Do you think an LED is a resistor?

[Sredni]

I am curious. How many of you people think an LED is a resistor? I should add: from the point of view of circuit theory, not from that of technology and underlying physics.

(This question originates from a question in another forum, where polls are not possible).

[switchabl]

Is this one of those trick questions where you pull a small-signal model out of your hat when everyone's distracted or explain later that when you said "resistor" you didn't actually mean resistor?

[xrunner]

No. It has a different name, so it is not a resistor

A rose by any other name ... ?

[hexreader]

This poll makes no sense

[Stray Electron]

  What a weird question.  And NO.  The answer to your question is in the last name of the part. The name that starts with a "D".

[switchabl]

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

[coppercone2]

its more like an antenna

[themadhippy]

There the cats whiskers when compared to many other illuminating devices

[Sredni]

Is this one of those trick questions where you pull a small-signal model out of your hat when everyone's distracted or explain later that when you said "resistor" you didn't actually mean resistor?

No, no trick question at all.
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.

A diode is a resistor in the same way an incandescent lamp is a resistor. Both are nonlinear and both have exponential V-I characteristics. And yet everyone has no problem in computing power dissipated in circuits with incandescent lamp treating them as resistors, while when it comes to diodes the 'hard' exponential seems to create a mental block.

I just wanted to see how widespread this misconception is.

[CatalinaWOW]

When you use one word for everything you lose the ability to discriminate. 

Your construct is silly.  Or personal.  Most people have trouble determining power in non linear circuit elements because of the difficulty in determining the operating point.   While it is simple to do with the linear time independent component normally called a resistor.

[Sredni]

Well, if you give a circuit with a 12V battery and a 20W lamp to any high school student, they will immediately find the operating point and the resistance of the lamp. Is the lamp a resistor? Of course it is.
And yet the resistance is non linear, and the value found is Vop/Iop and has nothing to do with the local slope of the vi curve. But it is the resistance shown by the device in the circuit.

But for some reason, if you give a diode (and a means to determine the operating point), even if you end up doing the same thing, finding a resistance that has nothing to do with the local slope of the vi curve, there seems to be a mental block to accept that as the resistance shown by the device in the circuit.


Maybe it is due to the fact that in small signal analysis we use the differential resistance rd, and since this is different from the Vop/Iop value found, this latter value is not considered a resistance?

[CatalinaWOW]

The answer is in your comment.  When you specify 20 W incandescent lamp you set the operating point.  I think if you gave your students a 10 mW LED they might do better.

[Zero999]

The poll is missing an option: this is a troll poll. 

A resistor is designed to have an Ohmic response i.e.the current is proportional to the voltage. An LED is a type of diode, which in this case is meant to produce light so the voltage, current characteristicis unimportant.

[Bud]

A Resistor is reciprocal. An LED is not reciprocal.

[Sredni]

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.

I can go even further: a transistor is a resistor. It is a nonlinear, multi terminal resistor. And I guess this notion would encounter even more... resistance than the diode one. (I was kind of surprised when I first heard about it, but then when you reason about it it becomes clear why it is so - if we neglect parasitic effects).

As I said before I just wanted to see how widespread this notion is (even with a small population sample).

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.

[Sredni]

A Resistor is reciprocal. An LED is not reciprocal.

A linear resistor is reciprocal.
A linear resistor show direct, bilateral proportionality between voltage and current. A nonlinear resistor does not.

Are you arguing that nonlinear resistors are not resistors?
Or that nonlinear resistors do not exist?

[fourfathom]

Is the OP deliberately trying to confuse students? A diode has resistance, but that doesn't make it a resistor.  Eliminate the "time invariant" part and you can call a capacitor a resistor. 

[switchabl]

In the case of the incandescent bulb, the resistance is meaningful in the sense that it is the physical resistance of the filament contained inside (at the operating temperature). If you were to heat filament externally and measure again with a small test current, you would get the same result.

But even so, when it comes to circuit analysis, this value is not particularly helpful. The small signal resistance on the other hand happens to change relatively slowly throughout the hot/"on" region (the resistance change is only about ~P^0.25 due to the Stefan-Boltzmann law). So it actually predicts even larger shifts in the operating point reasonably well.

You are of course free to define a nominal resistance for the LED at a particular operating point. I'm just not sure what you would do with it. If it is for quick-and-dirty circuit analysis, approximating the LED as a voltage source is usually the most helpful option.

[Bud]

A Resistor is reciprocal. An LED is not reciprocal.

A linear resistor is reciprocal.
A linear resistor show direct, bilateral proportionality between voltage and current. A nonlinear resistor does not.

Are you arguing that nonlinear resistors are not resistors?
Or that nonlinear resistors do not exist?

A Reciprocal 2-pole means properties are same in both directions. Show me LED resistance being same in both directions.

[Sredni]

A Resistor is reciprocal. An LED is not reciprocal.

A linear resistor is reciprocal.
A linear resistor show direct, bilateral proportionality between voltage and current. A nonlinear resistor does not.

Are you arguing that nonlinear resistors are not resistors?
Or that nonlinear resistors do not exist?

A Reciprocal 2-pole means properties are same in both directions. Show me LED resistance being same in both directions.

How could I? A diode is a NONlinear resistor with an exponential characteristic and as such it is not reciprocal.

[IanB]

An LED behaves as a resistive component in a circuit because its instantaneous power dissipation is equal to the instantaneous product of voltage and current. It has no significant reactive component in its circuit behavior. In contrast, capacitors and inductors have reactive (energy storage) behaviors that dominate over the resistive contributions.

An LED is not "a resistor" (that word being reserved for components whose resistance is substantially constant), but it does behave resistively.

[schmitt trigger]

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.

I know that this explanation won’t deter you from asserting differently. If you want to believe that…Be my guest. But please don’t attempt to convince us of your superior knowledge. It’s pedantic.

[IanB]

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?

[Sredni]

Is the OP deliberately trying to confuse students? A diode has resistance, but that doesn't make it a resistor.  Eliminate the "time invariant" part and you can call a capacitor a resistor.

No, a capacitor is a circuit element that relates charge (the integral of current) to voltage. It can be time-invariant in the sense that its q-V characteristic does not change with time, still remaining a dynamic element.

And to answer other objections: sure, all components have parasitic elements but we name them in base of their dominant trait. A capacitor has significant capacitance and negligible esr and inductance. And a resistor has dominant resistance (even if it is 0.1 ohms) compared to the reactances of its capacitive and inductive parasitics at the operating frequencies.

In all this discussion I am assuming zero parasitics.
And no, the resistance offered by a diode is not a parasitic or second order effect: it is ALL it does.
Small resistance (not incremental resistance! Resistance plain and simple) when forward biased; large resistance when reverse biased.

[CatalinaWOW]

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.

I can go even further: a transistor is a resistor. It is a nonlinear, multi terminal resistor. And I guess this notion would encounter even more... resistance than the diode one. (I was kind of surprised when I first heard about it, but then when you reason about it it becomes clear why it is so - if we neglect parasitic effects).

As I said before I just wanted to see how widespread this notion is (even with a small population sample).

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.