Introduction: Build a Zener Diode Identifier

About: Gian is a computational biologist,maker, author, humorist. He holds degrees in Molecular/Cellular Biology, Biochemistry, Computer Science. He has a collection of 8-bit microcontrollers and a room full of compu…

If your workbench or electronics studio looks anything like mine, you've organized your components in a step-wise fashion. That is, you didn't purchase all the shelving, component compartments, drawers, boxes, pull-out bins, etc on the first day you set up shop. Instead, as your electronics component inventory grew, you expanded your existing organizational scheme piecemeal. That has worked out well for me, as I now have shelves of components and nice pull-out drawers containing my resistors, capacitors, inductors, LEDs, MCUs, linear and discrete logics, etc. The biggest drawback, though, has to be when my inventory of a particular component outgrew the others, leaving me with a cramped compartment jam-packed full of, for instance, diodes (don't even get me started on resistors).

One of the problems with my diode overflow is that some kinds of diodes are not marked, except for the cathode lead, in particular, zener diodes. These have been the bane of my existence, as if I don't mark them somehow (usually with tape and a sharpie) I will never remember or know what the zener voltage is. You see, zeners aren't color coded like resistors and inductors, they don't have helpful numbers like capacitors, and they rarely -- if ever -- have any component numbers marked on their shiny, tubular bodies.

After a few different angles of attack, I think I have found the best way to not only determine the zener's breakdown voltage of an unidentified zener independent of changes in the supply voltage, and, in the same fell stroke, am able to determine if a zener diode is fried or otherwise faulty.

If you have a box, compartment, bag, or drawer full of unidentified zeners that you haven't been able to use for sake of identification, this instructable will be right up your alley. It could easily become a standard part of your electronic testing and measurement arsenal due to its simplicity and ease of use.

Go to the next step and let's begin!

Step 1: Zener Diodes



A diode is a two-lead semiconductor that acts as a one-way gate to electric current flow. When a diode's anode lead is made more positive in voltage than its cathode lead -- known as forward biasing -- current is permitted to flow through the device. However, if the polarities are reversed (that is, the cathode is made more positive than its anode) -- a condition known as reverse biasing -- the diode will act to block the flow of electrons.

Diodes are used in circuits that convert, or rectify, AC voltage into DC voltage. They are also commonly found in voltage-multiplier circuits, voltage-shifting circuits, voltage-clamping circuits, and voltage regulator circuits. For a novel design showing what you can do with regular old diodes, check out my instructable Make a Solar Panel Using Diodes!


Forward Voltage (Vf): Current flows in the direction of the arrow (from anode to cathode) provided the voltage across the diode, the forward voltage, exceeds its junction threshold voltage.

Junction Threshold Voltage: In many cases, the p-n junction composing the diode has a 0.6V threshold, while other special types of diodes, like germanium diodes exhibit 0.2V thresholds, and Schottky diodes often can be found with 0.4V thresholds. Don't carve these values in stone though, as production values may skew these thresholds off by upwards of a volt.

Forward Biasing: When the diode's anode is more positive in voltage than its cathode

Reversed Biasing: When the diode's anode is more negative in voltage than its cathode.

Note my hand drawings below to see this graphically represented. Keep in mind that not all diode junctions are p-n junctions, specifically Shottkys which use a metal junction.

Peak Current Rating (Iomax):  Also called Max Forward Current, this is the maximum current that must flow through the diode for safety reasons, lest you burn it out.

Peak Inverse Voltage (PIV) Rating: When reverse biased and the reverse voltage is negative, this negative voltage must be less than the peak inverse voltage rating of the diode.

Kinds of Diodes

As mentioned, there are numerous types of diodes to be found and that are useful in various projects you may work on. Here are some of  the top ones and a brief description of what they do.

