Re: delta V = I R

[Rondo Jeffery <RJEFFERY@WEBER.EDU>]

Many people have contributed to this thread, but I will quote just Michael Edmiston's here, since he asks a question that deserves an answer.  This tread, which I guess I started last week, has headed in a different direction from just how to teach Ohm's law.  For ohmic devices, I = (delta V)/R really is a definition of the resistance, and most appropriately applies to devices which have a linear relation between I and delta-V.  For non-ohmic devices, the ratio dV/dI still has some usefulness, namely with respect to Zener diodes used as voltage regulators.

If you look in the spec sheets of Zener diodes, you will find a quantity variously called the dynamic resistance, dynamic impedance, or Zener impedance.  This is the inverse of the slope, dI/dV, of the exponential relation between current and voltage drop in the Zener conducting region (reverse bias, past the breakdown point).  (So, Rz = dV/dI at this point.)  This is useful for estimating how much the Zener voltage changes for a given change in the current through the Zener diode.  It is given at the nominal operating point, Vz and nominal current, Iz.  Since the relation between I and V across the diode is non-linear (exponential), this dynamic resistance is not constant.  Still, it serves a useful function of estimating how good you can expect the voltage regulation to be with the Zener diode.  For example, if Vz = 5.6V and Rz = 11 ohms at 20 mA (value for 1N5232B), we have:
delta-Vz = Rz(delta-Iz).  For a delta-Iz of 10 mA, for example, this gives delta-Vz = 0.1 V.
Hope this helps.

Rondo Jeffery
Weber State University
Ogden, UT 84408-2508
rjeffery@weber.edu


> > > Michael Edmiston <edmiston@BLUFFTON.EDU> 11/08/99 09:00AM >>>
> > Bob Sciamanda points out that we can define non-constant parameters.

> We can define non-constant values if there is utility in doing so.
> What would be the utility of calculating and define a "resistance" for
> a non-ohmic device?  It cannot be used in I = (delta-V)/R to predict
> the current at any value of delta-V... it only works for one value.  A
> non-ohmic device needs a different (non-linear) equation to relate
> current to potential difference.  Or, if you wanted to tabulate values
> of R for various delta-V's, I guess you could do a "look-up table."
> Please tell me what utility one would gain from knowing the
> "resistance" of a non-ohmic device given that the value is only valid
> at one potential difference.

> Michael D. Edmiston, Ph.D.              Phone/voice-mail:       419-358-3270
> Professor of Chemistry & Physics        FAX:                    419-358-3323
> Chairman, Science Department            E-Mail                  edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817