# Zener dynamic resistance Rz: Is this graph or the values for Rz correct?

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: 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?

• Vertical line means that Rz = 0 Ohms and 1/Rz = 00 Conductance – G36 Mar 1 '18 at 15:10
• @G36 Exactly. However,the textbook states that the minimal Rz is 1 Ω giving a slope of 45 degrees. This is quite different from the vertical line in the graph. Does this mean that the line is actually not vertical? – John Katsantas Mar 1 '18 at 15:15
• The voltage and current scales are usually different (current in mA, voltage in volts) which will give a different visual slope. I have some curves where the current is on a log scale and voltage on a linear scale. – Peter Smith Mar 1 '18 at 15:22
• "the textbook states that the minimal Rz is 1 Ω" - no, it doesn't. "...giving a slope of 45 degrees." - there are no scales on that graph, so how can you know what the slope should be? – Bruce Abbott Mar 1 '18 at 15:28
• @BruceAbbott I assumed it is Amps and Volts. Minimal Rz was given and its value made me see something was wrong in my head. The scales are clearly(now), as Peter mentioned, mAmps and Volts. – John Katsantas Mar 1 '18 at 15:33

## 1 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.

• @John Katsantas. I apologize for posting this answer while the very same content was already posted in the comments to the question. The fact is that I started editing it while no comment was posted yet, and I did not refreshed the sight: should I delete it? – Daniele Tampieri Mar 1 '18 at 16:14
• If a question is answered in the comments section, then you have every right to "steal" those comments for your answer that you have posted here. You may keep this ;) After all, the answer box is utilized for solutions to the original question. You're providing a solution whereas the comments do not provide a solution. The comments are literally just commentary to add some aspects to the original question. – KingDuken Mar 1 '18 at 16:58
• @Daniele: as I noted the scale factors, you certainly have my blessing on the answer; I was a bit too busy to do an answer justice. – Peter Smith Mar 1 '18 at 17:16