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SOLVED: Zener Diode rated Voltage Drop "depends"...

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Axel1973:
Hi there Zener Experts :-) !

this is my story about building a crapy zener diode tester and some "findings" i made using it, puting up some questions i like to ask here.

few weeks ago i was sorting out my diode junk heap and determined zener diodes from other standard diodes. there i have been facing that my all new Fluke DMM in Diode testing mode does not always produces a voltage high enough to test zeners properly (~3V i think it is).

So i needed some test-equipment that produces a low current, high voltage output where i can put my DMM to it measuring the output voltage and plug in the unknown zener diode. And because turning around a zener diode between crocodile clips is wasted time i also wanted something that does switch polarity itself or at least by pressing a simple button.

I took a piece of stripe-circuitboard, some old 2x3 relais (2x 3pin switch inside + 2 pin coil) that works from round about +5V up to +30v with no remarkable problems, some junk button, 4 banana plugs and some ancient 300KOhm resistors 5-10% or so.

So I put together a devise that takes 5-30+ Volt DC via banana plugs and puts out an positive voltage (Uout=Uin) to the "testing banana plugs", and when pressing the button on the circuit board the polarity of the output voltage is been reversed without re-wiring the test-diode. the 300Kohm resistor is been put in series with the test-diode plugs to the current through the zener is limited.

Now i can test more easily unrated or unknown zeners with less re-wiring and with a much wider voltage range than before.

BUT... and here comes my findings:

it seemed to me that the ratings on several zener diodes are pretty much "off" compared to my measurements. I assumed that a 6.2V or a 12V zener would show a pretty precise voltage drop around that rating. but i often measured voltages off -+ 200-500mV as far i remember now.

I than though about that and though it might be the pretty large limiting resistor. so i put another two 300KOhm resistors in parallel with the existing one (now resulting in ~100Kohm) and the readings "seem to be slightly better". however still not perfect.

Question is now: Looking to a Zener U/I Curve, WHERE EXACTLY is the breakdown point of a Zener Diode? Where is the point the Diode is rated at? more where the "edge" is starting or somewhere in the middle? or at the higher end (higher reverse currents) ??

It also seems the "reverse biased edge/curve " is not always that precipitous as expected. sometimes its pretty "flat"  (still NOT talking about forward direction! which is a common diode curve)

Any good links/articles explainations/tips about that observation i made?

Thanks for any Advice/Tips!

73 de Axel (DG2GWE)

amspire:
Zener diode voltages are often specified at 5mA or 10mA. The knee is often around 1mA, so you were using far to little current.

Just as an example, for a typical 6.2V zener diode, is has an AC resistance at 5mA of 4 ohms, and at 1mA is is 200 ohms.

Also you have to look at the accuracy. 5% is common which for a 6.2V zener, that is +/- 0.31 volts. Over about 50V, the zeners might be specified at a lower current. Better then 5% is not common.

Also the zeners that work the best are in the range of about 5.6V to about 15V. Below 4.7volts, zeners are breaking down with zener breakdown that has a fairly soft curve. 6.8V and above are starting to get avalanche breakdown that has a sharper knee. For large zeners, the lighter doping means more resistance.

Around 6.2V is optimal for a zener - it will have the lowest temperature coefficient. This is why when you see a zener used for a voltage reference, you usually see between 4.7V and 8.2 volts. Also if you are serious about zener a stability, you will use a constant current through the zener. If you just have a resistor to the positive supply, it will not be as stable.

Richard.

SeanB:
Years ago I built a zener tester, using an old transformer and making a 100V psu, then added a constant current source to it to give 5mA into any load. Was good to test diodes from 0 - 75V, and was simple to use, even if the open circuit voltage was high enough to shock.

Recently I wanted to voltage limit the 6V charge circuitry in some OneHungLow cheep n cheerful lights/battery burners. Got 10 of each of 2.7, 3.0, 3.2, 3.3 and 3.5 V 1W zener diodes, and then tested them at 200ma to mix and make some sets up to limit the charge voltage to 6.8V. These were then added in the cheep and horrid power supply area, as they only had a 9VAC transformer along with a bridge rectifier and a resistor as charge control ( wonder why the SLA batteries only lasted 6 months at best) to voltage limit the battery. I had to select the diodes at 200mA as they had a very different voltage than at 50mA. If the power is disconnected these diodes will draw current and discharge the battery in around 1 day, but as these are emergency lights not a problem.

Axel1973:
in the meanwhile i put some schematics together in EAGLE for those who are troubling with my bad english.

Axel1973:
Im starting to LOVE THIS FORUM and the peoples here!
Almost always getting a "comprehensive answer" here! Just like this time again.

Thanks amspire and all!

I almost always forget about tolerances and i didnt did the math on that. of course i didnt realized that zeners usualy are rated 5% tolerance which explains a lot here.

also thank you both for the tip using a constant current source rated somewhere about 1-20mA (?).

BTW: dave once mentioned to use a lin.voltage regulator as a programmable constant current source. can non adjustable volt regulators like 7805/7812 also be used as a programmable/variable constant current source as well? need to look this up at google.com . but this is another topic.

73, Axel

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