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How to Measure The Voltage Rating Of A Zener Diode

By on September 15, 2015
zener diode tester









In this document, I show you how to measure the voltage rating of a zener diode, when there is no labeling on or if it is unreadable. Be carefully, because I show you a method with a lab power supply, and if you not really know the elementary of current and voltage, this explanation can be very dangerous. This can harm you, burn your hands, or maybe much worse. Be careful what you do, it could be very hot and warm!
I didn’t go down to explain how a zener works and what for it is. Here I will only show how can you figure out the voltage rating of the zener if you have no coding on it.

zener diode tester 2

Today I got a nice question from the son of my friend, who learns the basics of electronic. “How is it possible to know the voltage rating of a zener diode, if there is no coding or if it is not readable?” Let see what we can do.

I got a bad board from a UPS or whatever that was and desoldered some zeners. We got a bunch of unidentified zener diodes. There are two of them on the picture but we got more. Lets say we only knows does these diodes are zeners because of the marking on the board. ZD1 and ZD4. Here are the complete zeners what we took out from the board:

zener diode

I used one of my lab power supplies to see what voltage ratings are these zeners.
zener diode testing

  1. Put all the potentiometers on a zero stage. The current and the voltage potentiometers.
  2. Put croco clips to the output terminals of the PSU.

In my case the yellow is the positive terminal and the green is the negative terminal.

zener diode tester 5

  1. Short the two terminals of the crocos and adjust the current limiter potentiometers so does the PSU will limit the current around 10-20mA.

Be careful! do not play with this shorted crocos, if your PSU have no over current protection then the crocos can be mold if you put several amps to the output !!!

It could be end up in fire too!

Set up your PSU to delivery only 10-20mA and take apart the crocos.

Now you should have on the display all of zeros.

This means no current flow through the crocos because the circuit is open.

  1. Now put a zener diode between the crocos. The positive clip should be on the cathode on the diode, and the negative croco to the anode of the diode like on the picture below.

zener diode tester 6

Remember, in my case the yellow clip is the positive and the green clip is the negative terminal.

  1. Now start to rotate the voltage potentiometer slowly to add some voltage to the circuit.

Rotate until the current rise to the max setup current on the PSU. In our case around 10-20mA.

zener diode tester 7

Until I rotated the voltage potentiometer a bit upper then 20.7V the red light C.C on the current side goes on, so the unit told me I reached the max current consumption around 10-20mA. Then I rotated a bit back the voltage potentiometer and the C.C led goes off.

As you can see, the voltage is 20.7 V and the current is 0.01 A what means around 10 mA. This means, this zener is a zener close of 20 V. Let see what is marked on the zener:

20 volt zener diode

As you can see, on the zener is marked a number of 20. This zener diode is a 20V rating zener.

Here is another example:

zener diode tester 9

This zener has no labeling on it. Let see what is the voltage reference for this guy.

zener diode tester 10

The max current reached, the C.C is on, the voltage is 3.3V. This zener is a 3.3V zener.

Check the picture below. On the diode I found only a marking with “C-” or “-C”. Nothing else, even if I have a possibility to magnify the component I did not realize any knowing marking. However, one think is sure, this zener is a 3.3V rating zener diode I give +-5% tolerance.

zener diode checker



This is one of the methods how to get the voltage rating of a zener diode. This method is not in circuit usable. CMOS components or TTL-s could wake up on the board and made some mess because of the testing voltages. There is other method too of course but I use this type of testing in my practice.

This article is for basic skilled repairer and novices in the world of electronic repairing. For further learning the technique how to check electronic components please refer to the book by Mr. Jestine Yong, who made a well explained “Testing Electronic Components” guides with great pictures and explanations.

I hope you will enjoy this article.



This article was prepared for you by Christian Robert Adzic from Novi Knezevac-Serbia.

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  1. Ehsan Murad

    September 15, 2015 at 11:36 am

    Thank you.

  2. Mohamed Sittiq M

    September 15, 2015 at 12:17 pm

    Dear sir,
    super tutorial
    thanks and regards
    Mohamed Sittiq M

  3. Ulises Aguilar Pazzani

    September 15, 2015 at 12:19 pm

    Hey that was pretty good good hint

  4. Awais

    September 15, 2015 at 1:00 pm

    Dear justine
    Very guud day to you
    And bundle of thanks for this article i was waiting for this for a long time

  5. Orlando

    September 15, 2015 at 1:38 pm

    Another easy way to test voltage of a zener is to place it in serie-circuit with a 47k resistor and both conected between + and - of main condenser in any
    voltage source.Reading voltage between diode terminals is the right test.

