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In a previous question, it was brought to me that LEDs do not obey the Ohm's law. (See Calculate expected voltage around a resistor)

Simply put: how is that?

What makes them behave so differently? How should we treat them in a circuit and calculations?

Are there other components with similar behavior?

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Antoine_935
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    The non-ideal diode model has an exponential term in it. More importantly, Kirchoff's laws are satisfied, and those always apply. – Matt Young Apr 16 '13 at 19:31
  • @MattYoung just for clarity, the ideal diode has an exponential term, and the threshold model is just a very rough simplification – clabacchio Apr 16 '13 at 19:40
  • Try applying a certain variable voltage to water. What you'll find is that the resistance changes with voltage. Air also doesn't obey Ohm's Law - you've got gigantic voltages floating in the air. But there's almost no current until the voltage reaches a certain level. What you observe then is a spark in the form of a lighting. Ohm's law applies only to resistive materials - by definition. What does not obey Ohm's Law is not a resistor. – Christoph B Apr 16 '13 at 19:45
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    You have just discovered the principle of linear vs non-linear devices. Expect non-linear behavior from all the semiconductors. – gbarry Apr 16 '13 at 20:48

4 Answers4

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They do - they just do not have a "fixed" resistance. If you look at it from a standpoint of having a fixed forward voltage drop (which they sort of do - depending on operating region) - look at them more as having a fixed voltage across them. Therefore, as different currents go through them, their voltage will stay (relatively) constant, but the resistance will change.

This is a simplistic answer - but I think you're talking at this level.

Brad
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  • I am indeed talking at a relatively "newbie" level. But I'm willing to get the details of this. What makes their resistance change? – Antoine_935 Apr 16 '13 at 19:36
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    Well, it is a diode, which is a semiconductor which inherently means it does not have a fixed conductance, like a normal conductor. The properties of this (and other) semiconductors are complex. They do different things in different operating regions. It's resistance is more of an artifact of it's operation at any specific point - as opposed to a fixed quantity. See "Voltage-Current Characteristic" here: http://en.wikipedia.org/wiki/Diode – Brad Apr 16 '13 at 19:44
  • Waw, I don't really get all this already, guess I need to study a little more :) thanks for this good answer, it helps anyway! – Antoine_935 Apr 16 '13 at 20:13
  • Definitley look at the Wikipedia article on Diodes. Another good reference: http://www.allaboutcircuits.com/vol_3/chpt_3/1.html – Brad Apr 16 '13 at 20:15
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    This is not a good answer. If a circuit element obeys Ohm's law, the voltage is proportional to the current, i.e., the voltage across is a linear function of the current through - full stop. Moreover, this answer conflates the notion of resistance, V/I, and dynamic resistance, dv/di. See, for example, http://www.youtube.com/watch?v=QF6V74D2hbY – Alfred Centauri Apr 16 '13 at 21:42
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    I don't agree. Ohm's Law does not assert that resistance cannot be a function. To deny this means that, for instance, a potentiometer or rheostat do not obey Ohm's Law, because someone can turn the knob. – Kaz Apr 16 '13 at 23:16
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    @Kaz, linearity and time invariance are distinctly different. You're conflating the two. It they were the same, we wouldn't need to separately specify, for example, linear time-invariant system. A variable resistor is, at any moment of time, a resistor with resistance that is constant with respect to the voltage across it or the current through it. – Alfred Centauri Apr 17 '13 at 00:42
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    @AlfredCentauri But so is a diode, at any moment in time. It's like a rheostat, except that the daemon inside the diode which turns the knob is looking at the forward voltage rather than time. – Kaz Apr 17 '13 at 00:49
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    @Kaz, no at any moment in time, the current through the diode is a non-linear function of the voltage across the diode. For a resistor, variable or not, at any moment in time, the voltage across the resistor is a linear function of current. Think of a 3-D space with voltage, current, and time as axes. Take any constant time slice through that space to get the I-V curve. It is either linear or it isn't. – Alfred Centauri Apr 17 '13 at 00:53
  • Kaz's analogy is good. The rheostat obeys ohms law, even though the daemon in the box is acting in a non-linear fashion. The function of the box can both be non-linear, but yet obey ohms' law. There is nothing (except Wikipedia) which says R has to be a constant. – Brad Apr 17 '13 at 01:56
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    @Brad, from hyperphysics: "The ratio of voltage to current is called the resistance, and if the ratio is constant over a wide range of voltages, the material is said to be an "ohmic" material." – Alfred Centauri Apr 17 '13 at 02:09
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    @Brad, from Britanica: "Ohm’s law, description of the relationship between current, voltage, and resistance. The amount of steady current through a large number of materials is directly proportional to the potential difference, or voltage, across the materials. Thus, if the voltage V (in units of volts) between two ends of a wire made from one of these materials is tripled, the current I (amperes) also triples; and the quotient V/I remains constant.". – Alfred Centauri Apr 17 '13 at 02:11
  • @Brad, you wrote: "There is nothing (except Wikipedia) which says R has to be a constant." Evidently, that statement is false. – Alfred Centauri Apr 17 '13 at 02:12
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    I'm with you up to the point where a non-"ohmic" material "violates ohms law". – Brad Apr 17 '13 at 02:31
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    This answer is outright incorrect, as it dismisses the very basis of Ohmic relationship, perhaps due to limited conceptual understanding. – Anindo Ghosh Apr 17 '13 at 05:30
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Ohm's law applies to resistance. All resistive aspects of a device will behave according to OHm's law.

