Beginner question on Z-Diode behaviour - Page 1
Forum section: electronics / beginners

author: 8008
author_rank: Contributor
timestamp: July 30, 2023, 10:28:19 am
content: Hi,I am having trouble understanding the behaviour of my zener diodes. I suppose I am missing some pracitical understanding how this thing really works.Maybe someone can explain, would be great.I have here a whole bunch of 3.3 volts zener diodes. They are sold with name "ZF 3.3" by some german vendor (reichelt.de), but these are in fact BZX79C 3V3 .My simple understanding is, that a Zener diode regulates output voltage between Kathode and Anode. If a voltage is put there (plus to Kathode, Minus to Anode). And if the current has some required value,the output voltage is around the value printed on the diode.From the BZX79C datasheet, I can see that the required current is 5mA. Then the diode should regulate and output a voltage between 3.1 and 3.5 volts.So I have created a simple test, with a 330 ohms resistor, the zener diode and put some voltage on it. I have high quality power supply and measurement devices, so I played around to get 5mA current flow.The voltage measued then between K and A of the diode was 3.936 volts. Expected was, as said, something between 3.1 ... 3.5 volts.I tested several of my zener diodes, they all have a behaviour like this. So it is not due to a single broken one.I tried this with 3 different power sources, all linear regulators. AC ripple can be ruled out I think.To summarize. I set up the diode to get the current it needs for the desired output. But voltage output is higher. Why? How to set up the diode that it outputs/regulates to 3.3 volts?Try to visualize the test curcuit:Vcc, about 5.64 volts o------R=330 Ohms ---(K)--|<|---(A)----o GNDAmperemeter is between Vcc and resistor, voltmeter between K and A.Dennis

author: MrAl
author_rank: Super Contributor
timestamp: July 30, 2023, 10:34:59 am
content: Quote from: 8008 on July 30, 2023, 10:28:19 amHi,First, zener diodes are not very good at regulating the voltage across them. Some of them seem to be hardly regulating at all, especially the ones under about 6 or 7 volts. Here are some suggestions.1. Double check the current. If it is 5ma then try lowering to see how you can get the voltage to read 3.3v or whatever. Then, do a quick curve test. Try different current levels and plot the voltage across the diode, then post here so we can take a look. That will tell us more.2. Try measuring some other zeners if you have them. The lower voltage ones like 3.3v, and the higher voltage ones like 7v or above. Plot the voltage vs current for those and compare.3. If you can get ahold of a voltage reference diode, measure that the same way. Note that the voltage stays very very constant unlike the zeners. This is the more modern way to implement something like a zener diode. A voltage reference diode works much much better for this.

author: iMo
author_rank: Super Contributor
timestamp: July 30, 2023, 11:06:26 am
content: BZX 3V3 should show you a voltage around 3.3V at 5mA in your setup. I rarely see BZX zeners be so off.Looks like the 3V9 version. Double check the markings.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 30, 2023, 12:01:25 pm
content: Quote from: iMo on July 30, 2023, 11:06:26 am+1If the markings do indicate 3v3, are you sure about the accuracy of your voltmeter?

author: madires
author_rank: Super Contributor
timestamp: July 30, 2023, 12:15:06 pm
content: Zener diodes are not what you think, by DiodeGoneWild:

author: 8008
author_rank: Contributor
timestamp: July 30, 2023, 12:15:27 pm
content: I did some measurements and created a chart with Libreoffice.In the chart I have input voltage (Vin), applied to resistor and zener. Also output voltage Vzener.X-axis is current, I measured currents between 0.1 and 15 mA.There are also two horizontal lines (yellow fort 3,3 volts and green for 3,9 volts).It looks like for 5mA, the value 3.9 volts is touched quite close.For 3.3 volts, a current of only 1.1 mA is needed.My eyes are not the best, the 3V3 is super small on the diode. The diodes are still in delivery box from the vendor, and on that box "ZF 3.3" is listed. It may be wrong. I will check the printing on the diode with a microscope.But just from the chart, it is more a 3V9 diode, right?My meters should be ok (Keithley 177 and Thurlby 1905A, I also checked with few other meters . 99.9% sure :-) ). Also the lab supplies (HP 6111A and 6112A), all linear supplies.Have also attached datasheet of BZX79C where I got the 5mA and other data from.

author: 8008
author_rank: Contributor
timestamp: July 30, 2023, 12:16:05 pm
content: I will consume the video, thanks. Did post before seing that.

author: madires
author_rank: Super Contributor
timestamp: July 30, 2023, 12:36:49 pm
content: Quote from: 8008 on July 30, 2023, 12:15:27 pmThen it should be a 3.9V Zener.

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2023, 12:42:52 pm
content: Is reichelt.de an authorized reseller, maybe its a fake or mislabeled china part.Another thing to consider, I don't know how good your multimeter is but some cheaper meters start reading voltages higher than they really are when the battery is getting flat.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2023, 02:17:56 pm
content: The datasheethttps://cdn-reichelt.de/documents/datenblatt/A900/SMDZF%23VIS.pdfspecifies the dynamic resistance rZIless than 90\$\Omega\$, while in the measured yellow plot the slope is at least 500\$\Omega\$.That's 5-10 times worst than specified in the datasheet, so that yellow plot can not be of a ZF3.3 in good working condition.

author: iMo
author_rank: Super Contributor
timestamp: July 30, 2023, 02:54:23 pm
content: When you look at his table - the voltage at 4mA and 5mA the Rd=92ohm.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 30, 2023, 03:02:05 pm
content: Hi RoGeorge, hi Dennis,the yellow plot is just the input-voltage.If i remember correctly, Z-diodes under 6.2 volts can't be made, so they use multiple diodes in series.Maybe Dennis could conduct a test for U-flow anode positive and cathode negativ?Ciao4now

author: iMo
author_rank: Super Contributor
timestamp: July 30, 2023, 03:09:21 pm
content: Until Dennis doublechecks the markings on the diodes, we will just hypothesize..It looks like normal 3V9 diode, imho, it happens the distributor simply throws something similar in the box and sends to the customer. It happened to me many times already.First have a look at the markings.. 3V3 3V6 3V9 are all similar looking with limited eyesight.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 30, 2023, 03:17:07 pm
content: Quote from: Roehrenonkel on July 30, 2023, 03:02:05 pmThen those diodes would not measure 0.6Vf! But they doI've replaced a ton of 3v3 zeners lately in old equipment, they had drifted to 3.5-3.6V.

author: madires
author_rank: Super Contributor
timestamp: July 30, 2023, 03:59:25 pm
content: Below 5.6V the Zener effect dominates (IV curve rounded, negative temperature coefficient), and above 5.6V it's the avalanche effect (IV curve less rounded around breakdown, positive temperature coefficient).

author: 8008
author_rank: Contributor
timestamp: July 30, 2023, 04:32:38 pm
content: Hi all,I had a look with microscope. These are 3V9 ones. I have them in my parts box for several years now, so I cannot remember how this happens. Most probably I mixed the diodes in wrong box. Sorry for confusion. But to my excuse, the text on diodes was so small that my large magnifying glass lamp also did not make it possible for me to read it. And "3" looks close to "9"Thanks for all replies, which gave me anyway great insight. The video was great to see too.And I was forced to create a U/I diagram for that diode I did that kind of work last time about 40 years ago when studying at university :-)Have all a good sunday and thanks again!Dennis