# LED forward voltage drop with temperature

Started by April 22, 2020
```On Thu, 23 Apr 2020 22:05:27 +0200, habib <h.bouazizviallet@free.fr>
wrote:

>Le 23/04/2020 &#2013265920; 16:06, jlarkin@highlandsniptechnology.com a &#2013265929;crit&#2013266080;:
>> 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.
>Ok I see, MMBTH81 is not working at the same temperature than the LED,
>is it ?

It was just a little breadboard. The temps on all the parts were
similar.

>>
>> 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.

I don't have a Spice model of any of the LEDs that we have in stock.
I'd have to measure one over current and temperature and verify a
Spice model. It was easier to build the circuit. I'd want to do that
anyhow to verify the model.

The trick of this circuit is that the LED has more voltage drop than
the transistor be junction, but a similar absolute 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.
>In the dark as you say ;-)

Light doesn't seem to matter. Makes sense. The LED is making a lot of
light, and it's very close to the LED.

>Yeah resistors have few ppm/&#2013266096;C but the transistor has Vbe drift
>-2.2mV/&#2013266096;C along with large drift of h21e intrinsic gain parameter

Exactly. The LED tempco mostly cancels the Vbe tempco.

>>
>> Transistor self-heating would be a problem if one wanted serious
>> stability. Base current too.
>
>Thank you for the topic. H
--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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

```
```Le 23/04/2020 &agrave; 22:50, John Larkin a &eacute;crit&nbsp;:
> On Thu, 23 Apr 2020 22:05:27 +0200, habib <h.bouazizviallet@free.fr>
> wrote:
>
>> 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 ?
>
> It was just a little breadboard. The temps on all the parts were
> similar.
>
>
>
>
>>>
>>> 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.
>
> I don't have a Spice model of any of the LEDs that we have in stock.
> I'd have to measure one over current and temperature and verify a
> Spice model. It was easier to build the circuit. I'd want to do that
> anyhow to verify the model.
>
> The trick of this circuit is that the LED has more voltage drop than
> the transistor be junction, but a similar absolute 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.
>> In the dark as you say ;-)
>
> Light doesn't seem to matter. Makes sense. The LED is making a lot of
> light, and it's very close to the LED.
>
>> 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
>
> Exactly. The LED tempco mostly cancels the Vbe tempco.
Are you sure ? LED is not a silicon based component so I'm not sure they
have the same tempco.
>
>
>>>
>>> Transistor self-heating would be a problem if one wanted serious
>>> stability. Base current too.
>>
>> Thank you for the topic. H

```
```On Thu, 23 Apr 2020 23:21:33 +0200, habib <h.bouazizviallet@free.fr>
wrote:

>Le 23/04/2020 &#2013265920; 22:50, John Larkin a &#2013265929;crit&#2013266080;:
>> On Thu, 23 Apr 2020 22:05:27 +0200, habib <h.bouazizviallet@free.fr>
>> wrote:
>>
>>> Le 23/04/2020 &#2013265920; 16:06, jlarkin@highlandsniptechnology.com a &#2013265929;crit&#2013266080;:
>>>> 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.
>>> Ok I see, MMBTH81 is not working at the same temperature than the LED,
>>> is it ?
>>
>> It was just a little breadboard. The temps on all the parts were
>> similar.
>>
>>
>>
>>
>>>>
>>>> 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.
>>
>> I don't have a Spice model of any of the LEDs that we have in stock.
>> I'd have to measure one over current and temperature and verify a
>> Spice model. It was easier to build the circuit. I'd want to do that
>> anyhow to verify the model.
>>
>> The trick of this circuit is that the LED has more voltage drop than
>> the transistor be junction, but a similar absolute 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.
>>> In the dark as you say ;-)
>>
>> Light doesn't seem to matter. Makes sense. The LED is making a lot of
>> light, and it's very close to the LED.
>>
>>> Yeah resistors have few ppm/&#2013266096;C but the transistor has Vbe drift
>>> -2.2mV/&#2013266096;C along with large drift of h21e intrinsic gain parameter
>>
>> Exactly. The LED tempco mostly cancels the Vbe tempco.
>Are you sure ? LED is not a silicon based component so I'm not sure they
>have the same tempco.

It works.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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

