Do you think an LED is a resistor?

[Sredni]

A resistor has an identical resistance value for both DC and AC voltages (disregarding real world parasitics). Simple as that.
An incandescent light bulb is a temperature dependent resistor, but the temperature rise is identical for DC and the same AC RMS voltage.
Another way of viewing it: a resistor has unity power factor.
A diode or LED doesn’t have those properties.

You are differentiating between resistors with linear, nonlinear symmetrical and nonlinear asymmetrical characteristics. A linear resistor and an incandescent lightbulb have symmetrical VI chars , a diode does not have a symmetrical VI char.

And no, it is not MY knowledge. I have given a well respected reference for that.

[bdunham7]

I can even show my hand: it is a resistor, a nonlinear resistor to be precise. Why? because its state is determined by the present values of voltage and current (and not their derivatives and integrals like capacitors, inductors, and memristors).

You could distinguish between resistors and nonresistors just based on your stated criteria, but then neither actual resistors nor LEDs would qualify since their previous history determines their thermal profile (temperature and internal gradients) and probably other things--thus their states are not determined solely by present values.  Also, you'd have to define the range of time, current, voltage, frequency and temperature values that apply to your attempts to classify the LED.  Depending on other parameters, an LED could be a capacitor or inductor, a photocell, a detonator or an antenna.  And so on.

But a lot of people have trouble in recognizing that.

I just wanted to see how widespread this misconception is.

Some ways of understanding things are more useful than others.  I think most of us understand the operation of an LED well enough to fully utilize and accomodate its characteristics without classifying it as a non-linear resistor.  Can you explain how considering it a non-linear resistor is helpful?

[Sredni]

For example, in this other forum (or whatever it is called) it was argued that you can't use a multimeter measure the resistance of an LED because "it is not a resistor", while in fact the incapability to return a value is that at the small testing current used by the DMM either the voltage had to exceed the compliance of the current source, or the resistance to measure would be out of range.

You don't even follow your own rules. A DMM does measure the "resistance" of a diode, including Leads AT THE OPERATING POINT DEFINED BY THE DMM'S DESIGN..  No confusing nonsense about compliance points etc. 

This is why the definition of resistance you propose is not particularly useful.

Of course it measures the resistance at the DMM operating point. The DMM returns a single value and since the resistance has infinite values (unlike in the linear case) you have to select one.

If you want to know the resistance (not the incremental one) offered by a diode in a circuit you can use load lines.

Oh, BTW, the same is true for the threshold voltage of a diode (which has infinite values as well) or the hFE of a transistor.

[bdunham7]

Can you show that an LED does not have unity power factor?

PF is calculated over time, at least a complete cycle of AC, otherwise it is meaningless.  Do you think an LED will have a unity power factor with an applied AC voltage with a peak greater than its V_f?

[Zero999]

Another way of viewing it: a resistor has unity power factor.
A diode or LED doesn’t have those properties.

Can you show that an LED does not have unity power factor?

Only linear resistive loads have a unity power factor. An LED is non-linear, so it will not have unity power factor.

[magic]

Here's another one:

I think it's obvious that MOSFETs are resistors too, but are they current controlled or voltage controlled resistors?
I think they are current controlled, because without current flowing to the gate their resistance doesn't change.

Would you agree?

[Sredni]

I can even show my hand: it is a resistor, a nonlinear resistor to be precise. Why? because its state is determined by the present values of voltage and current (and not their derivatives and integrals like capacitors, inductors, and memristors).

You could distinguish between resistors and nonresistors just based on your stated criteria, but then neither actual resistors nor LEDs would qualify since their previous history determines their thermal profile (temperature and internal gradients) and probably other things--thus their states are not determined solely by present values. 