If you invert your question you see that every thing that behaves according to Ohm's law must be a resistor. There is only so much that one can do with pure resistance. So logically the anything that doesn't behave according to ohms law isn't a resistor. Or any thing that isn't a resistor won't behave according to ohms law.

I believe that is called a Tautology.

In circuit design we have many different devices all having unique properties to be able to implement different things/functions.

placeholder
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    I think your anwswer needs to be highlighted more as it is the only correct one (at the moment I am writing this). Ohm's law is empirical and was originally derived from obserwing the behaviour of wires of different length. Water doesn't obey Ohm's Law, air doesn't - only conductive materials do, and even then not always. – Christoph B Apr 16 '13 at 19:42
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    If I had a black box, and ran a current through it - then measured the voltage across it - I could calculate it's resistance at that point in time. It' doesn't matter what is in the black box. – Brad Apr 16 '13 at 19:55
  • @Brad exactly, and then how would you use that single data point to then tell you what would happen at say 2X the current? WIth Ohms law and a resistor you'd be able to say 2X the voltage. You are talking about an equivalent resistance at a single operating point. that does not describe a device. – placeholder Apr 16 '13 at 20:15
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    Exactly - like I said: A diode does not have a "fixed resistance". I could however argue that it does have a known resistance for a given current. The question was about if it obeys ohms law, and it does. It just doesn't have a constant resistance. – Brad Apr 16 '13 at 20:18
  • @Brad I won't reply any further after this. Just because you CAN map the device to be a variable resistor doesn't mean you should. It doesn't reveal any inherent behaviour and is misleading. You could as easily model the device as a variable capacitor with even less explanatory power. The I vs. V characteristics are THE defining aspect and are fundamental to what the device is. And even in this aspect you're wrong because you should define a device in it's small signal resistance NOT large signal as you have done. You must deal with partial differentials to reconstruct the I/V curve. – placeholder Apr 16 '13 at 20:31
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    @Brad, it doesn't obey Ohm's law - period. For Ohm's law, V and I are proportional, i.e., the ratio is constant. – Alfred Centauri Apr 16 '13 at 22:16
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    You are describing the fact the the response is nonlinear - and I agree it is not. However that was not the question. By this logic a "variable resistor" would also disobey ohms law. Ohms law defines the relationship between resistance, current and voltage as an equation - proportions to each other. It merely states a change in one will require at least a change on one other to remain valid. You are insisting R must maintain constant while only V and I change for a device. This would describe a device which is linear and purely resistive - but not the only one which would apply. – Brad Apr 16 '13 at 22:46
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    @Brad, don't you see that you're generalizing the concept of Ohm's law into simply "if you take the voltage and divide it by the current, you get a number"? Of course you do. But there's more to Ohm's law than that! Further, by introducing explicit time variation, you're muddying the waters. The crucial point here is this: if a circuit element obeys Ohm's law, the ratio of voltage to current does not depend on the particular voltage or current value. (cont.) – Alfred Centauri Apr 16 '13 at 23:01
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    @Brad, (cont.). From Wiki: Ohm's law states that the current through a conductor between two points is directly proportional* to the potential difference across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship... More specifically, Ohm's law states that the R in this relation is constant, independent of the current.* – Alfred Centauri Apr 16 '13 at 23:12
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    "Proportional" - yes. "R is a constant" - nope, it doesn't state that. Why does it "state this" for R and not for V and I. "Proportionality" is akin to "linearity", which I do agree. Wikipedia may state that - but ohms law does not. It's call ohms "law" for a reason. It is universal, not just for purely resistive, nonlinear devices. – Brad Apr 16 '13 at 23:23
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    Then again, maybe Alfred has a point. For instance, do you still call it Hooke's Law, if you have a spring that has a non-linear force with respect to elongation? – Kaz Apr 16 '13 at 23:26
  • It's a "law" - you can't violate it! – Brad Apr 17 '13 at 00:44
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    But, say, Hooke's law can be violated. Any elastic thing whose elongation/compression is not proportional to the displacing force does not obey Hooke's spring law. – Kaz Apr 17 '13 at 00:52
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    So it's a matter of word semantics. To me, resistance is an important quantity; it's more than just some number that pops out when you divide voltage by current. It has meaning. – Kaz Apr 17 '13 at 02:54
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    @Brad Perhaps you need to re-read Ohms law, and see why it doesn't apply to non-Ohmic (yes, google that term) devices. That might provide some clearer fundamental understanding of the law, rather than a debate based on applying specific state-variants. – Anindo Ghosh Apr 17 '13 at 05:28
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Simply put, because the're not resistors but p-n junctions, and because of that their V-I ratio is exponential.

It doesn't mean that you can't calculate their current, just that it's not as simple as for resistors. For instance, you can treat them with a threshold model, with a fixed voltage drop. Then the current will be set by external resistors or active components.

The LEDs are diodes, so that's the obvious similarity. Also the base-emitter junction of a bipolar transistor is a diode, and behaves similarly. The only difference with diodes is that their threshold voltage is higher due to the different materials and doping.

clabacchio
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A lightbulb, on first examination may not appear to obey ohms law. Measure its resistance with a multimeter and it might be 5 ohms. Connect it to a power supply capable of illuminating it and measure current and voltage and its resistance will have considerably risen (maybe 20 or 30 ohms). Its still a resistor but its resistance changes with power delivered to it.

A light dependent resistor is another example - its resistance changes with incident light - it's still a resistor and obeys ohms law - but it takes a little bit more than a linear volt-current graph to figure things out.

Andy aka
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