Do you think an LED is a resistor?

[SiliconWizard]

Yes, but as I said earlier, that may just be a rhetorical question involving modeling components.

Can a diode be mathematically modeled with a variable resistor, the resistance of which is a non-linear function of voltage? For a simplified model, yes. Does that make it a resistor?

From the usual definition, no, as Ohm's law defines a resistor as a constant relationship between voltage and current. It can't vary, linearly or non-linearly. Now, real resistors have a resistance which does vary, at least with temperature (and thus, with current, due to damn Joules).

Models.

[magic]

Nah, it's another common and utterly boring case of somebody learning about resistors and insisting that everything that passively resists current flow must be a resistor, on purely linguistic grounds.

The OP hasn't posted any single mathematical model of anything.

edit
Except for the idea that you can define "momentary resistance" of a diode as the ratio of its momentary Vf and If, which is an utterly useless notion and I'm pretty sure it is not the topic of any reputable sources that the OP might bring up to justify himself.

[switchabl]

So a nonlinear resistor is NOT a resistor?

In an ordinary (non network theory) sense, no. Blame it on the theorists who couldn't be bothered to come up with a new name for their extended concept. The physical component and it's idealized (linear) version definitely came first. And nobody wants to keep specifying "linear resistor" when that's what they mean 99% of the time.

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?

Vf exists in an approximate sense because there is a limited useful current range between "does not visibly light up" and "catches fire". As a consequence of the exponential characteristic, the voltage does not actually vary that much throughout this range. For design work you would usually start there, then check the actual I-V curve or do a simulation later if you need something more accurate.

Except for the idea that you can define "momentary resistance" of a diode as the ratio of its momentary Vf and If, which is an utterly useless notion and I'm pretty sure it is not the topic of any reputable sources that the OP might bring up to justify himself.

It is unfortunately a thing in (non-linear) network theory to call everything a "resistor" that can be defined in terms of f(v, i, t) = 0. A regular resistor would actually be a "linear, time-invariant resistor" in that terminology. I have never before met anyone who tried to push that definition in engineering.

It's a bit like going on a computer graphics forum, showing a picture of a circle and asking people if it's "not a sphere, because it's a circle" or "just a different kind of sphere". Indeed, in mathematics the word "sphere" is usually generalized to include "spheres" of arbitrary dimensionality of which both the circle (the 1-sphere) and the actual sphere (the 2-sphere) are just special cases. And you can find many reputable sources which will back this up. But that doesn't make more appropriate or less confusing in a different context.

[tom66]

Yes.

But only when you burn it out and resistance approaches zero.  That makes it a Darkness Emitting Diode (DED).

[Zero999]

I wonder how many people here chose electronics to avoid philosophy?

This discussion is no longer about engineering or science. We might as well talk about fascism, Marxism, feminism, gender ideology, critical race theory etc. 

At this point, discussing critical social justice seems far more productive.

Let's start by looking at systemic racism and patriarchal influence in engineering. The fact that the colours brown and black in the resistor colour code indicate low values 0 and 1 and white is the highest at 9, demonstrates that resistors are racist and white supremacist. Male and female connectors are a sign of heteronormativity, which marginalizes gay and non-binary voices. 

[tom66]

OK, this thread has officially jumped the shark with that post.   

[Berni]

Anything can be a resistor if you want it to look like a resistor.

Inductors have internal resistance so at DC they are low value resistors.
Capacitors have leakage current so at DC they are a high value resistor.
Memrisors are also just a resistor if you apply DC across it.
Heck even a mono audio jack can be a resistor since the insulating materials inside it have a very very high but not quite infinite resistance.
Or even all 2 terminal devices could be considered resistors since they can have a voltage U and current I across it so you can apply ohms law.

The question is more "is it appropriate to think of it as a resistor?"

In some cases it definitely is appropriate to think of a diode as a resistor. For example if you are analyzing the feedback loop of a LED or laser diode driver or something you will be interested in the didoes dynamic resistance, since it is what affects the loop gain and might decide if your loop is stable or not.

But in the vast majority of cases it is better to consider the diode being well.. a diode.Especially in simple calculations like calculating the correct resistor for a LED. It is definitely not appropriate to wave this idea around in an educational setting as this would only serve to confuse people into thinking they can use a diode as a resistor in a circuit. After all what datasheet specifies the characteristic in Ohms? They all show the IV curves instead, those make more sense.

Imagine the diode as whatever helps you get the job done. Just don't expect others to imagine it the same way because they might be doing the same job with that diode

[Siwastaja]

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).
...
But a lot of people have trouble in recognizing that.
...
I just wanted to see how widespread this misconception is.

A very INTERESTING and internally coherent and logical way to define a resistor, but you can't just arbitrarily come up with new meanings for existing words and then say others have "trouble" recognizing your invention, or call this misconception.

There is a simple solution to all of this: just call an LED LED. Then anyone can check out the characteristics, such as voltage-current transfer curve, amount of capacitance and inductance, from the literature, or, more exactly, from a particular datasheet. This is to avoid obfuscation and word trickery.

