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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.
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.
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 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.
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.
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
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.
Hmm? Are you asking about the voltage drop versus current
or vs temperature.
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.
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
The V:I curve should have a zero tempco point.
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. 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 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.
Hmm? Are you asking about the voltage drop versus current
or vs temperature.
** Try reading the damn heading - dickwad !!
Well dick head, I did,
and everyone seemed to be answering the
question of voltage drop vs current.
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
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?