Timeline for Why do LEDs not obey Ohm's law?
Current License: CC BY-SA 3.0
17 events
| when toggle format | what | by | license | comment | |
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| Apr 17, 2013 at 5:30 | comment | added | Anindo Ghosh | This answer is outright incorrect, as it dismisses the very basis of Ohmic relationship, perhaps due to limited conceptual understanding. | |
| Apr 17, 2013 at 2:31 | comment | added | Brad | I'm with you up to the point where a non-"ohmic" material "violates ohms law". | |
| Apr 17, 2013 at 2:12 | comment | added | Alfred Centauri | @Brad, you wrote: "There is nothing (except Wikipedia) which says R has to be a constant." Evidently, that statement is false. | |
| Apr 17, 2013 at 2:11 | comment | added | Alfred Centauri | @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.". | |
| Apr 17, 2013 at 2:09 | comment | added | Alfred Centauri | @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." | |
| Apr 17, 2013 at 1:56 | comment | added | Brad | 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. | |
| Apr 17, 2013 at 0:53 | comment | added | Alfred Centauri | @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. | |
| Apr 17, 2013 at 0:49 | comment | added | Kaz | @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. | |
| Apr 17, 2013 at 0:42 | comment | added | Alfred Centauri | @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. | |
| Apr 16, 2013 at 23:16 | comment | added | Kaz | 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. | |
| Apr 16, 2013 at 21:42 | comment | added | Alfred Centauri | 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, youtube.com/watch?v=QF6V74D2hbY | |
| Apr 16, 2013 at 20:15 | comment | added | Brad | Definitley look at the Wikipedia article on Diodes. Another good reference: allaboutcircuits.com/vol_3/chpt_3/1.html | |
| Apr 16, 2013 at 20:13 | vote | accept | Antoine_935 | ||
| May 6, 2013 at 8:10 | |||||
| Apr 16, 2013 at 20:13 | comment | added | Antoine_935 | 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! | |
| Apr 16, 2013 at 19:44 | comment | added | Brad | 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: en.wikipedia.org/wiki/Diode | |
| Apr 16, 2013 at 19:36 | comment | added | Antoine_935 | I am indeed talking at a relatively "newbie" level. But I'm willing to get the details of this. What makes their resistance change? | |
| Apr 16, 2013 at 19:30 | history | answered | Brad | CC BY-SA 3.0 |