[Sredni]

So, it seems that the main objection comes down to:

Since Ohm's law is defined as a linear relationship, nonlinear *resistors* are not resistors.

Fascinating.

[langwadt]

So, it seems that the main objection comes down to:

Since Ohm's law is defined as a linear relationship, nonlinear *resistors* are not resistors.

Fascinating.

if you see everything you can stick the probes of an ohmmeter on and get a result as a resistor then everything is a resistor, making the word pointless

[Rick Law]

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).
...
But a lot of people have trouble in recognizing that.
...
I just wanted to see how widespread this misconception is.

A very INTERESTING and internally coherent and logical way to define a resistor, but you can't just arbitrarily come up with new meanings for existing words and then say others have "trouble" recognizing your invention, or call this misconception.

There is a simple solution to all of this: just call an LED LED. Then anyone can check out the characteristics, such as voltage-current transfer curve, amount of capacitance and inductance, from the literature, or, more exactly, from a particular datasheet. This is to avoid obfuscation and word trickery.

Adding to Siwastaja's point:  When one can (re)define resistor that way, then everything on earth is a resistor.  You sure can call "Air" a resistor, just give it enough voltage, we see thunder lightning.

Current flow is a matter of electrons transfer.  We all know that electrons can go anywhere physical condition dictates, even in vacuum.  I am hard pressed to think of any material that entirely block the transfer of electrons from point A to point B in this universe we know. 

I personally think, best definition for resistor is "Something design to pass a specific amount of current directly proportional to the voltage applied (ohm's law)."

[Nominal Animal]

When no specific context or definition is supplied, one should default to the dictionary meaning for maximum chance of successful communication.  Otherwise, it is almost certain that the discussion will be about the definition or context, and thus superficial; never reaching the actual interesting stuff.  It is like arguing about when to argue about a practical thing, instead of arguing about the practical thing directly.

For a resistor, this is "an electric component that transmits current in direct proportion to the voltage across it" (Wiktionary); "a part of an electrical circuit designed to produce a particular amount of resistance to the flow of current" (Cambridge dictionary); "electrical component that opposes the flow of either direct or alternating current, [...] Resistors can have a fixed value of resistance, or they can be made variable or adjustable within a certain range, in which case they may be called rheostats, or potentiometers." (Britannica).

In other words, by default, when one uses "resistor" without a specific context or definition, it refers to a circuit element with fixed resistance, and thus a linear relationship (direct proportion) between voltage and current over it.

None of this is about electrical engineering per se; this is purely about how to communicate with others effectively.  If you disagree, you're basically offering your own theory about effective communication between humans, so you better have a ton of proof about your assertions.  A couple of eminent scientists or half-nut Nobel laureates is not enough; they are so focused on their own area of expertise that their opinions about human communications are basically irrelevant.  (In mathematics, physics, et cetera, this is amply evidenced by these scientists often developing their own notations, because they fail to convey the concepts effectively using existing notations.)

Now, if one bothers to additionally compare the dictionary references to "nonlinear resistors", one will see that terms "nonlinear element" or "nonlinear component" are preferred and more widely used; see for example Wikipedia, Britannica/memristor, and so on.

Considering all of the above:

I for one fully reject the term "nonlinear resistor", and insist you use the standard term, "nonlinear element" or "nonlinear component" instead.

If we were to accept such twisted terms, we should accept "enhanced bicycle" for an automobile, "wide-ended stick" for a shovel, "multi-prong spoon" for a fork, and so on.  Such terms reduce the likelihood of concepts being conveyed without degradation, without bringing anything positive to the discussion; only allowing semantic games and trickery, which leads to discussions where nobody learns anything new.  Therefore, such twisted terms should be rejected with maximum prejudice.

[Sredni]

Anything can be a resistor if you want it to look like a resistor.

As I said before, I am neglecting secondary or parasitic effects.
A resistor is a device that primarily behaves as a resistor. Yes, it may have some parasitic capacitance and inductance but these are minor effects unless you push the conditions too far.
A piece of coiled wire is a shunt resistor until I push the frequency so high that its inductive reactance becomes appreciable. And if I push it even further it might even show a significant capacitive reactance.

But I made clear that I am neglecting secondary effect. I am here talking about a diode completely described by it exponential Shockley equation. That is, soleley by its V-I relationship. I am even neglecting the effect of junction capacitance (which is there) because unless I push the frequency too far (or build a diode with abnormous junction capacitance to exploit that effect), its effect in the qV plane is negligible compared to the *resistive* behavior in the VI plane.

And what a diode with negligible capacitance does is... showing a resistance (which is not constant but depends on the voltage or current). That is all it does, from the point of view of the circuit. It does not matter that the power it takes out goes all in IR heat, visible light or aura vibrations. The circuit sees a variable resistance (a fixed resistance once you fixed the operating point).

And yet people cannot see it is behaving as a resistor. I find it fascinating. (I have just watched an old ST convention on YT so I'm in Spock mode, now).


(Funny that nobody has noticed I called a varicap a nonlinear capacitor...)

