# LED forward voltage drop with temperature

Started by April 22, 2020
```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?
```
```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.

--

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

```
```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.
>

Thanks, John.

```
```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.

```
```On 4/22/2020 18:55, 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?

It depends on the LED colour to a huge extent. Not the direction, that
is, never checked that, but the stability.
A red LED is remarkably stable over temp, I investigated (and
used during initial prototyping as a reference voltage). I had seen it
ised as a reference in some competitor product so I tried; I used
a green first (unlike the red they had used) and it was just a temp
sensor, not a reference. Tried the red one and it worked.
This was almost 30 years ago though, LEDs today may not be the same
like the LEDs used to be back then. It was a 5mm red LED, this is all
I recall.

Dimiter

======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

```
```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

--
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 4/22/2020 12:14 PM, 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.
>
>
>
>
>

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
```
```On Wed, 22 Apr 2020 12:04:19 -0500, 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.

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

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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

```
```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.
```
```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.

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.
```