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LED forward voltage drop with temperature

Started by John S April 22, 2020
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
George Herold wrote:

===================
> > > Hmm? Are you asking about the voltage drop versus current > or vs temperature. >
** Try reading the damn heading - dickwad !! .... Phil
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
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.
On Wednesday, April 22, 2020 at 6:22:27 PM UTC-4, Phil Allison wrote:
> George Herold wrote: > > =================== > > > > > > Hmm? Are you asking about the voltage drop versus current > > or vs temperature. > > > > ** Try reading the damn heading - dickwad !! > > > > .... Phil
Well dick head, I did, and everyone seemed to be answering the question of voltage drop vs current. GH
John Larkin wrote:
> > The V:I curve should have a zero tempco point.
But where is that point relative to the useful current range?
On Wednesday, April 22, 2020 at 6:45:45 PM UTC-4, George Herold wrote:
> On Wednesday, April 22, 2020 at 6:22:27 PM UTC-4, Phil Allison wrote: > > George Herold wrote: > > > > =================== > > > > > > > > > Hmm? Are you asking about the voltage drop versus current > > > or vs temperature. > > > > > > > ** Try reading the damn heading - dickwad !! > > > > > > > > .... Phil > > Well dick head, I did, and everyone seemed to be answering the > question of voltage drop vs current. > > GH
Please excuse me. Getting mad online is not constructive. George H.
On Wednesday, April 22, 2020 at 6:41:21 PM UTC-4, boB 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 > > > 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.
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.
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.&Acirc;&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.
That's interesting. All the diode lasers I know of tune towards the blue at low temperature.
> 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
George Herold is a Dope wrote:

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

> > > > > > Hmm? Are you asking about the voltage drop versus current > > > or vs temperature. > > > > > > > ** Try reading the damn heading - dickwad !! > > > Well dick head, I did,
** Then you ignored it.
> and everyone seemed to be answering the > question of voltage drop vs current. >
** So you wrote a pile of drivel about the colour changing with low temps. Yaawwwwnnn.... Context has no meaning in your bubble. .... Phil
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