```
```On 2020-04-23 17:21, habib wrote:
> Le 23/04/2020 &agrave; 22:50, John Larkin a &eacute;crit&nbsp;:
>> On Thu, 23 Apr 2020 22:05:27 +0200, habib <h.bouazizviallet@free.fr>
>> wrote:
>>
>>> 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 ?
>>
>> It was just a little breadboard. The temps on all the parts were
>> similar.
>>
>>
>>
>>
>>>>
>>>> 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.
>>
>> I don't have a Spice model of any of the LEDs that we have in stock.
>> I'd have to measure one over current and temperature and verify a
>> Spice model. It was easier to build the circuit. I'd want to do that
>> anyhow to verify the model.
>>
>> The trick of this circuit is that the LED has more voltage drop than
>> the transistor be junction, but a similar absolute 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.
>>> In the dark as you say ;-)
>>
>> Light doesn't seem to matter. Makes sense. The LED is making a lot of
>> light, and it's very close to the LED.
>>
>>> 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
>>
>> Exactly. The LED tempco mostly cancels the Vbe tempco.
> Are you sure ? LED is not a silicon based component so I'm not sure they
> have the same tempco.

Different LEDs have slightly different tempcos.  The bandgap goes up as
the lattice contracts, and the thermal effect goes the other way.

LEDs are direct-bandgap devices, so there are radiative as well as
nonradiative branches, which depend differently on the details of the
band shapes.

LEDs and lasers nearly always tune towards the blue at low temperatures,
but George's one tunes the other way.

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

```
```habib wrote:

============

>
> Are you sure ? LED is not a silicon based component so I'm not sure they
> have the same tempco.
>

**FYI:

Silver, copper, gold, aluminium, magnesium, tungsten, zinc, nickel, tin, iron, platinum, mild steel, lead plus some alloys ALL have the same positive tempco of resistance.

Close to .004% per C.

....  Phil
```
```Le 24/04/2020 &agrave; 00:58, Phil Allison a &eacute;crit&nbsp;:
> habib wrote:
>
> ============
>
>>
>> Are you sure ? LED is not a silicon based component so I'm not sure they
>> have the same tempco.
>>
>
>
> **FYI:
>
>   Silver, copper, gold, aluminium, magnesium, tungsten, zinc, nickel, tin, iron, platinum, mild steel, lead plus some alloys ALL have the same positive tempco of resistance.
>
> Close to .004% per C.
Sure although it is not "%" in that case.

Anyway we did not speak about metals, it was about tempco of IV-V
(silicon) and III-V materials.

H
>
>
> ....  Phil
>

```
```On 4/24/2020 2:51 AM, habib wrote:
> Le 23/04/2020 &agrave; 22:50, John Larkin a &eacute;crit&nbsp;:
>>
>> Exactly. The LED tempco mostly cancels the Vbe tempco.
>
> Are you sure ? LED is not a silicon based component so I'm not sure they
> have the same tempco.

It's a fairly well known fact.
```
``` habib wrote:

============

> >>
> >> Are you sure ? LED is not a silicon based component so I'm not
>> sure they> have the same tempco.
> >>
> >
> >
> > **FYI:
> >
> >   Silver, copper, gold, aluminium, magnesium, tungsten, zinc, nickel, tin, iron, platinum, mild steel, lead plus some alloys ALL have the same positive tempco of resistance.

> >
> > Close to .004% per C.
>>
> Sure although it is not "%" in that case.

** Pedant.

>
> Anyway we did not speak about metals,

** Yes we did and I mentioned alloys.

Anyhow the fact is YOU are 100%  WRONG !!

.....   Phil
```
```On 2020-04-24 11:20, Phil Allison wrote:
>   habib wrote:
>
> ============
>
>>>>
>>>> Are you sure ? LED is not a silicon based component so I'm not
>>> sure they> have the same tempco.
>>>>
>>>
>>>
>>> **FYI:
>>>
>>>    Silver, copper, gold, aluminium, magnesium, tungsten, zinc, nickel, tin, iron, platinum, mild steel, lead plus some alloys ALL have the same positive tempco of resistance.
>
>>>
>>> Close to .004% per C.
>>>
>> Sure although it is not "%" in that case.
>
> ** Pedant.

Pointing out a factor of hundred error is being pedantic?

Interesting.

Jeroen Belleman

```
```Jeroen Belleman wrote:

=======================
>
> >
> >>>>
> >>>> Are you sure ? LED is not a silicon based component so I'm not
> >>> sure they> have the same tempco.
> >>>>
> >>>
> >>>
> >>> **FYI:
> >>>
> >>>    Silver, copper, gold, aluminium, magnesium, tungsten, zinc, nickel, tin, iron, platinum, mild steel, lead plus some alloys ALL have the same positive tempco of resistance.
> >
> >>>
> >>> Close to .004% per C.
> >>>
> >> Sure although it is not "%" in that case.
> >
> > ** Pedant.
>
> Pointing out a factor of hundred error

** Only a pedant would bother.

Someone like you.

....  Phil

```