  6. Paris Azis

    September 15, 2015 at 2:03 pm

    Hello Christian

    This is the method I use for many years up to now, only slightly different from yours, and this due to safety reasons either for protecting the zener diode or the PSU itself.
    So, I adjust my PSU to 20V (:it's the maximum of my home made PSU). Then, no matter its current limit setting (which is reasonably low, but irrelevant) I use a 4,7KOhms resistor connected in series with the positive supply cable. Then I feed the zener diode. This equals to approximately 5mA of current supplying the device under test, having its one terminal disconnected from the PCB. Next I measure the zener voltage across the diode with my digital multimeter (because I use analog instruments in my PSU and therefore they are not as accurate as a digital meter).
    So I fully agree with this method you use. It is very reliable because it is a dynamic test simulating the real working condition of a zener in a circuit.
    Good job and keep it up!

    Best Regards

    • Chris

      September 16, 2015 at 1:42 am

      I appreciate the way how you test the zener.
      It's nice and safe.
      Let's see how other people tests, measure the zener.
      My best regards.

      • Paris Azis

        September 16, 2015 at 8:24 pm

        Hi Chris

        I don't think that there are other altenatives about this topic due to the nature of the function of this diode.
        Perhaps the only one would be a transistorized constant current generator to feed the d.u.t (device under test). I have seen many relevant circuits, but practically either your method or mine are exactly the same thing!
        You feed the diode directly with constant current using the current limiter of your PSU and I feed it with constant voltage through a resistor which finally limits the current like you do. No difference at all. What else of a method can be applied in this case? (I also wonder as you do)!

      • Robert Calk

        September 24, 2015 at 6:18 pm

        Hi Guy's,
        I was thinking about using my Variable Isolation Transformer as a power source, feeding through a Bridge Rectifier, and using a circuit like yalls for testing high voltage Zeners. I forgot about it until just now. I got to doing other stuff and forgot about it.

  7. Saman Dolawatta

    September 15, 2015 at 2:58 pm

    Thanks for Valuable hint!!!!!

  8. Albert van Bemmelen

    September 15, 2015 at 3:28 pm

    You explained it very well Christian Robert. But one thing must not be forgotten. Because the very low voltage Zeners are in fact never Zeners. But just one or more Diodes in conductivity. And in these cases any normal Diode without any marking can be seen as a Zener without really being one. As I remembered because of the Low Voltage Zeners that are almost impossible to make the way the Zener diode is Manufactured. I guess it also means that the reverse Zener curve of a low voltage Zener may be a little bit different from a real (and higher voltage) Zener Diode. Although one can also use the serial resistor method to limit the Zener current to be able to measure the exact voltage, nowadays I use my new digital tester for this. It even handles opto-couplers, Leds and Displays.

    • Chris

      September 16, 2015 at 1:53 am

      Please can you share the model of your digital tester with as?
      My best regards.

      • Albert van Bemmelen

        September 17, 2015 at 2:12 am

        Yes sure Chris. I hope this Ebay link works for you:
        This is my latest tester I bought. It has a lot of great test possibilities but is less accurate on the TTL/CMOS IC type test.

        But I also own another Tester a bit less expensive and this one doesn't have a zener test to name one test less but is very accurate on the TTL/CMOS test:
        But I wouldn't miss anyone of them. Both have their unique ways.

        • Albert van Bemmelen

          September 17, 2015 at 2:23 am

          I have also the PDF instruction files of both Digital testers.
          So I also could send them to you but then I need your email address. But maybe you are also able to download them from Ebay from the Digital Tester articles.
          They both can also test Opto-couplers besides some Amplifiers and Opamps also. But the cheapest one operates from just one mini-penlight.

    • Paris Azis

      September 16, 2015 at 8:51 pm

      Hello Albert

      You are absolutely right. As far as I know about this matter, it is techologically very difficut to produce zeners below 2,7V. That's why we use the tricks you refer to, using normal diodes instead of zeners wherever needed.
      I have also seen LEDs being used as voltage stabilizers (they normally work around 1,6V and have excellent drift characteristics for this purpose).
      In any case, we talk about normal diodes which, therefore, are used in forward bias. Not in reverse, as zeners are.

      Best Regards

      • Albert van Bemmelen

        September 17, 2015 at 2:38 am

        Hi Paris, I also have seen ordinary Transistors being used as a 2 pole zener. But of course we are able to make a thyristor with 2 Transistors or even a High Frequency (about 250MHz) TunnelDiode (or Esaki diode) oscillator. They did that in Elektor (then still called Elektuur) September 1979. In a Gate Dipper project. I've build it , it really works!

  9. Allan

    September 15, 2015 at 4:08 pm

    Hi Christian, this is really a cool testing method for unidentified zener diodes.I really like this. Thanks for guys like you.

  10. Mark

    September 15, 2015 at 5:37 pm

    Hey Christian,

    Thanks for your testing method - quick and easy.

    A few years back when I bought Jestine's Testing Electronic Components E-book, one of the bonus reports was how to make a diode tester. It didn't take too long to build and was a good project. It works well and can be used for testing Zener Diodes, batteries, continuity, voltage regulators, LEDs, Schottky diodes and non polarity capacitors. Another useful tool in the electronics lab!

