Do you think an LED is a resistor?

[SiliconWizard]

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.

Yes.
Beyond the definitions, there's an underlying thought that any model of a reality equates this reality - for which, incidentally, your example also works.

[Sredni]

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

Had I defined the terms, there would be no reason for a poll. What I am polling is other people's definition of diode and resistor, so there is no point in suggesting mine (even if I gave it in the ensuing discussion).
Maybe I should rephrase the poll as

Are nonlinear resistors resistors?

Or

Do nonlinear resistors exist at all? And if they do, why can't they have an exponential VI characteristics?

[PlainName]

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

You are misreading it. They say it's not showing the diode resistance, it is just showing a resistance reading one way and not the other, which you can use to assume the diode is working. Like if you shove a large voltage up a digitial thermometer input it will show a temperature reading. That's not the temperature of whatever is supplying the voltage, it is merely using the temperature reading to indicate a voltage is present.

[Sredni]

It is reading the *static* resistance at the operating point set by the constant current supplied by the DMM.

(It is one value among infinitely many, but it shows that a multimeter can measure the resistance of a diode when used in ohmmeter mode on the device alone).

[switchabl]

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.

Because that is not a common definition in an engineering context where resistor (without further qualification) very strongly implies "ohmic". And this is an EE forum. Theory papers and books sometimes use it as a shorthand for anything that lives in the I-V plane. But they are usually very careful to make that explicit.

[Sredni]

So a nonlinear resistor is NOT a resistor?

[NiHaoMike]

A LED is not a resistor. An incandescent bulb is a resistor, one that emits light and has a rather high temperature coefficient. (So those should be called LERs?)

[amyk]

Since no one has said it yet: yes, it turns into a resistor if you pass enough current through it for long enough.

[bdunham7]

It is reading the *static* resistance at the operating point set by the constant current supplied by the DMM.

(It is one value among infinitely many, but it shows that a multimeter can measure the resistance of a diode when used in ohmmeter mode on the device alone).

Those DMMs that work with a fixed precision test current (which is not all of them b/t/w) are actually displaying the voltage that results from the test current or sometimes a computed ratio of that voltage.  One older model I have specifically refers to diode testing using the 2k resistance range, which uses a 1mA test current and then displays voltage with 2V full scale.  So a typical diode will read like a 600 ohm or so resistor, as you'd expect.  Can I claim that resistors are actually symmetrical linear diodes (diode means two wires, after all) with a highly current-dependent V_f?  Or would that be ridiculous and confusing?

[IanB]

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.

Since at this point you appear to be trolling, let us cut through your trollish behavior.

A resistor is a device with the primary purpose of introducing resistance into a circuit. If that is not its primary purpose, then it is not a resistor, it is something else.

Many devices exhibit resistance, but not all of them are resistors.

If you start calling diodes or other things resistors, then you have made the word "resistor" useless, since that word can no longer be usefully used to identify the things which are sold, labeled, used, and drawn on schematic diagrams as resistors.

[Sredni]

Vf does not exists. It is a figment of our imagination. Plot the exponential characteristic on a logarithmic scale and you will see a straight line. Where is Vf?

But anyway, it's not that nonlinear functions are specializations of linear functions. Among all characteristics in the VI plane, the really special ones are the linear ones - because they allow you to apply superposition. This is why we try to linearize problems wherever we can (small signal analysis, and dynamic resistance which is locally linear).

I have to ask again: is a nonlinear resistor NOT a resistor?

[Sredni]

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.

Since at this point you appear to be trolling, let us cut through your trollish behavior.

Again with this trolling. Did you not see the reference I gave? Leon O. Chua is an authority in the field (I believe he was the one who postulated the existence of the memristor, he's the Chua of Chua's circuit). Both he and Charles A. Desoer taught at MIT. And MIT is the absolute excellence in the EE field. Do you think they are trolls?
Desoer and Kuh also wrote the previous high level bible of circuit theory, used in universities all over the world. No trolling, believe them.

You think that a resistor is defined upon its primary purpose. That's a nice definition for vocational school students. But when you raise the level and want to systematize the resolution of general nonlinear circuits containing any sort of elements you must be a little more cunning. You need to identify the true nature of a component from the point of view of the state variables. And this leads to generalization of concepts. As I said, there are four type of elements: resistors, capacitors, inductors and memristors. A diode? It's a nonlinear resistor. A varicap? It's a nonlinear capacitor. A Josephson junction? It's a nonlinear inductor.
And this is not classification per se: you need to place your elements variables in the correct part of the vector and matrices used to systematically solve these circuits. You don't have slots for diodes, Josephson junctions and so on. You have elements that depends on certain quantities, their derivatives or integrals.

I acknowledge that, so far, the majority of members seem to think that diodes are not resistors, it's fine. I am a bit puzzled by the lack of stance on the existence of nonlinear resistor according to them.

Do nonlinear resistors exists? And why can't they have an exponential VI characteristic?

[bdunham7]

Vf does not exists. It is a figment of our imagination. Plot the exponential characteristic on a logarithmic scale and you will see a straight line. Where is Vf?

Well, that isn't really true in my world(there may be a straight-ish section but not the whole VI plot) but that's beside the point so lets talk about your world.  If you can define 'resistance' as a ratio that changes with applied voltage and/or current and thus can be any number (infinite values, right) and only determined for each individual operating point, then I can claim the same for V_f.  It's not a constant, it just is what it is under any specific condition that one chooses to test it at.  So if I test my newly defined diode (formerly "resistor") with a test current of 1mA and then 2mA, V_f is simply twice as high in the second case.  It has infinite possible values.

