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LED forward voltage drop with temperature

Started by John S April 22, 2020
Am 22.04.20 um 19:52 schrieb bitrex:

> 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
These are my measurements from a few years ago (just 2 types): http://www.hoffmann-hochfrequenz.de/downloads/Led_tk.pdf BTW if you want to abuse a LED as a _very_ low voltage noise reference, Avago HLMP-6000 is the best by far, whoever owns that now. It's a quite dimm bulb. I have the impression that optical efficiency goes with noise. cheers, Gerhard
On Wed, 22 Apr 2020 18:46:53 -0400, "Tom Del Rosso"
<fizzbintuesday@that-google-mail-domain.com> wrote:

>John Larkin wrote: >> >> The V:I curve should have a zero tempco point. > >But where is that point relative to the useful current range? > >
Depends. It's rare to see a diode data sheet that shows the VI curve at different temperatures, much less at even rated max current. I've deliberately run some small schottky diodes at the zero TC point, which was about 15 mA. Big power diodes sometimes show that point on the data sheet. That counters the conventional wisdom about thermal runaway current hogging of diodes in parallel. Some of the big power diode bricks are diodes in parallel. Here's one, zero TC at 200 amps, well past rated current in this case. https://www.dropbox.com/s/aes9hav7rfymuwh/SD41.pdf?dl=0 It would be interesting to test an LED. People make cheap flashlights with a battery and an LED, so some must be substantially ohmic. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
On 4/22/2020 11:05 PM, Gerhard Hoffmann wrote:
> Am 22.04.20 um 19:52 schrieb bitrex: > >> 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 > > These are my measurements from a few years ago (just 2 types): > > http://www.hoffmann-hochfrequenz.de/downloads/Led_tk.pdf > > > BTW if you want to abuse a LED as a _very_ low voltage noise reference, > Avago HLMP-6000 is the best by far, whoever owns that now. > > It's a quite dimm bulb. > I have the impression that optical efficiency goes with noise. > > cheers, Gerhard
Thank you Gerhard. I will save your pdf for my future reference.
On 4/22/2020 9:38 PM, Phil Allison wrote:
> > 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 >
Thanks, Phil. It looks like LTSpice is giving me what I suspected was incorrect info. Spice is showing about 4mV/C positive voltage with temperature at 2mA. I need to make bench measurements and not depend on spice.
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
> > 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. > > > > >
On 4/23/2020 6:49 AM, Jasen Betts wrote:
> On 2020-04-22, 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. >> > > Look at John's circuit, the LED voltage is matched by the V_be and > resittor, for it to compensate both V_be and LED must move in the same > direction. >
Thanks, Jasen. Yes, I studied his circuit and understand how it works.
On Wednesday, April 22, 2020 at 7:25:49 PM UTC-4, Phil Hobbs wrote:
> 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.&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.
So I think it's green LED's that 'go the wrong way'. https://www.youtube.com/watch?v=wmdIFLoBYOU And I'm assuming the forward voltage changes too. George H.
> > > 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
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. 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. 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. Transistor self-heating would be a problem if one wanted serious stability. Base current too. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
On Thu, 23 Apr 2020 05:35:30 -0500, John S <Sophi.2@invalid.org>
wrote:

>On 4/22/2020 9:38 PM, Phil Allison wrote: >> >> 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 >> > >Thanks, Phil. It looks like LTSpice is giving me what I suspected was >incorrect info. Spice is showing about 4mV/C positive voltage with >temperature at 2mA. I need to make bench measurements and not depend on >spice.
Right. There are all sorts of LED chemistry and fabs around. I wouldn't trust any of the LT Spice LED models very far. My green LED really seemed to be an oddball. I don't think my transistor was oscillating, but that's possible. I really like the color of the orange Osram, so I used that one. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
On 4/23/2020 10:06 AM, jlarkin@highlandsniptechnology.com wrote:
> 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. > > 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. > > 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. > > Transistor self-heating would be a problem if one wanted serious > stability. Base current too.
Any "basic math" you do on that circuit will result in a transcendental equation that can't be solved in closed-form, anyway so you'll have to resort to numerical methods in any event. The only way to solve just the diode-resistor circuit exactly for the R-D junction voltage in "closed form" is with the Lambert W function <https://en.wikipedia.org/wiki/Lambert_W_function> if the base current assumed to be 0 a simple model would be: <something involving lambert W and the diode Is and the supply voltage> = Is log (Vbe/Vt) + I*R_e. The diode and transistor Is's are non-linear functions of temperature so unless they cancel neatly like in an a differential pair with matched transistors leaving only the secondary temperature dependence on Vt, without further data about Is this equation doesn't tell you much about real-world temperature stability.