  11. Yogesh Panchal

    September 15, 2015 at 8:31 pm

    Thanks for informative article.

  12. corriete

    September 15, 2015 at 8:32 pm

    cool way to test.thanks

  13. g wells

    September 16, 2015 at 1:21 am

    Thanks, Good to know

  14. Humberto

    September 16, 2015 at 1:39 am

    A very good tutorial Mr. Christian. Keep up.

  15. Erik

    September 16, 2015 at 2:48 am

    What if you dont know if you have a normal diode or a Z diode?
    What if you have power Z diode, more then 10-20mA?

    Why is there not a zenerdiode projekt which handmated/ automatically put current for 10mA ---- 1 amp, and automatically drive up the Voltage

    • Paris Azis

      September 16, 2015 at 7:57 pm

      Hi Erik

      A) Zeners operate when being reverse biased. That is, following Cristian's testing method, increasing the voltage you will never see a sudden increase in current as a zener "knee" does not exist with normal diodes. (Except if you reach the breakover voltage of the diode which is very high and catastrophic as well). So normal diode = open circuit with this test.
      B) Higher power zeners will act exactly the same as the low power. (We are trying to identify the zener voltage here. Not to execute a power endurance test of a diode. Higher test current is (under the light of this purpose) not necessary, and
      C) There are in the market testers like the one you describe, but remember that they are expensive instruments, used normaly for research and development departments of design centers of big companies.
      Moreover, if you want to test the stability of a zener there are other alternatives. For example you can use the "minimum-maximum recording" of a good multimeter which will show you any voltage decline from the nominal zener voltage.

      Best Regards

  16. Robert Calk

    September 16, 2015 at 8:57 am

    Thanks Chris for the helpful article. I use my Peak Atlas Zen 50 that works great. It tests with 4 different currents and also gives the slope resistance.

    • Chris

      September 16, 2015 at 6:48 pm

      I like the tools from Peak Atlas but I newer got one.
      In my country that was hard to buy.
      It's a nice tool.
      Thanks for sharing.

      • Albert van Bemmelen

        September 17, 2015 at 2:49 am

        Hi Robert and Chris. My young friend Chafik who ownes a repairshop bought also a quite expensive Peak Atlas Transistor Tester.
        He had it about a year or so and recently the Battery (12V) was only 3.4V when he tried to use it. So I guess it is much better to remove the battery afterwards when you don't need the Peak Atlas. And these Batteries are expensive if you need to buy them every year again. And testing Thyristors is not always possible either because of the too high Gate current that mostly is needed and can't be delivered by the Peak Atlas. I have a cheaper China build, and a few other testers that probably does just as much for less money. And do not use any 12V battery.

        • Robert Calk

          September 18, 2015 at 12:38 pm

          Hi Albert,

          Maybe your friend had a bad battery. I have never had any problems with my Peak Atlas Analyzers, and I have 4 of them.
          The DCA75 Pro will only check Thyristors that have gate currents up to about 10mA. For thyristors with larger gate currents you need to get the SCR100 Triac & Thyristor Analyzer. They are great tools and you may be able to order them from their website in England.

          • Robert Calk

            September 18, 2015 at 3:40 pm

            Also, the DCA75 Pro uses a AAA 1.5V Alkaline battery. I hope your friend didn't put a 12V battery in it.

  17. iskansdar

    September 16, 2015 at 9:26 pm

    good job thanks

  18. Samantha Perera

    September 16, 2015 at 11:50 pm

    Nice lesson thank you

  19. Abdul

    September 17, 2015 at 9:11 am

    A very good and straight forward tutorial.
    You explained in simple terms.
    Sometimes we have difficulties in identifying the zener diodes. Yes this is the way I like it.
    Thank you. Hope more articles will come.

  20. Arnab Kumar Chatterjee

    September 17, 2015 at 5:58 pm

    Dear Christian Robert Adzic sir
    Can you please put an article on " Another method of checking LM317 IC ".

    • Chris

      September 18, 2015 at 4:45 am

      I will check if I have one LM317. If I found one then I will see how can I make the checking method and I will post it.
      Thanks for supporting my article.

  21. Gurpreet

    September 17, 2015 at 8:04 pm

    Very good and easy way to understand about zener diodes. Thanks for sharing this article.

  22. mahmoud

    September 18, 2015 at 4:39 am

    thanks mr.chris this is the best methode for knowing the voltage of a unknowning zener diode.

  23. Owais Akhter

    October 31, 2017 at 4:14 am

    Nice article. I use this method and most of the time I use my zener diode tester which design I got from Mr. Jestine Yong. The tester is more fast and accurate and save time and risk of overheating of Zener diode. But I can say that both methods of testing Zener diode is accurate and powerful because we supplied full operating voltage and safe current in order to test Zener diode accurate.

    • Jestine Yong

      October 31, 2017 at 9:41 am

      Hi Owais,
      It's been a long time. Good to hear from you again.




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