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LED forward voltage drop with temperature

Started by John S 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.
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.