# LED forward voltage drop with temperature

Started by April 22, 2020
```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 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>>> 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.

```
```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 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>> 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.

```
```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.&#2013266080; 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.
>>
>> 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
```
```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.
>>
>> &nbsp; A red LED has about 2 or 3mV per degree drop @ 2mA.
>>
>> &nbsp; Close to a regular Si diode.
>>
>>
>> ...&nbsp;&nbsp; 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 &#2013265920; 18:14, jlarkin@highlandsniptechnology.com a &#2013265929;crit&#2013266080;:
>>>> 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

```
```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.&nbsp; 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?)
```
```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 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>>> 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

```
```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 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>>>> 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.&nbsp; 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'.
```
```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 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>>>> 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.

```
```Le 23/04/2020 &agrave; 16:06, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
> On Thu, 23 Apr 2020 12:39:28 +0200, habib <h.bouazizviallet@free.fr>
> wrote:
>
>> Le 22/04/2020 &agrave; 18:14, jlarkin@highlandsniptechnology.com a &eacute;crit&nbsp;:
>>> 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.
Ok I see, MMBTH81 is not working at the same temperature than the LED,
is it ?
>
> 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.
Why not? Spice directives ".op" and ".step Temp 25 100" 1 would help to
figure out that TC is (always?) negative. AFAIK III/V components (e.g.
Gallium/Arsenide) have a negative TC coefficient.

>
> 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.
In the dark as you say ;-)
Yeah resistors have few ppm/&deg;C but the transistor has Vbe drift
-2.2mV/&deg;C along with large drift of h21e intrinsic gain parameter
>
> Transistor self-heating would be a problem if one wanted serious
> stability. Base current too.

Thank you for the topic. H
```