Q: how find specific LED

The idea is to have a battery monitor light that will not unduly run
down the battery.

How about blinking the LED at 25 percent duty cycle...or even lower

 

[George Herold]

On Wednesday, March 11, 2020 at 11:13:35 PM UTC-4, Robert Baer wrote:

George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4, jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

George H.



I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

A good LED is visible in modest office light at 1 uA.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

What did you use ("photocurrent")? Motofotoplier?
The best i had for sensitivity was that ugly CdS fotoresistor.

This would have been a photodiode into a TIA... (no bias voltage :^)
and fet opamp. With 1 Gig or 100 Meg ohm FB resistor for the lowest
ranges.
You can get 1 Gig R's for ~$0.2 in onsies. 10 gig cost several
bucks... just went shopping.

George H.

 

bulegoge@columbus.rr.com wrote in
news:d1b625a6-7c9c-4cf4-bed2-81de72fff2a9@googlegroups.com:

The idea is to have a battery monitor light that will not
unduly run
down the battery.

How about blinking the LED at 25 percent duty cycle...or even
lower

Yeah... I know... I know... I still thought it was funny.

<https://www.youtube.com/watch?v=PsUsByrOveE>

Don't know what it will unduly run down, but it won't be your
battery.

 

[Phil Hobbs]

On 2020-03-12 10:20, George Herold wrote:

On Wednesday, March 11, 2020 at 11:13:35 PM UTC-4, Robert Baer wrote:
George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4, jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

George H.



I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

A good LED is visible in modest office light at 1 uA.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

What did you use ("photocurrent")? Motofotoplier?
The best i had for sensitivity was that ugly CdS fotoresistor.

This would have been a photodiode into a TIA... (no bias voltage :^)
and fet opamp. With 1 Gig or 100 Meg ohm FB resistor for the lowest
ranges.
You can get 1 Gig R's for ~$0.2 in onsies. 10 gig cost several
bucks... just went shopping.

George H.

We used a 50G 30% 0805 awhile back, $3 in onesies. It was for that nice
surface voltage tool for semiconductors.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-03-11 09:11, George Herold wrote:

On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

Mouser online catalog was not helpful, and i seriously doubt DigiKey
would be better.
I see specs like 45mcd unknown current, 5000mcd 30mA, 2000mcd 20mA,
high intensity red, etc.

I got a little desperate, bought various Cree 5mm clear red LEDs
apparently all 500mcd 20mA...but dim as all heck at 50uA.
One of their standard LEDs was twice as bright as some of their high
intensity LEDs.
And i stupidly thought that Cree had brighter LEDs.
ROHM and Everlight look better, but i need to use 250uA to drive them
to visually similar intensity WRT my "reference".

Hell, the LED in the Dollar Tree LED Flashlight and Lantern1-3/4 LE
Combination SKU: 286458 visually equaled the intensity at 25uA (white,
clear, gigantic size).

Would like red, 5mm, bright at 50uA.

Ideas?
Thanks

Digikey's search engine is ~100x better than mouser's.
I'm not sure any place will give you the intensity at such low current.

Is there some reason you can't increase the current?

George H.

IME modern T1-3/4 LEDs usually have a knee at about 10 uA and are pretty
linear above that. So once the OP figures out what he means by
'reasonably bright', especially the solid angle/brightness tradeoff,
it's reasonable to take the datasheet candela value, multiply by the ratio

(50 uA - 10 uA knee)/(datasheet current).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, March 12, 2020 at 5:58:39 AM UTC-4, Martin Brown wrote:

On 11/03/2020 15:28, George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4, jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

I suspect the curve tends towards a slope of 2 when the drive current is
low and source impedance becomes comparable with the leakage resistance
in the device so that both I and V are falling faster and together.

Your real physical LED is an ideal perfect LED in parallel with a high
but not quite negligible leakage resistance which acts as a potential
divider once the drive current gets small.

Hi Martin, that's a nice simple model, but I'm not sure it explains the data.
For the green led the slope is ~3/2 from ~100 uA to 100 nA.
I mumble, hand wave and say 'non-radiative recombination', but I don't
know. The only sorta model I can imagine is a bunch of resistors
and series batteries with the battery voltage near the band-gap voltage.
All that in parallel with the ideal LED.

But still not very satisfying.

At the physics level I think about the density of carriers
in the conduction band and some time diffusing around
looking for a hole to recombine with. If you think about
each electron being in some area, that might give you a
term at low current that goes as I_led^2.
The LED manufacturers might have a good picture of what's going on...
but it's hard to find that info on the web.

George H.

