LED forward voltage drop with temperature

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.

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

Thanks, John.

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

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

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

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

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

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

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.

e

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.

ode

There's this from the British journal of anesthesia. :^) (it's weird what you find with search engines.)

GH

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

** Try reading the damn heading - dickwad !!

.... Phil

Sure is weird and wonderful what you can find and in this case, where ! But very appropriate ! Nice article actually if you're interested in that kind of stuff, which I am.

Well dick head, I did, and everyone seemed to be answering the question of voltage drop vs current.

GH

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

Please excuse me. Getting mad online is not constructive. George H.

e

ts.

diode

ts

Oh, I'm sure someone at Bells labs, or some where did it in the

60's 70's...?

The 780 nm laser diodes I know tune about 1 nm with 4 deg C/K.

Things change with temperature, it's both a blessing and a curse. :^)

George h.

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

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

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

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

** Then you ignored it.

** So you wrote a pile of drivel about the colour changing with low temps.

Yaawwwwnnn....

Context has no meaning in your bubble.

.... Phil

** Nope.

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

Close to a regular Si diode.

... Phil