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Do you think an LED is a resistor?

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magic:

--- Quote from: Sredni on April 23, 2024, 11:31:38 pm ---From: A.C. Fischer-Cripps, in Newnes Interfacing Companion, 2002
3.3.7 Log amplifier
A non-linear resistor is connected into the feedback circuit. In practice, this can be a diode, but a transistor connected as a diode is used since the forward biased transfer function is more accurately exponential. The exponential nature of the forward biased diode leads to a logarithmic decrease in gain of the circuit as the input signal is increased.


--- End quote ---

Nonsense.

This circuit has almost linear decrease in gain as the input signal increases. Consider for example R1 = 1kΩ:

Vin = 1V, I = 1mA, Vout = -600mV, gain = -0.6
Vin = 2V, I = 2mA, Vout = -618mV, gain = -0.31
Vin = 5V, I = 5mA, Vout = -642mV, gain = -0.13
Vin = 10V, I = 10mA, Vout = -660mV, gain = -0.066

DenzilPenberthy:
This thread is a truly dumb premise.  So what OP is saying is that literally every single electrical/electronic component that exists is a 'resistor' and that's somehow a useful or interesting observation?

Presumably OP would even pedantically call a superconductor a 'resistor' just 0 Ohms?

This is the same sort of person who spends the whole evening in the pub with friends strenuously arguing that breakfast cereal is a type of soup.


Sredni:

--- Quote from: TopQuark on April 24, 2024, 04:23:48 am ---There's no reason why you couldn't call every 2-leaded device as a resistor:

- A diode is a non-linear resistor that conducts current exponentially with applied voltage, and only in one way
- A capacitor is a giga-ohm resistor with two electrodes that conducts through dielectric materials, it also happen to store charge too. 
- An inductor is a low-value resistor made with coils of copper wire wrapped around something, that also happens to store energy in the magnetic field.

 
--- End quote ---

No.
Capacitors and inductors are FUNDAMENTALLY different from resistors. How many times do I have to repeat it? There are only four (three in practice) fundamental circuit elements: resistors, capacitors, inductors,(and memristors, but let's forget about them).

I mean, this is the basis of circuit theory.
It's all about what are the state variables that describe their behavior. All you need to describe the state of a circuit at any time are the values of charge q, magnetic flux phi and their derivatives, the current i=dq/dt and the voltage v=dphi/dt.

(Pure) Resistors are components described by a relationship in the V-I plane. If this relationship is a straight line they are linear resistors (but we call them just "resistors" for simplicity). If this relationship is not a straight line they are nonlinear resistors and depending on the particular shape of the characteristic they take many names such as incandescent lamp, varistor, diode, zener diode, tunnel diode, and so on.

(Pure) Capacitors are components described by a relationship in the q-V plane. If this relationship is a straight line they are linear capacitors (we call them "capacitors" for brevity). If the relationship is not a straight line they are nonlinear capacitors. You cannot describe the state of a capacitor by using the values V and I; you need v and the integral of I, that is charge.

(Pure) Inductors are components described by a relationship in the phi-I plane. If this relationship is a straight line they are linear inductors (and in introdoctury courses we call them "inductors" for brevity). If the relationship is not a straight line (because of saturation and hysterisis) they are nonlinear inductors (and we still call them "inductors" because they are very widespread, like the I in SEPIC).
You cannot describe the state of an inductor by just using the values of V and I; you need I and the derivative of V, that is the magnetic flux phi.

And before you say, if I give you the v(t) and I(t) functions I can still find the value of C and L, Think again: you need a function to extract information on the integral or derivative.

What is the value of a (linear) capacitor that has a voltage of 1 volt and a current of 1 mA? You can't find it. But if I tell you that when it has 1 volt across it, it's charge is 1 uC you will immediately tell me it's a 1uF capacitor.

So, no. You CAN'T call a (pure) capacitor a resistor because it is NOT a resistor.
This is the absolute bare minimum of circuit theory.

But you can call a (pure)diode a resistor because it IS a (nonlinear) resistor. Offering a voltage dependent resistance is what it FUNDAMENTALLY does. It's not a matter of naming, it's about it's findamental nature.


--- Quote ---You can invent your own method of describing a circuit. You can invent your own mathematical system. You can invent your new language. We simply agreed on what we commonly use to get everyone on the same page, and not having to explain everything from scratch every single time.

--- End quote ---

You people should start read some books. This is NOT "my" method of describing circuits. This is not "my" language. This is how circuits are described in universities all over the world. It's just that not all textbooks join the dots for you.

TopQuark:
This is stupid.

https://article.murata.com/en-global/article/insulation-resistance-and-leakage-current-of-capacitor

Capacitors can leak, dielectrics can conduct, it has a I-V curve at DC, so it could be resistive. Your pure, ideal capacitor in your head does not leak, but I look at all the capacitors in my parts storage, and all of them has a leakage current (I) when high enough voltage is applied to it (V). Should I relable it as all resistors?


--- Quote ---(Pure) Resistors, (Pure) Capacitors, (Pure) Inductors
--- End quote ---

Resistance, capacitance, inductance are the words you are looking for. Those are the the theoretical physical properties that are well defined. Resistor, capacitor and inductors are the devices that does mostly what it says in the name, but we all know it isn't perfect. When I put a capacitor symbol in my schematics, I don't mean to spec in two magically floating metal plates separated by a perfect vacuum, I am placing a tube containing a couple strips of foil separated by paper soaked in electrolyte, a.k.a. an electrolytic capacitor on a PCB.

You can say a diode has resistance, which I am sure you can wrangle your math to proudly show it is so, fine, but that does not mean "an LED is a resistor". The ability to use maths and quote books to make pointless arguments proves you have linguistic intelligence, but it does mean not you are an intelligent person.

If you walk up to a straight dude, say he is gay, get punched in the face. You can argue "dude you are dumb, I saw you laughing, and according to the dictionary gay could mean happy, so I was correct. Did you even study English?". Who's the dumb one in this story?  ::)

Sredni:
I believe this is the sixth time I have to repeat it: in all this discussion I am neglecting parasitic effects. Because I am discussing the FUNDAMENTAL nature of the diode.

A silicon diode described by the exponential Shockley relation, with negligible junction capacitance and negligible inductance of its terminals is FUNDAMENTALLY just a nonlinear resistor, that is: a resistor whose resistance is a function of its voltage.
I also showed the R = R(V) curve in LTSpice.

It does nothing more than that. The nonlinear resistance is its primary and intended function. It is what it makes it a diode.
Unlike the ESR of capacitors and inductors, which is only an undesirable parasitics.

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