--
Regards,
Martin Brown

 

[George Herold]

On Thursday, March 12, 2020 at 11:21:51 AM UTC-4, Phil Hobbs wrote:

On 2020-03-11 09:11, George Herold wrote:
On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

Mouser online catalog was not helpful, and i seriously doubt DigiKey
would be better.
I see specs like 45mcd unknown current, 5000mcd 30mA, 2000mcd 20mA,
high intensity red, etc.

I got a little desperate, bought various Cree 5mm clear red LEDs
apparently all 500mcd 20mA...but dim as all heck at 50uA.
One of their standard LEDs was twice as bright as some of their high
intensity LEDs.
And i stupidly thought that Cree had brighter LEDs.
ROHM and Everlight look better, but i need to use 250uA to drive them
to visually similar intensity WRT my "reference".

Hell, the LED in the Dollar Tree LED Flashlight and Lantern1-3/4 LE
Combination SKU: 286458 visually equaled the intensity at 25uA (white,
clear, gigantic size).

Would like red, 5mm, bright at 50uA.

Ideas?
Thanks

Digikey's search engine is ~100x better than mouser's.
I'm not sure any place will give you the intensity at such low current.

Is there some reason you can't increase the current?

George H.

IME modern T1-3/4 LEDs usually have a knee at about 10 uA and are pretty
linear above that. So once the OP figures out what he means by
'reasonably bright', especially the solid angle/brightness tradeoff,
it's reasonable to take the datasheet candela value, multiply by the ratio

(50 uA - 10 uA knee)/(datasheet current).

Hi Phil, Got a name/ part number of a data sheet that I could look at?
At one point in the past I wanted a light source that would be fairly
linear in light output.... (which was why I took the LED data above.)
It would be nice if some LED was linear in current from 10 uA to 1 mA...
All the data sheets I look at have a lower current of 1 mA.

George h.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-03-12 11:38, George Herold wrote:

On Thursday, March 12, 2020 at 11:21:51 AM UTC-4, Phil Hobbs wrote:
On 2020-03-11 09:11, George Herold wrote:
On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

Mouser online catalog was not helpful, and i seriously doubt DigiKey
would be better.
I see specs like 45mcd unknown current, 5000mcd 30mA, 2000mcd 20mA,
high intensity red, etc.

I got a little desperate, bought various Cree 5mm clear red LEDs
apparently all 500mcd 20mA...but dim as all heck at 50uA.
One of their standard LEDs was twice as bright as some of their high
intensity LEDs.
And i stupidly thought that Cree had brighter LEDs.
ROHM and Everlight look better, but i need to use 250uA to drive them
to visually similar intensity WRT my "reference".

Hell, the LED in the Dollar Tree LED Flashlight and Lantern1-3/4 LE
Combination SKU: 286458 visually equaled the intensity at 25uA (white,
clear, gigantic size).

Would like red, 5mm, bright at 50uA.

Ideas?
Thanks

Digikey's search engine is ~100x better than mouser's.
I'm not sure any place will give you the intensity at such low current.

Is there some reason you can't increase the current?

George H.

IME modern T1-3/4 LEDs usually have a knee at about 10 uA and are pretty
linear above that. So once the OP figures out what he means by
'reasonably bright', especially the solid angle/brightness tradeoff,
it's reasonable to take the datasheet candela value, multiply by the ratio

(50 uA - 10 uA knee)/(datasheet current).

Hi Phil, Got a name/ part number of a data sheet that I could look at?
At one point in the past I wanted a light source that would be fairly
linear in light output.... (which was why I took the LED data above.)
It would be nice if some LED was linear in current from 10 uA to 1 mA...
All the data sheets I look at have a lower current of 1 mA.

Haven't made a homebrew optocoupler in a decade or so, but these ones
look pretty good.

http://www.kingbrightusa.com/images/catalog/SPEC/WP7113LZGCK.pdf

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Thu, 12 Mar 2020 07:20:45 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Wednesday, March 11, 2020 at 11:13:35 PM UTC-4, Robert Baer wrote:
George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4, jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

George H.



I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

A good LED is visible in modest office light at 1 uA.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

What did you use ("photocurrent")? Motofotoplier?
The best i had for sensitivity was that ugly CdS fotoresistor.

This would have been a photodiode into a TIA... (no bias voltage :^)
and fet opamp. With 1 Gig or 100 Meg ohm FB resistor for the lowest
ranges.

