LED forward voltage drop with temperature

[Gerhard Hoffmann]

Am 22.04.20 um 19:52 schrieb bitrex:

In my own tests of lots of LEDs for forward voltage vs current and
temperature I found that 3mm diffused yellow LEDs, the old-timey type
made similar to red LEDs on gallium arsenide I believe, were on average
the best

These are my measurements from a few years ago (just 2 types):

http://www.hoffmann-hochfrequenz.de/downloads/Led_tk.pdf


BTW if you want to abuse a LED as a _very_ low voltage noise reference,
Avago HLMP-6000 is the best by far, whoever owns that now.

It's a quite dimm bulb.
I have the impression that optical efficiency goes with noise.

cheers, Gerhard

 

On Wed, 22 Apr 2020 18:46:53 -0400, "Tom Del Rosso"
<fizzbintuesday@that-google-mail-domain.com> wrote:

John Larkin wrote:

The V:I curve should have a zero tempco point.

But where is that point relative to the useful current range?

Depends. It's rare to see a diode data sheet that shows the VI curve
at different temperatures, much less at even rated max current.

I've deliberately run some small schottky diodes at the zero TC point,
which was about 15 mA.

Big power diodes sometimes show that point on the data sheet. That
counters the conventional wisdom about thermal runaway current hogging
of diodes in parallel. Some of the big power diode bricks are diodes
in parallel.

Here's one, zero TC at 200 amps, well past rated current in this case.

https://www.dropbox.com/s/aes9hav7rfymuwh/SD41.pdf?dl=0


It would be interesting to test an LED. People make cheap flashlights
with a battery and an LED, so some must be substantially ohmic.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[habib]

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :

On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

At the right current, the voltage tempco of the LED cancels the tempco
of the transistor Vbe. This current source tempco was probably
dominated by the emitter resistor.

 

[John S]

On 4/22/2020 11:05 PM, Gerhard Hoffmann wrote:

Am 22.04.20 um 19:52 schrieb bitrex:

In my own tests of lots of LEDs for forward voltage vs current and
temperature I found that 3mm diffused yellow LEDs, the old-timey type
made similar to red LEDs on gallium arsenide I believe, were on
average the best

These are my measurements from a few years ago (just 2 types):

http://www.hoffmann-hochfrequenz.de/downloads/Led_tk.pdf


BTW if you want to abuse a LED as a _very_ low voltage noise reference,
Avago HLMP-6000 is the best by far, whoever owns that now.

It's a quite dimm bulb.
I have the impression that optical efficiency goes with noise.

cheers, Gerhard

Thank you Gerhard. I will save your pdf for my future reference.

 

[John S]

On 4/22/2020 9:38 PM, Phil Allison wrote:

John S wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?


** Nope.

A red LED has about 2 or 3mV per degree drop @ 2mA.

Close to a regular Si diode.


... Phil

Thanks, Phil. It looks like LTSpice is giving me what I suspected was
incorrect info. Spice is showing about 4mV/C positive voltage with
temperature at 2mA. I need to make bench measurements and not depend on
spice.

 

[John S]

On 4/23/2020 6:49 AM, Jasen Betts wrote:

On 2020-04-22, John S <Sophi.2@invalid.org> wrote:
On 4/22/2020 11:14 AM, jlarkin@highlandsniptechnology.com wrote:
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

At the right current, the voltage tempco of the LED cancels the tempco
of the transistor Vbe. This current source tempco was probably
dominated by the emitter resistor.

I guess I have not reached that point yet since the voltage drops as the
heat builds. As I said, contrary to my measurements.


Look at John's circuit, the LED voltage is matched by the V_be and
resittor, for it to compensate both V_be and LED must move in the same
direction.

Thanks, Jasen. Yes, I studied his circuit and understand how it works.

 

[Jasen Betts]

On 2020-04-22, John S <Sophi.2@invalid.org> wrote:

On 4/22/2020 11:14 AM, jlarkin@highlandsniptechnology.com wrote:
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

At the right current, the voltage tempco of the LED cancels the tempco
of the transistor Vbe. This current source tempco was probably
dominated by the emitter resistor.

I guess I have not reached that point yet since the voltage drops as the
heat builds. As I said, contrary to my measurements.

Look at John's circuit, the LED voltage is matched by the V_be and
resittor, for it to compensate both V_be and LED must move in the same
direction.

--
Jasen.

 

[George Herold]

On Wednesday, April 22, 2020 at 7:25:49 PM UTC-4, Phil Hobbs wrote:

On 2020-04-22 17:58, George Herold wrote:
On Wednesday, April 22, 2020 at 4:47:39 PM UTC-4, John S wrote:
On 4/22/2020 12:52 PM, Phil Hobbs wrote:
On 2020-04-22 11:55, John S wrote:
LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Depends on the drive current.  At low current it looks like a diode
(NTC), whereas at high current the resistance (PTC) dominates.

Cheers

Phil Hobbz


Thank you one and all for your help.

I guess I can't go any further without making some actual measurements
rather than relying SPICE sims. I hope to do that soon.

Hmm? Are you asking about the voltage drop versus current
or vs temperature.

Versus temperature I know that different LED's change color
in different directions when dunked into LN2... so there are
mechanisms that go both ways.

That's interesting. All the diode lasers I know of tune towards the
blue at low temperature.

So I think it's green LED's that 'go the wrong way'.
https://www.youtube.com/watch?v=wmdIFLoBYOU

And I'm assuming the forward voltage changes too.

George H.

My first order idea is that temperature causes the crystal to
expand. And the effect of a bigger x-tal spacing is a
lowering of the bandgap energy.. Which says
LED's shift to longer wavelengths when you heat them.
(at constant current.. I'm assuming the forward voltage
is some measure of the bandgap energy.)
Which agrees with my experience... but there are some LED's that go
the other way, and I don't know the mechanism.

Phosphide LEDs hardly tune at all with bias current, but nitride ones
tune strongly towards the blue as bias current increases.

http://www.ka-electronics.com/images/pdf/Junction_Temperature_LED_Tempco..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

 

On Thu, 23 Apr 2020 05:35:30 -0500, John S <Sophi.2@invalid.org>
wrote:

On 4/22/2020 9:38 PM, Phil Allison wrote:

John S wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?


** Nope.

A red LED has about 2 or 3mV per degree drop @ 2mA.

Close to a regular Si diode.


... Phil


Thanks, Phil. It looks like LTSpice is giving me what I suspected was
incorrect info. Spice is showing about 4mV/C positive voltage with
temperature at 2mA. I need to make bench measurements and not depend on
spice.

Right. There are all sorts of LED chemistry and fabs around. I
wouldn't trust any of the LT Spice LED models very far.

My green LED really seemed to be an oddball. I don't think my
transistor was oscillating, but that's possible. I really like the
color of the orange Osram, so I used that one.






--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr>
wrote:

Le 22/04/2020 ŕ 18:14, jlarkin@highlandsniptechnology.com a écrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.








--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[bitrex]

On 4/23/2020 11:48 AM, bitrex wrote:

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.

Any "basic math" you do on that circuit will result in a transcendental
equation that can't be solved in closed-form, anyway so you'll have to
resort to numerical methods in any event.

The only way to solve just the diode-resistor circuit exactly for the
R-D junction voltage in "closed form" is with the Lambert W function

https://en.wikipedia.org/wiki/Lambert_W_function

if the base current assumed to be 0 a simple model would be:

something involving lambert W and the diode Is and the supply voltage
= Is log (Vbe/Vt) + I*R_e.

Ooops, I meant:

<something involving lambert W and the diode Is and the supply voltage>
> = Vt * log (I/Is) + I*R_e.

 

[legg]

On Wed, 22 Apr 2020 15:22:13 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Wednesday, April 22, 2020 at 5:58:28 PM UTC-4, George Herold wrote:
On Wednesday, April 22, 2020 at 4:47:39 PM UTC-4, John S wrote:
On 4/22/2020 12:52 PM, Phil Hobbs wrote:
On 2020-04-22 11:55, John S wrote:
LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Depends on the drive current.  At low current it looks like a diode
(NTC), whereas at high current the resistance (PTC) dominates.

Cheers

Phil Hobbz


Thank you one and all for your help.

I guess I can't go any further without making some actual measurements
rather than relying SPICE sims. I hope to do that soon.

Hmm? Are you asking about the voltage drop versus current
or vs temperature.

Versus temperature I know that different LED's change color
in different directions when dunked into LN2... so there are
mechanisms that go both ways.

My first order idea is that temperature causes the crystal to
expand. And the effect of a bigger x-tal spacing is a
lowering of the bandgap energy.. Which says
LED's shift to longer wavelengths when you heat them.
(at constant current.. I'm assuming the forward voltage
is some measure of the bandgap energy.)
Which agrees with my experience... but there are some LED's that go
the other way, and I don't know the mechanism.

George H.

There's this from the British journal of anesthesia. :^)
(it's weird what you find with search engines.)
https://www.bjanaesthesia.org.uk/article/S0007-0912(17)47891-3/pdf

GH

That's wavelength stability, not voltage drop.

RL

 

[bitrex]

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:

On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.

It's about the best discrete current-source you can build for the money.
To build one using discretes that improves on it substantially would
require a lot more parts and probably matched transistors.

 

[bitrex]

On 4/23/2020 6:35 AM, John S wrote:

On 4/22/2020 9:38 PM, Phil Allison wrote:

John S wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?


** Nope.

  A red LED has about 2 or 3mV per degree drop @ 2mA.

  Close to a regular Si diode.


...   Phil


Thanks, Phil. It looks like LTSpice is giving me what I suspected was
incorrect info. Spice is showing about 4mV/C positive voltage with
temperature at 2mA. I need to make bench measurements and not depend on
spice.

LTSpice model may just assume a constant value for the LED Is parameter,
and this works well enough for many purposes. but it's a strong function
of temperature in reality like all junction semiconductor devices.

Doing math computerized or otherwise with the diode or transistor
equations assuming Is is constant expecting to get meaningful tempco
results is hopeless.

 

On Thu, 23 Apr 2020 12:03:52 -0400, bitrex <user@example.net> wrote:

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 ŕ 18:14, jlarkin@highlandsniptechnology.com a écrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.



It's about the best discrete current-source you can build for the money.
To build one using discretes that improves on it substantially would
require a lot more parts and probably matched transistors.

You could do better if you allow more voltage drop in the emitter
resistor.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[bitrex]

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:

On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.

Any "basic math" you do on that circuit will result in a transcendental
equation that can't be solved in closed-form, anyway so you'll have to
resort to numerical methods in any event.

The only way to solve just the diode-resistor circuit exactly for the
R-D junction voltage in "closed form" is with the Lambert W function

<https://en.wikipedia.org/wiki/Lambert_W_function>

if the base current assumed to be 0 a simple model would be:

<something involving lambert W and the diode Is and the supply voltage>
= Is log (Vbe/Vt) + I*R_e.

The diode and transistor Is's are non-linear functions of temperature so
unless they cancel neatly like in an a differential pair with matched
transistors leaving only the secondary temperature dependence on Vt,
without further data about Is this equation doesn't tell you much about
real-world temperature stability.

 

[George Herold]

On Thursday, April 23, 2020 at 12:02:23 PM UTC-4, legg wrote:

On Wed, 22 Apr 2020 15:22:13 -0700 (PDT), George Herold
ggherold@gmail.com> wrote:

On Wednesday, April 22, 2020 at 5:58:28 PM UTC-4, George Herold wrote:
On Wednesday, April 22, 2020 at 4:47:39 PM UTC-4, John S wrote:
On 4/22/2020 12:52 PM, Phil Hobbs wrote:
On 2020-04-22 11:55, John S wrote:
LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Depends on the drive current.  At low current it looks like a diode
(NTC), whereas at high current the resistance (PTC) dominates.

Cheers

Phil Hobbz


Thank you one and all for your help.

I guess I can't go any further without making some actual measurements
rather than relying SPICE sims. I hope to do that soon.

Hmm? Are you asking about the voltage drop versus current
or vs temperature.

Versus temperature I know that different LED's change color
in different directions when dunked into LN2... so there are
mechanisms that go both ways.

My first order idea is that temperature causes the crystal to
expand. And the effect of a bigger x-tal spacing is a
lowering of the bandgap energy.. Which says
LED's shift to longer wavelengths when you heat them.
(at constant current.. I'm assuming the forward voltage
is some measure of the bandgap energy.)
Which agrees with my experience... but there are some LED's that go
the other way, and I don't know the mechanism.

George H.

There's this from the British journal of anesthesia. :^)
(it's weird what you find with search engines.)
https://www.bjanaesthesia.org.uk/article/S0007-0912(17)47891-3/pdf

GH

That's wavelength stability, not voltage drop.

RL

Right. (Sorry just my first hit in google)
There are two 'pieces' to the forward voltage drop.
One from the current ... the diode equation (and any real resistances)
And then from the bandgap of the LED.
(Phil H. posted a nice article that covered both.)
(And of course the current heats the junction so the two interact
strongly.)

George H.
reading the article Phil linked to... I'm confused by some of it.
(fig 2 shows led wavelength getting shorter at higher led currents?)

 

[Phil Hobbs]

On 2020-04-23 11:48, bitrex wrote:

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.

Any "basic math" you do on that circuit will result in a transcendental
equation that can't be solved in closed-form, anyway so you'll have to
resort to numerical methods in any event.

Nah, you just apply a suitable sleazy analytic approximation and press
on. Binomial expansion, perturbation, asymptotic analysis, Taylor
series, no worries.

The only way to solve just the diode-resistor circuit exactly for the
R-D junction voltage in "closed form" is with the Lambert W function

https://en.wikipedia.org/wiki/Lambert_W_function

if the base current assumed to be 0 a simple model would be:

something involving lambert W and the diode Is and the supply voltage
= Is log (Vbe/Vt) + I*R_e.

The diode and transistor Is's are non-linear functions of temperature so
unless they cancel neatly like in an a differential pair with matched
transistors leaving only the secondary temperature dependence on Vt,
without further data about Is this equation doesn't tell you much about
real-world temperature stability.

Tractable analytic approximations are much more useful than some opaque
exact result, and way more useful than a stack of simulations--you can
optimize analytically, for instance, or set good reliable upper and
lower bounds on performance.

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

 

[bitrex]

On 4/23/2020 12:26 PM, jlarkin@highlandsniptechnology.com wrote:

On Thu, 23 Apr 2020 12:03:52 -0400, bitrex <user@example.net> wrote:

On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.



It's about the best discrete current-source you can build for the money.
To build one using discretes that improves on it substantially would
require a lot more parts and probably matched transistors.

You could do better if you allow more voltage drop in the emitter
resistor.

Wonder how a dual-gate MOSFET would work and hang an LED with opposite
tempco off each gate.

I don't think P-channel duals are easy to come by no mo' and dual-gate
FETs aren't particularly cheap in general though.

There's an example of a discrete band-gap made with the transistors in
e.g. a CA3046 array, for high supply voltages perhaps, there was
probably a time when those were cheap and available but most
multi-transistor chip arrays seem out of production, now. My ~70 MHz
Kikuisi analog scope circa 1982 uses two CA3046 in the input circuit.

 

[bitrex]

On 4/23/2020 1:50 PM, Phil Hobbs wrote:

On 2020-04-23 11:48, bitrex wrote:
On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr
wrote:

Le 22/04/2020 à 18:14, jlarkin@highlandsniptechnology.com a Êcrit :
On Wed, 22 Apr 2020 10:55:04 -0500, John S <Sophi.2@invalid.org
wrote:

LTSpice says that the forward voltage drop of LEDs have a positive
coefficient. That is contrary to my thinking and to my measurements.

Am I doing something wrong?

Probably depends on the current. Low current follows the diode
equation, ntc, but at high current voltage drop is dominated by the
ohmic component, with a positive TC. Basically all diodes do that.

Hi John,
https://www.dropbox.com/s/d4ntmq7fdzah69a/LED_Isrc_data.JPG?raw=1

Are you sure the current flowing through the LED is constant ? i.e.
temperature independent.
AFAIK current should be kept constant to evaluate voltage drift of Vf
over temperature.

Some basic math on your circuit would be nice to be explained. Please.

H

The LED current is set by the voltage drop across the 2K resistor. The
current TC is what I measured for the whole circuit.

I also measured power supply sensitivity, which isn't bad at all.
Variation in power supply voltage directly (actually worse than
directly) changes the LED current. A tweak might null out power supply
sensitivity, but I had a good supply in my application.

There's no serious math here. There can't be without knowing a lot
more about the LEDs than is available. It was easier to build it and
test it.

You could Spice it to see the basic functionality, but I wouldn't
trust a simulation to predict TC.

It could certainly be better, with a thinfilm emitter resistor and
some more tweaking. It was good enough for my product so I moved on.
But there are much better circuits if one wants a super stable current
source. This was just sort of fun, and it glows in the dark.

Transistor self-heating would be a problem if one wanted serious
stability. Base current too.

Any "basic math" you do on that circuit will result in a
transcendental equation that can't be solved in closed-form, anyway so
you'll have to resort to numerical methods in any event.

Nah, you just apply a suitable sleazy analytic approximation and press
on.  Binomial expansion, perturbation, asymptotic analysis, Taylor
series, no worries.

The only way to solve just the diode-resistor circuit exactly for the
R-D junction voltage in "closed form" is with the Lambert W function

https://en.wikipedia.org/wiki/Lambert_W_function

if the base current assumed to be 0 a simple model would be:

something involving lambert W and the diode Is and the supply
voltage> = Is log (Vbe/Vt) + I*R_e.

The diode and transistor Is's are non-linear functions of temperature
so unless they cancel neatly like in an a differential pair with
matched transistors leaving only the secondary temperature dependence
on Vt, without further data about Is this equation doesn't tell you
much about real-world temperature stability.

Tractable analytic approximations are much more useful than some opaque
exact result, and way more useful than a stack of simulations--you can
optimize analytically, for instance, or set good reliable upper and
lower bounds on performance.

Cheers

Phil Hobbs

Middlebrook developed a whole framework for obtaining tractable results
for on-paper feedback amplifier design; once you get beyond a couple
transistors the exact results become hopeless nobody could make sense of
them if written out.

<https://en.wikipedia.org/wiki/Extra_element_theorem>

Besides the EET I can't say I've seen it used much in practice. Most
designers except those who work at semiconductor cos don't design their
own multi-stage discrete feedback amplifiers no mo'.