How to Measure The Voltage Rating Of A Zener Diode
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.
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:
- Put all the potentiometers on a zero stage. The current and the voltage potentiometers.
- 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.
- 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.
- 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.
Remember, in my case the yellow clip is the positive and the green clip is the negative terminal.
- 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.
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:
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:
This zener has no labeling on it. Let see what is the voltage reference for this guy.
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.
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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