Are you confident that the photodiode was linear? That could be tested
with filters.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[whit3rd]

On Wednesday, March 11, 2020 at 7:33:10 PM UTC-7, mpm wrote:

We recently made a small power distribution board. Nothing special, just an "in" and several "outs", all thru-hole connectorized to make things neat and tidy. This was for low-voltage DC on the order of a couple of amps per output, so some decent size connectors one could easily solder to the boards.

I asked the design engineer to place a simple LED & dropping resistor on the power rail so we could tell at a glance whether the board was powered-up, once installed in the product.

You guessed it: He picks a tiny 0402 size LED and resistor that you need a microscope to solder.

There's TWO reasons that was bad design; not only is it hard to hand-place the parts,
but a connector puts stress on the board, bending it slightly whenever your
connectors are manipulated. A surface-mount part usually doesn't have much strain
relief, you can crack the parts in normal operation. A bent-leads through-hole resistor
has a lot of compliance to deal with stress (and with a suitable standoff, or lead-forming,
so does a leaded LED).

 

[Jon Elson]

On Tue, 10 Mar 2020 20:43:14 -0700, Robert Baer wrote:

I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA and
it failed. Have no other than this.



Would like red, 5mm, bright at 50uA.

Get the smallest physical diode you can. So, you should look for lower
maximum current, that might indicate a smaller diode. Don't look for the
"brightest" at some test current.

A few manufacturers do have luminance vs. current graphs, so you might
search for those and try to extrapolate.

Jon

 

[John Larkin]

On Thu, 12 Mar 2020 09:52:18 +0000, Martin Brown
<'''newspam'''@nezumi.demon.co.uk> wrote:

On 11/03/2020 14:49, jlarkin@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.

It shouldn't make any photons once the terminal voltage drops below the
potential difference needed to emit the photon (plus a tiny bit).

That is about 3v for a blue photon and a shade under 2v for red. Like
the photoelectric effect with metals but in reverse.

One could argue that if there is any current, then electrons are
crossing the bandgap, then photons will happen. Maybe. A good LED has
picoamps of reverse leakage in the dark, so there may be not much
ohmic shunting to prevent the electrons from navigating the bandgap.

I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

A good LED is visible in modest office light at 1 uA.

Although there are plenty of not that good LEDs about. Even two decades
ago the standard white and some blue and green LED dies were visible in
daylight at about 10uA. the best have improved an order of magnitude
since. Some of those intended for high current maximum brightness seem
to have parallel resistive leakage which hammers low current efficiency.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.

Increasingly efficient modern LEDs with the drive current not adjusted
from their nominal 10mA are blindingly bright indoors.

We used to use the original blue Cree LEDs as VME access indicators,
at 50 mA. As they improved, we were really annoying people. We run the
Osram blues at 1 mA.





--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Thursday, March 12, 2020 at 2:43:22 PM UTC-4, John Larkin wrote:

On Thu, 12 Mar 2020 07:20:45 -0700 (PDT), George Herold
ggherold@gmail.com> wrote:

On Wednesday, March 11, 2020 at 11:13:35 PM UTC-4, Robert Baer wrote:
George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4, jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA. So you can
compare them at a standard current, like
20mA. Did you consider beam width? Your
bright LED may have had a beam-narrowing
lens. Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

George H.



I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

A good LED is visible in modest office light at 1 uA.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

What did you use ("photocurrent")? Motofotoplier?
The best i had for sensitivity was that ugly CdS fotoresistor.

This would have been a photodiode into a TIA... (no bias voltage :^)
and fet opamp. With 1 Gig or 100 Meg ohm FB resistor for the lowest
ranges.

Are you confident that the photodiode was linear? That could be tested
with filters.

Huh, I've never questioned PD linearity at low light levels.
I've done at lot of switched gain testing (switching FB R)
and it's always been linear. (you might have to subtract
off various offsets at low current.)
Reading of recombination models they are all proportional to the
(minority) carrier density.. like here.
tps://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-772-compound-semiconductor-devices-spring-2003/lecture-notes/Lecture18v2.pdf

Which I think implies it's linear (light level with current) all the way down.

So color me confused as usual.

It might be there is a lot of non-radiative recombo near the edges...
that might give a 3/2 power... a one dimensional density of defects.

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Robert Baer]

Phil Hobbs wrote:

On 2020-03-11 09:11, George Herold wrote:
On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

    Mouser online catalog was not helpful, and i seriously doubt DigiKey
would be better.
    I see specs like 45mcd unknown current, 5000mcd 30mA, 2000mcd 20mA,
high intensity red, etc.