[bdunham7]

Do you think they are trolls?

Do nonlinear resistors exists? And why can't they have an exponential VI characteristic?

Nobody is accusing them of being trolls.  Some are accusing you of being a troll.  That's a pretty simple concept and quoting respected authoritative people doesn't change anything either way.

What you are calling a "nonlinear resistor" certainly does exist, but that doesn't mean we have to accept either your terminology or your assertion that failing to acknowledge some theoretical connection between these devices and what is more typically known as a resistor is some big educational failing or lack of understanding.  The definition of a resistor doesn't need to be rigidly defined by a concise theoretical concept either.  Again, if you like, call them all "marklar". 

[magic]

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

Considering both the question of the poll and the choices of answers, why not both?

[electrodacus]

Vf does not exists. It is a figment of our imagination. Plot the exponential characteristic on a logarithmic scale and you will see a straight line. Where is Vf?

How do you define an ideal diode?
Also for contrast how do you define an ideal resistor ?

[Sredni]

Vf does not exists. It is a figment of our imagination. Plot the exponential characteristic on a logarithmic scale and you will see a straight line. Where is Vf?

How do you define an ideal diode?
Also for contrast how do you define an ideal resistor ?

My point of view on the non existence of a Vf is summarized here
https://electronics.stackexchange.com/questions/655076/forward-bias-voltage-of-diode/655157#655157

It applies to exponential functions.
An ideal diode has a piecewise vi characteristic that has a definite Vf of zero volts. It's kind of a special case of nonlinear resistor.

[Sredni]

What you are calling a "nonlinear resistor" certainly does exist,

Ok. So if nonlinear resistors do exists and if their vi char can be any (reasonable, non-pathological) function, then we can imagine a nonlinear resistor with exponential characteristic exactly identical to that of a diode.
Is this nonlinear resistor a... resistor?

but that doesn't mean we have to accept either your terminology

I do not ask anyone to accept "my" terminology (again it is not mine), I am just curious to see how this misconception is widespread.

[electrodacus]

Vf does not exists. It is a figment of our imagination. Plot the exponential characteristic on a logarithmic scale and you will see a straight line. Where is Vf?

How do you define an ideal diode?
Also for contrast how do you define an ideal resistor ?

It applies to exponential functions.
An ideal diode has a piecewise vi characteristic that has a definite Vf of zero volts. It's kind of a special case of nonlinear resistor.

I think it will be useful if you can define what an ideal diode is and see if it has anything in common with an ideal resistor.

Let me know of you agree with this ideal diode definition:
Perfect conductor in forward bias and zero volt drop.
Open circuit in reverse bias

An ideal resistor will be defined as purely resistive.

If you agree with this definitions then diodes are clearly not resistors.

[Kim Christensen]

Here I present to you, the "Sredni Common Emitter Amplifier"...

Notice how the voltage divider formed by R1 and R2, sets the base voltage perfectly. These are Blue and Red LEDs respectively. (You may have to stack multiple blue LEDs. This task is left to the students)
R4 (1N4007) and R3 (1N4736) are selected so that Q1 operates in it's linear region and the quiescent voltage at the collector is 1/2Vcc. Input coupling is provided by capacitor C1 and output coupling by C2. There will be some capacitor leakage.

For the students,
Q: If Vcc is 12V and V1 is a perfect 1Vpk-pk sinewave, what is the output waveform's Pk-Pk value and THD?
Bonus Q: model Q1 as an amplifying resistor network. Show your work.

[Sredni]

Kim, do you consider all diodes to be equal? Can you use a tunnel diode where a Schottky diode is needed, or a zener where an LED is required?

I hope not.
Each diode has a specific vi characteristic, just like each linear resistor of different value has one. I consider them all resistors, but I choose them with the right characteristic for their application.

And I am supposed to be the one trolling?
😁

[Kim Christensen]

Kim, do you consider all diodes to be equal? Can you use a tunnel diode where a Schottky diode is needed, or a zener where an LED is required?😁

Those aren't diodes.
They are resistors.

[bdunham7]

Is this nonlinear resistor a... resistor?
I do not ask anyone to accept "my" terminology (again it is not mine), I am just curious to see how this misconception is widespread.

You can call it that if you like.  In the case of an LED, it does 'resist' to a degree so I suppose I can't say it is wrong, just that it is an unusual and unhelpful degeneration of the term 'resistor'.  But what exactly is this 'misconception' that you keep bleating about?  That your 'nonlinear resistor' devices shoud be lumped in with linear ones as kindred spirits and not have a separate category of their own?  Meh.

[DimitriP]

TL;DR
...but this has a smell of an upcoming veritasium video.

I am a resistor of most things "rather stupid".
We should all be resistors.

[IanB]

It's mainly a problem of language and logic.

Properties of circuit elements include resistance, capacitance and inductance.

All resistors have resistance. Not all devices with resistance are resistors.

All capacitors have capacitance. Not all devices with capacitance are capacitors.

All inductors have inductance. Not all devices with inductance are inductors.

If the topic title were changed to something like "Do LEDs exhibit resistance to electric current?" then the whole thread would not exist.