Electrical – Zener dynamic resistance Rz: Is this graph or the values for Rz correct

diodeszener

The textbook I'm studying from states that the typical dynamic resistance ranges from 1 Ohm to 50 Ohms. Seeing the following graph something doesn't make sense: enter image description here

Since the slope in the breakdown region is 1/Rz and the graph is almost a vertical line the typical values I'm given are not justified. In the best case scenario and for Rz=1 the slope is 1. Looking at the graph the slope should be a very big number.

Is this graph a supposedly ideal zener diode? Is the slope smaller in reality? Or have I utterly failed my high school math teacher?

Best Answer

The misleading occurrence in the diagram of the Zener diode characteristic you are looking is that the the anode current axis in the diagram itself is probably graduated in milliamperes (\$\mathrm{mA}\$) while the Anode voltage axis is graduated in volts (\$\mathrm{V}\$). When you work in the breakdown region, beyond \$V_A\simeq V_Z\$, a variation of \$0.1\mathrm{V}\$ for a Zener diode with \$R_Z\simeq 1\Omega\$ causes a variation in anode current of nearly \$100\mathrm{mA}\$, probably out of sight since out of the ranges of diagram. The same thing happens even if the scale is logarithmic.