    I got a little desperate, bought various Cree 5mm clear red LEDs
apparently all 500mcd 20mA...but dim as all heck at 50uA.
    One of their standard LEDs was twice as bright as some of their high
intensity LEDs.
    And i stupidly thought that Cree had brighter LEDs.
    ROHM and Everlight look better, but i need to use 250uA to drive
them
to visually similar intensity WRT my "reference".

    Hell, the LED in the Dollar Tree LED Flashlight and Lantern1-3/4 LE
Combination SKU: 286458 visually equaled the intensity at 25uA (white,
clear, gigantic size).

    Would like red, 5mm, bright at 50uA.

    Ideas?
    Thanks

Digikey's search engine is ~100x better than mouser's.
I'm not sure any place will give you the intensity at such low current.

Is there some reason you can't increase the current?

George H.

IME modern T1-3/4 LEDs usually have a knee at about 10 uA and are pretty
linear above that.  So once the OP figures out what he means by
'reasonably bright', especially the solid angle/brightness tradeoff,
it's reasonable to take the datasheet candela value, multiply by the ratio

(50 uA - 10 uA knee)/(datasheet current).

Cheers

Phil Hobbs

Does not explain the WIDE discrepancy i saw:
Suggest you look at the "comparison" of a 45 mcd VS 5000 mcd LED.
Everlight 45 mcd: 10uA reads 18.5Meg, 25uA reads 4.56Meg, 50uA reads
1.73Meg.
ROHM 5000 mcd: 10uA reads 7.6Meg, 25uA reads 1.86Meg, 50uA reads 0.41Meg.
Note the 45 mcd at 50uA reads near what the 5000 mcd at 25uA.

 

Robert Baer <robertbaer@localnet.com> wrote in
news:lGEaG.362476$Mj1.242991@fx39.iad:

bulegoge@columbus.rr.com wrote:

The idea is to have a battery monitor light that will not
unduly run
down the battery.

How about blinking the LED at 25 percent duty cycle...or even
lower

A 2.2Meg resistor takes no extra electronic fluff, and at 24V
supply
the 10uA is very close to zero load.

Also unable to illuminate the LED. D'OH!

Use the RIGHT current limit resistor and use THIN PWM 'on' pulses
to make for 'dim' operation.

 

[Robert Baer]

bulegoge@columbus.rr.com wrote:

The idea is to have a battery monitor light that will not unduly run
down the battery.

How about blinking the LED at 25 percent duty cycle...or even lower

A 2.2Meg resistor takes no extra electronic fluff, and at 24V supply
the 10uA is very close to zero load.

 

[Robert Baer]

Robert Baer wrote:

George Herold wrote:
On Wednesday, March 11, 2020 at 10:49:32 AM UTC-4,
jla...@highlandsniptechnology.com wrote:
On 11 Mar 2020 06:38:53 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

  Would like red, 5mm, bright at 50uA.

The LED brightness should be proportional
to current, even down to 50uA.  So you can
compare them at a standard current, like
20mA.  Did you consider beam width?  Your
bright LED may have had a beam-narrowing
lens.  Also, LEDs with scattering material
in the plastic are easily visible from all
angles, but will appear much dimmer.

I've wondered if, or when, an LED just stops making photons as the
current (and voltage) go down. Any shunt ohmic component would shut it
down as the terminal voltage gets too low.
The slope for the low current end is about 3/2 (100 times the current
gives 1000 times the light.) (note log-log plot)
But here's data from an IR led.
https://www.dropbox.com/s/i1am61wga5fhpax/IR-PD.BMP?dl=0

Which seems to poop out faster.

I'm guessing newer LED's are better.. but don't know.
(data posted to dropbox in 2013)

George H.



I have seen light from a green LED at 1 nA, and the threshold may well
have been my eyes. A good LED and a PMT and a lockin would be
interesting.

* A Huggins lamp (mercury relay pulsing an open coax) has light output
linear with voltage and used to test risetime of a PMT.
The eye sensitivity seems superior to the PMT; turning down the
driver/coax voltage will show weaker and weaker light pulses,even 5nSec
wide...until it manifestly drops out at about 30V; the ionization
potential of mercury.
Use pulses, the brain is a superior correlator.
Bet the eye wins.

A good LED is visible in modest office light at 1 uA.

A plastic machine screw might make a decent light pipe for the front
panel of a rackmount box. Just a few mA of drive will make a blinding
amount of light from a good LED, so optical efficiency doesn't matter.



--

John Larkin         Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over
Reason"

  What did you use ("photocurrent")?  Motofotoplier?
  The best i had for sensitivity was that ugly CdS fotoresistor.

 

[Robert Baer]

mpm wrote:

On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

Interesting situation.
It might be helpful if everyone reading this went into their grab-bag of T-1 3/4 (5mm) red LED's to see if they can find a particularly bright one at 50 uA.