Well, that is true for everything. In all circuit theory textbooks the essential traits of a components are isolated, and the secondary effects are neglected (until later chapters where all effects are combined).
I mean, this is the ordinary way of teaching circuit theory everywhere:

Resistors, it's voltage vs current
Capacitors, it's voltage versus charge
Inductors, its flux versus current
Memristors, usually not mentioned (but its flux versus charge)

But a lot of people have trouble in recognizing that.

I just wanted to see how widespread this misconception is.

Some ways of understanding things are more useful than others.  I think most of us understand the operation of an LED well enough to fully utilize and accomodate its characteristics without classifying it as a non-linear resistor.  Can you explain how considering it a non-linear resistor is helpful?

I am a black swan hunter, and misconceptions like these are the main food of black swans. I see this as a failure of connecting the dots between different chapters of the same book: it is in a separate chapter, it has a different name, so it has to be something different. Nah, it's just in a different chapter and has a different name.

(Minor) case in point, in this other venue it was stated that the DMM could not read the resistance of the diode (alone) because the diode is not a resistor (I have to double check the phrasing). This is a misunderstanding of the nature of the component and of the operation of the DMM.

[Rick Law]

All these times, I thought resisters are those who fight against the enemy while under enemy occupation....

This is a language/semantic issue.  Meaning of words varies from group to group, region to region, country to country.  In Europe, I may well want "Gas" in my bottled water.  In the USA, I would prefer "sparkle" or "carbonated water".  You put "Gas" in your car, and expel "gas" when you eat certain food.

This as an electronics forum, one can take some assumptions -- but this forum has such wide audience that if one wants that language precision, one must express it so or request it so.  Otherwise, with such wide audience, it would be merely "precision leaning" but not "precision conforming".

Precision use of language has it minuses.  I think those trained in engineering or physical sciences have a tendency to demand the same precision in wording from others.  This may be why in most businesses-side folks and tech-side folks don't communicate well with "the other side".

[bdunham7]

This is a misunderstanding of the nature of the component and of the operation of the DMM.

No it isn't a misunderstanding of that sort at all but simply a different use of the terminology, an assumption that the terms resistor and resistance mean simple linear resistance that heeds Ohms law.  A VI curve gives you a good understanding of the nature of the component even if you don't use the term "nonlinear resistor". 

[bdunham7]

I am a black swan hunter, and misconceptions like these are the main food of black swans.

You think that some calamity will result because an otherwise qualified engineer that understands the VI curve of an LED is going to make some ridiculous error because they don't consider the LED to be a resistor?  I asked you how this revelation that LEDs are actually (nonlinear) resistors could be useful.  Could you be less grandiose and more specific? 

[bdunham7]

All these times, I thought resisters are those who fight against the enemy while under enemy occupation....

Then the LED is a "collaborator"--it pretends that it will resist and then capitulates.

Precision use of language has it minuses.  I think those trained in engineering or physical sciences have a tendency to demand the same precision in wording from others.  This may be why in most businesses-side folks and tech-side folks don't communicate well with "the other side".

Actually I think the precision wording is more important in a legal setting where the words are literally the substance.  In physical sciences, I think one indicia of intelligence is when one can recognize concepts and principles when they are phrased differently or use different terminology.  In the metrology forum we can't even agree on what "calibration" means and there are reliable reference sources that differ on that matter.  There's no need to agree as long as you're clear on which definition you mean.  In physical sciences, words are describing a very precise reality as best we can and there's not one absolutely correct way of doing it.  There may be well-established conventions, but that's a different matter.

[Sredni]

I don't consider this to be a mere semantic issue. It is the signal that something is amiss in the educational process.
It reminds me of that chapter in "Are you joking Mr. Feynman ?" when Feynman was in Brazil and found that physics education there was fundamentally notionistic.

The diode-resistor issue seems to point to a compartmentalized knowledge problem, where the student is not able to generalize a concept learned in a previous chapter (or course) because nobody has connected the dots for him/her.

[magic]

I think it simply is a matter of the ratio of voltage and current through a diode never being important for anything practical, unlike in a resistor, hence no one cares about it.

