can even vary
with one lamp or two on one dimmer. My present living room wall lights,
which I have just swapped to higher wattage LED Lumilife lamps, gave
[roblems with the existing dimmer, so I swapped the dimmer for a LAP
auto selecting positive or negative switch. That works fine for the one
with two lamps, but there is a single walllight too - That goes out
every few seconds briefly, on the full setting.

[Dave Plowman (News)]

In article <soqi70$10i$1...@dont-email.me>,

Harry Bloomfield wrote:
> It can happen with some dimmers and lamp combination, it can even vary
> with one lamp or two on one dimmer. My present living room wall lights,
> which I have just swapped to higher wattage LED Lumilife lamps, gave
> [roblems with the existing dimmer, so I swapped the dimmer for a LAP
> auto selecting positive or negative switch. That works fine for the one
> with two lamps, but there is a single walllight too - That goes out
> every few seconds briefly, on the full setting.

I have a V-Pro dimmer running a single LED - 100 watt equivalent. With the
V-Pro, you can programme the maximum and minimum setting. I've been able
to set the minimum one OK - but not the maximum. The LED comes up to full
brightness about 2/3rds of the knob travel. But if you put it to full, the
light will go out and come on again at random. There are two other modules
on the same plate that work as intended - but with a higher load. Not got
round to swapping them, to see if it's a fault or 'feature'.

--
*How does Moses make his tea? Hebrews it.*

[Unknown]

Dave Plowman (News) brought next idea :

> With the
> V-Pro, you can programme the maximum and minimum setting.

Mine start to show some light at about 10 degrees clockwise from off,
just the barest glimmer if the room is completely dark. There is a one
second delay from switching on, before the LED's light up, I guess
that's the 'automatic polarity' working out what to offer the LED.