If nothing else, it might be a welcomed change from everyone bitching about politics, or bickering over the "OT topic du-jour".

I can attest that these LED (below) are SUPER-BRIGHT... at 40 mA!!
Link: https://www.digikey.com/products/en?keywords=C503B-RCN-CW0Z0AA1-ND

* That was one of the many CREE red, clear lens LEDs i measured at 10uA,
25uA,and 50uA.
Useful at 10uA as an indicator, just bright enough to show.
Fairly bright at 25uA and "wow" at 50uA.
Would guess that it would blast the eyeballs at 40mA.

I was recently tasked with converting one of our products to allow for 24-hour battery back-up operation and in the process discovered that we were wasting about 3 watts just on these LED's (and about 4.25 watts total). The design engineer had simply spec'd the wrong dropping resistor, and worse, that resistor was a thru-hole SIP pack. You start to worry about I2R heat when they're all lit up.

Luckily, it turns out that both the LED's and the SIP's can handle the current, which I guess is why nobody ever noticed it until now, or why we never got any units returned under RMA's. But they sure are BRIGHT!!

I'll pull one out of inventory tomorrow and see if it will even light up at 50 uA and let you know. Maybe?? It's a nice LED, with even lighting - not sure why we originally selected it in this design (may have been availability, or we had some left over from some other project?)

* I recommend you try one out at 50uA, it is quite bright and should do
the job you want.

To do A-B LED visual comparison, put them in series, using a 500K
resistor minimum with variable PS.

 

[Robert Baer]

Phil Hobbs wrote:

On 2020-03-12 11:38, George Herold wrote:
On Thursday, March 12, 2020 at 11:21:51 AM UTC-4, Phil Hobbs wrote:
On 2020-03-11 09:11, George Herold wrote:
On Tuesday, March 10, 2020 at 11:43:23 PM UTC-4, Robert Baer wrote:
I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.

     Mouser online catalog was not helpful, and i seriously doubt
DigiKey
would be better.
     I see specs like 45mcd unknown current, 5000mcd 30mA, 2000mcd
20mA,
high intensity red, etc.

     I got a little desperate, bought various Cree 5mm clear red LEDs
apparently all 500mcd 20mA...but dim as all heck at 50uA.
     One of their standard LEDs was twice as bright as some of
their high
intensity LEDs.
     And i stupidly thought that Cree had brighter LEDs.
     ROHM and Everlight look better, but i need to use 250uA to
drive them
to visually similar intensity WRT my "reference".

     Hell, the LED in the Dollar Tree LED Flashlight and
Lantern1-3/4 LE
Combination SKU: 286458 visually equaled the intensity at 25uA (white,
clear, gigantic size).

     Would like red, 5mm, bright at 50uA.

     Ideas?
     Thanks

Digikey's search engine is ~100x better than mouser's.
I'm not sure any place will give you the intensity at such low current.

Is there some reason you can't increase the current?

George H.

IME modern T1-3/4 LEDs usually have a knee at about 10 uA and are pretty
linear above that.  So once the OP figures out what he means by
'reasonably bright', especially the solid angle/brightness tradeoff,
it's reasonable to take the datasheet candela value, multiply by the
ratio

(50 uA - 10 uA knee)/(datasheet current).

Hi Phil, Got a name/ part number of a data sheet that I could look at?
At one point in the past I wanted a light source that would be fairly
linear in light output.... (which was why I took the LED data above.)
It would be nice if some LED was linear in current from 10 uA to 1 mA...
All the data sheets I look at have a lower current of 1 mA.

Haven't made a homebrew optocoupler in a decade or so, but these ones
look pretty good.

http://www.kingbrightusa.com/images/catalog/SPEC/WP7113LZGCK.pdf

Cheers

Phil Hobbs

I like the detail they gave concerning lead bending/forming/protection.

 

[Mikko OH2HVJ]

Robert Baer <robertbaer@localnet.com> writes:

I had one red T-1 3/4 (5mm) clear LED that was rather bright at 50uA
and it failed. Have no other than this.
....
Would like red, 5mm, bright at 50uA.

I've used these for high efficiency indicators, although at 100uA.

The output is 15°, which makes the candela rating quite high, but
also the flux efficiency is great. And it's specified in datasheets,
which is rare for indicator LEDs.

(Eye is more sensitive to green, so I prefer those)

Green HLMP-CM1G 475lm/W 27-59cd @ 20mA
Red HLMP-EG1A 200lm/W 12-20cd @ 20mA

--
mikko