Rectifier diodes: These diodes are used to rectify (convert) AC voltage and current into DC voltage and current.
Signal/Switching diodes: Often used in circuits where small currents and high frequencies are involved, like radio and RF.
Fast recovery diodes: Ultrafast switching diodes used to conduct reactive load current and rectify AC, among other uses.
Schottky diodes: Using a metal conductor junction, these diodes have a lower Vf and are commonly found in voltage clamping and transistor saturation modulation circuits.
Germanium diodes: These have a low reverse resistance value giving a lower forward volt drop across the junction but have a higher forward resistance value because of their small junction area.
LEDs: Ah the light-emitting diode. The blinken light favorites of all of us.

Zener Diodes

Zener diodes, instead of acting like a one-way gate to current flow, act as a two-way gate for current flow. In the forward direction it's easy to break through, often 0.5 - 0.7V like a standard diode. But, in the reverse direction, current can flow but it's harder to push open. The voltage required is equal to the zener's breakdown voltage. This breakdown voltage can be anywhere from 1.8V to upwards of 200V. Power ratings vary.


Breakdown Voltage (Vz):The voltage required to push open and flow current in the reverse direction. This is, arguably, the primary characteristic you look for in a zener diode (outside of maybe power ratings).

Zener Diode Application Examples

1. Line regulation
2. Load regulation
3. Split supply from a non-tapped transformer
4. Waveform modification and limiter
5. Voltage shifter
6. Voltage regulator booster
7. Overvoltage protection

Step 2: Identifying an Unknown Zener

So we know Zeners come in many different breakdown voltages, but let me tell you this straight: I've not seen any marked with part number or voltage rating. Sure, maybe there are some out there, but I haven't had the luck to have found any. This goes doubly true if you're using MELF and mini-MELF packages for your diodes.

If you don't keep your diodes separately in different compartments, or in small baggies, or some other way, you run the risk like me of having a bin full of diodes you don't know the value of. At first blush, it may seem straight forward to make a resistive divider out of a resistor and a zener and read the voltage on the other side, but in hind-sight, it's got some glaring problems. First, you don't know what value to use as a resistor, as different ohmages (and current) will yield different Vout voltages.
The circuit I present below can be used to test not only for the zener breakdown voltage of an unknown zener, but it will also test the zener itself for faults. This straightforward circuit uses a constant current for the zener under test, adjustable by varying the R2 potentiometer, independent of changes in power supply voltage.

The OpAmp I use is a UA741 because, as the picture below indicates, I have a lot of them I got on the cheap at one point. The output voltage of the first op amp, IC1 can be determined by:
V0 = Vz(R3 + R4) / R4  (see the equation below for a better layout)
The zener current is given by:
Iz = (Vo - Vz) / (R1 + R2) (again, see the formula below for a better layout)

The output of the first opamp depends on the breakdown voltage for the zener  under test and the values of the resistors. It's important to use a single supply for your op amps as the circuit can go into a latch-up state when powered up since the circuit is based on positive feedback. So, just use GND as your negative or V- voltage.

Looking at the circuit below and the equations mentioned previously, if R3 = R4, then Vo = 2Vz . Since we can solve for Vz, we get Vz = Vo / 2, so the second op amp buffers the output and halves it with a resistive divider. Voltage can then be read with a multimeter across R6. This voltage will be the zener's breakdown voltage that you're testing. Any bad or wonky reading indicates a faulty zener. 

To get an accurate reading of the zener breakdown voltage you will have to adjust potentiometer R2 until the read-out voltage settles and indicates a constant voltage. Once this happens, you've got your zener's breakdown voltage.

Also attached is a Bill of Material (BOM) in PDF format, schematics and a board layout in PNG graphics format, as well as a zip file containing the actual *.brd and *.sch files I used in building mine.

Step 3: Conclusion

So, I've elucidated the problem domain: having unidentified zeners with unknown breakdown voltages, and gave you a brief background on diodes, in general, and zeners in particular (my apologies if this was old news to you).  I then stepped over a constant current circuit that will identify faulty zeners and the break down voltage of any zener under test.

I hope you have enjoyed my little instructable and, as always, I am open to your comments, concerns, and suggestions about this or any of my instructables. If you liked this instructable, I'd appreciate you rating it. You can always post any questions or comments you may have here, send them to my email on this site, or to my off-site email: jamesbl 'at'

Thanks for reading and have a great time building your projects!