[MK14]

(Bold and font size change, made by me)

And what a diode with negligible capacitance does is... showing a resistance (which is not constant but depends on the voltage or current). That is all it does, from the point of view of the circuit. It does not matter that the power it takes out goes all in IR heat, visible light or aura vibrations. The circuit sees a variable resistance (a fixed resistance once you fixed the operating point).

And yet people cannot see it is behaving as a resistor. I find it fascinating. (I have just watched an old ST convention on YT so I'm in Spock mode, now).

But, it is NOT behaving like a resistor (the diode), because it reacts in completely different ways (i.e. current flow or not), depending on the polarity across it (unlike a proper resistor, which is unpolarised).  I.e. reversing the polarity, will usually have a dramatic effect on the current flow.

Also, the current verses the voltage across it (non-ohmic), is usually not proportional, in the same way it would be with a real resistor.

Example:
To run down a battery, overnight.  If I gave you a small, brand new 9 volt battery, and a power diode.  Then, as a challenge, you needed to connect it up and leave it over-night.  Assuming I've rubbed off the polarity markings on the diode.  How would you be more than 50% sure, that you had connected it up the right way, to discharge the battery (assuming I don't give you enough time, to see if it or the battery, gets hot or not), overnight?

N.B. The DIFFERENCE matters.  Because that is why a diode is called a diode and not a fuse, short-circuit, open-circuit or resistor etc.

[Sredni]

It is not behaving like a *linear* resistor. It has an asymmetric nonlinear vi characteristic. As I said, the amount of resistance depends on the operating point, but it is resistance nonetheless.

I mean, it is not capacitance. Not inductance. Not memristance.
I don't think it diodance...

Now I am curious: are nonlinear inductors... Inductors?
If I take a coil in air with linear relationship phi = L I, that is an inductor, will it cease to be an inductor when I stick a piece of iron inside it making its characteristic in the phi-I plane nonlinear?

[MK14]

It is not behaving like a *linear* resistor. It has an asymmetric nonlinear vi characteristic. As I said, the amount of resistance depends on the operating point, but it is resistance nonetheless.

I mean, it is not capacitance. Not inductance. Not memristance.
I don't think it diodance...

Now I am curious: are nonlinear inductors... Inductors?
If I take a coil in air with linear relationship phi = L I, that is an inductor, will it cease to be an inductor when I stick a piece of iron inside it making its characteristic in the phi-I plane nonlinear?

A 'resistor' is normally taken, to mean a specific physical entity (component), that often has two (or more, e.g. four) terminals/leads/connections, and a (usually) somewhat precise value and other (datasheet) characteristics, such as specific dimensions, and maximum ratings.

[Kim Christensen]

We name things by their dominant property.
That's why we do not call diodes, capacitors, and inductors, resistors.   

[IanB]

but it is resistance nonetheless

Ah, you are starting to improve your English. If you change the verb and say that a diode "has resistance", then you will be doing even better.

[switchabl]

This thread has reminded me of a delightful little story I read a long time ago and it was a nice surprise to find that someone has translated it into English: A Table is a Table

[M0HZH]

Anything can be a resistor in very specific circumstances.

Nothing is a resistor if you use too much voltage.

For everything else, we use the most dominant caracteristic.

[Sredni]

We name things by their dominant property.
That's why we do not call diodes, capacitors, and inductors, resistors.   

The property being...?

Diodance?

[electrodacus]

As I said before, I am neglecting secondary or parasitic effects.

That is not a true statement else there will be no discussion here.


I asked you to define an ideal diode as that will have no resistance at all.
Above image shows a simple equivalent model of a real diode. If that resistor is non linear as most resistors are it has nothing to do with the diode (theoretical ideal diode).
If you say a diode is nothing other than a resistor you should be able to remove that "ideal diode" from the model.

[Kim Christensen]

We name things by their dominant property.
That's why we do not call diodes, capacitors, and inductors, resistors.   

The property being...?
Diodance?

Close. It's called rectification. Diodes are also called rectifiers. That's it's dominant property and one of it's main use cases. Hence we don't call them resistors.
 

[bdunham7]

The property being...?

Diodance?

No, "nonlinear resistance" is just fine if you insist on assigning a specific term to its properties.  That doesn't mean we need to call it a resistor anymore than the fact that I can locomote makes me a locomotive.

[Sredni]

I don't know how to say it more clearly. An ideal diode is a nonlinear resistor whose vi characteristic is a piecewise function formed by an horizontal line until V=0 and a vertical line for V=0.
It is a resistor that has infinite resistance (the horizontal part of the vi char) for V<0 and zero resistance for V>=0.
I am using resistance to specify its behaviour.

The device you show is a nonlinear resistor with piecewise linear VI characteristic that shows infinite resistance for V<Vf and a resistance that depends linearly from the operating point for V>Vf.

A silicon diode is a nonlinear resistor with exponential continuous VI characteristic. It shows very high resistance for negative voltages and an increasingly  smaller (operating point dependent) resistance for positive voltages.

It is always about the resistance it opposes to the flow of current.