If anything, it's dynamic resistance which may matter. Some diodes even specify it in the datasheet (at some nominal bias current).

[IanB]

I don't consider this to be a mere semantic issue. It is the signal that something is amiss in the educational process.
It reminds me of that chapter in "Are you joking Mr. Feynman ?" when Feynman was in Brazil and found that physics education there was fundamentally notionistic.

The diode-resistor issue seems to point to a compartmentalized knowledge problem, where the student is not able to generalize a concept learned in a previous chapter (or course) because nobody has connected the dots for him/her.

But what is amiss? What dots are not joined up?

A resistor is a circuit component with the primary purpose of acting as a resistor. A diode (LED or otherwise) is not such a component.

A resistor is a component with a power factor of unity in an AC circuit. A diode does not meet that criterion.

What I might grant you, is the often stated idea that a diode can magically drop 0.6 V in a circuit. What can be missed about this is the fact that the diode will necessarily dissipate heat equal to 0.6 V times the current flowing. The voltage drop is not "magic", it is resistive in nature.

[bdunham7]

I don't consider this to be a mere semantic issue. It is the signal that something is amiss in the educational process.

Could you be less grandiose and more specific?

[schmitt trigger]

Can you show that an LED does not have unity power factor?

. Power factor has two components, displacement angle and distortion.
An LED used as a load fed with an AC voltage, will rectify and distort the current waveform.

[Sredni]

I don't consider this to be a mere semantic issue. It is the signal that something is amiss in the educational process.

Could you be less grandiose and more specific?

I did, in the post you are quoting.
The inability to generalize a concept, namely the concept that resistors are described by voltage-current characteristics that do not need to be linear (nor symmetric).

[SiliconWizard]

its more like an antenna

Yes, an antenna that captures dark energy.

[Sredni]

A resistor is a circuit component with the primary purpose of acting as a resistor. A diode (LED or otherwise) is not such a component.
-snip-

What I might grant you, is the often stated idea that a diode can magically drop 0.6 V in a circuit. What can be missed about this is the fact that the diode will necessarily dissipate heat equal to 0.6 V times the current flowing. The voltage drop is not "magic", it is resistive in nature.

But a diode does act as a resistor: a giant (almost independent from operating point) resistance that impede current flow when reverse biased, and a small (operating point dependent) resistance that let current flow when forward biased. And you confirm it in your last paragraph.

It walks like a duck, it quacks like a duck and it dissipates power like a duck.

[xrunner]

But a diode does act as a resistor: a giant (almost independent from operating point) resistance that impede current flow when reverse biased, and a small (operating point dependent) resistance that let current flow when forward biased. And you confirm it in your last paragraph.

It walks like a duck, it quacks like a duck and it dissipates power like a duck.

Sure you can go that route, but then you render the term "resistor" almost meaningless to anyone.

Some beginner asks Mr. Sredni "What is a resistor?" Smartly he yelps, "Well it's anything at all: an LED, an apple, a quarter, a leaf, a jolly rancher, an ice cube, a cat - oh yea and also those little components with color bands by the way. Just tell those college boys that when they talk about 'em!"

Quack, quack. 

[hans]

I assure you, this is not a troll post.
From the point of view of circuit theory, a diode IS a nonlinear resistor. Period.
You can read, for example, Chua, Desoer and Kuh, "Linear and nonlinear circuits". Top level authors and respected university level textbook.

And an opamp configured in a peculiar way will be a negative resistor.

No its not a resistor. Its just modeled as with a resistive impedance. It "IS" not a resistor, as in the component class, not the modeled properties.

I agree that small signal models will have ''resistors'' pop up everywhere.. transistors, diodes, etc. But those model non-ideal properties (e.g. the channel length modulation of a MOS FET or "output impedance" of a BJT) and are not desired. An ideal diode would have Zd=0 Ohm when Vd>0V and Zd=Inf when Vd<0V

[bdunham7]

I did, in the post you are quoting.
The inability to generalize a concept, namely the concept that resistors are described by voltage-current characteristics that do not need to be linear (nor symmetric).

I think the debate over whether the term "resistor" (when referring to a discrete device) applies only to things that nominally follow Ohms law or applies more generally as you assert is a profoundly uninteresting question.  To me it is like debating whether the term "automobile" applies to pickup trucks.  You may think you know the right answer, but no actual knowledge of the physical universe attaches to these definitions. 

When people talk about the resistance of non-ohmic devices, they can mean either V/I or dV/dI.  And in almost all cases, for example the specified dynamic resistance of a zener diode at various currents, dV/dI is the meaningful number.  Negative resistance is another example, this means that dV/dI is negative, not V/I--unless you are going to expand the definition of "resistor" to include batteries.  Of course, the way you've stated it your definition could include batteries.  If we're going to reclassify electronic components, why not call them all "marklar"?

[switchabl]

I don't consider this to be a mere semantic issue. It is the signal that something is amiss in the educational process.

The diode-resistor issue seems to point to a compartmentalized knowledge problem, where the student is not able to generalize a concept learned in a previous chapter (or course) because nobody has connected the dots for him/her.

I am (mostly) fine with the generalization. In fact it seems I did guess what this was all about after all (if jokingly):

"Aren't all memoryless, time-invariant two-ports just voltage dependent resistors in the end?"
- Gustav Kirchhoff, probably

What is a very bad idea in my opinion is implicitly broadening the meaning of a word. You are effectively redefining the word "resistor" to encompass this generalization. In other words, you now want me to always say "linear resistor" if I want to avoid this new meaning. That can only lead to confusion and debates without substance.

And I might want to generalize the concept along a different axis. After all, in some ways, a resistor has more in common with a capacitor than a diode. They are both linear, passive, time-independent, reciprocal, symmetric. I'll just call it a reactive resistor. Which is really just another kind of resistor, right? Just make sure to call the regular one "resistive resistor" now.
[Yes, that's just a step removed from the idea of a "complex impedance". I think it's a good thing we ended up using a different word for this.]

The original question also suggests that there is such a thing as a "true" definition of a word. You are free to use your own definitions but if you want to communicate with others you have to be explicit about that. And then we can argue whether they are commonly used, useful, confusing, consistent, etc. But to me at least it is not meaningful to ask which one is "the correct one".

[Nominal Animal]

Making a poll about terms without defining those terms is either trolling, or rather stupid.

The entire question is only about one of implicit definitions, and has nothing to do with understanding.  Just replace "an LED" with "a male after surgery", and "a resistor" with "a woman", to see what I mean.

[Sredni]

I am (mostly) fine with the generalization. In fact it seems I did guess what this was all about after all (if jokingly):

"Aren't all memoryless, time-invariant two-ports just voltage dependent resistors in the end?"
- Gustav Kirchhoff, probably

What is a very bad idea in my opinion is implicitly broadening the meaning of a word. You are effectively redefining the word "resistor" to encompass this generalization.

No, why?
A resistor is a device whose state is defined by the current values of voltage and current.
That's it.
It applies to linear resistors, to symmetric nonlinear resistors (incandescent lamps), to asymmetric nonlinear resistors (diodes, LEDs...), etc.

One might use the specifier 'nonlinear' for the diodes, but that does not mean they cease to be resistors; they are just a specific kind of resistor.

Yes, you got it right in your first message.
To my knowledge there are only four types of passive circuital elements: resistors, capacitors, inductors,and memristors. All the rest can be modeled with combinations of these elements, including active devices and circuits.

To avoid making a further post, here is a note from Fluke where they show how to measure the (static) resistance of a diode. It's easy if the forward voltage at the supplied current stays below the compliance of the internal current source.

https://www.fluke.com/en/learn/blog/digital-multimeters/how-to-test-diodes