I think this is a fairly good for seeing how a diode behaves (as describing how the diode changes it's resistance over supply voltage hence is not conforming to ohm's law): http://www.allaboutcircuits.com/vol_3/chpt_3/1.html Take a look at the graphs below - the breakdown is where zener works and the foward where 'normal' diodes are. For thumb of rule you may can think diode as infinite resistance before the forward voltage region and infinite conductance after that (on that graph you can see that it is not exactly the case but for the sake of simplicity forget the small details for now). LED is not much different, however, you have a bigger forward voltage drop -- depending on your LED it can vary from 1.8V to 5V or maybe even more -- you need to check it on the datasheet of your LED to figure this out. It also describes the maximum continuous current it can take. All you need is to subtract the voltage drop from the input voltage and using ohm's law set the resistor to get the appropriate current. R = (Vsupp - Vdrop) / I You could use a constant current source as well but then it makes your circuit far too expensive and is not much better than a 1 cent worth of resistor calculated to a specific voltage supply. Tamas On Mon, Jul 6, 2009 at 7:08 AM, solarwind wrote: > On Mon, Jul 6, 2009 at 1:27 AM, Sean Breheny wrote: > > You have a sharp eye, Solarwind :) You caught the inconsistency in my > > explanation. > > > > In the example I gave, one could say that grip pressure and sweat > > change the resistance, but that the current to voltage relationship > > would still be linear. > > > > That might be true, I don't know enough about human skin. > > > > However, there ARE examples of materials and devices which do NOT > > follow Ohm's law. > > > > Diodes, for example. Their voltage drop is approximately the natural > > log of the current. > > > > Another example is a salt water solution with conductive electrodes > > stuck into it. I don't know what the voltage to current relation is > > here but I suspect that it is very nonlinear and, therefore, > > non-Ohmic. > > > > A spark gap is another. When a spark gap is NOT arcing, you can > > increase the voltage until it reaches the breakdown voltage (depends > > on the gap/electrode geometry, the gas they are in, presence of > > ionizing radiation, etc.). Up til this point, the current will be > > zero. Then, suddenly, the arc starts and the current begins to rise. > > As the current rises, the voltage actually DROPS. If you attach a > > current source to the arc, and you turn up the current, you will see > > the voltage go DOWN, instead of up as you would expect with an Ohmic > > material. > > > > I would suspect that skin has an Ohmic region (i.e., up to a certain > > voltage it will be mostly Ohmic), but that it behaves in a non-Ohmic > > fashion at higher voltages. > > > > Sean > > This is definitely something I should read up on. I thought Ohm's law > was universal. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.mcuhobby.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist