Measured zener voltages - Page 1
Forum section: electronics / projects

author: paulossant
author_rank: Newbie
timestamp: July 09, 2020, 10:08:28 am
content: I bought a pack of zener diodes from ebay a while back. Since it came from ebay, I didn't had any datasheet for those.Now I just tested a bunch of zener diodes voltages vs current, and got a really spread of expected values. I expected the rated value around 1mA, but some of those are really far from that.Are those really bad zeners, or this is expected?The only one that behaved very well was the 5V1 zener, as expected since around 5V they are usually very well behaved.All pictures and the .xls file are attached

author: madires
author_rank: Super Contributor
timestamp: July 09, 2020, 12:04:05 pm
content: Look for datasheets of Zener diodes with the same wattage or package to get an idea about the specs. For example, in case of 500mW low voltage Zeners the Zener voltage is often stated for a current of 5mA.

author: paulossant
author_rank: Newbie
timestamp: July 09, 2020, 01:00:11 pm
content: That's ok, but what surprised me, is that the IV curve hardly resembles a "knee" shape, as stated all over electronic books.

author: markce
author_rank: Regular Contributor
timestamp: July 09, 2020, 01:14:53 pm
content: The standard low voltage zener diodes are arround 400mW max, so mind the test current.Different types of zener diodes have quite a different temp coefficient.The internal resistance also influences the clamp voltage.

author: tszaboo
author_rank: Super Contributor
timestamp: July 09, 2020, 01:23:42 pm
content: Low voltage zeners are terrible. They always have been.

author: magic
author_rank: Super Contributor
timestamp: July 09, 2020, 01:25:37 pm
content: 5V1 looks like the textbook curve, 4V7 is not far behind.The high voltage ones could indeed have significant internal resistance. Connect a 47Ω resistor in series with 5V1 and you will probably see a similar curve above a few mA.3V3

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 09, 2020, 01:26:33 pm
content: I believe that the combination of a log scale for the vertical axis, and a very compressed range for the horizontal axis, is giving you those weird curves.Re-plot using a linear scale for the current, and start all the way from "0" in the voltage, and see how they look like.

author: macboy
author_rank: Super Contributor
timestamp: July 09, 2020, 01:28:43 pm
content: Low voltage Zeners (< ~ 5V) use a true Zener effect, and this has a relatively soft knee. Above that, they are actually using avalanche effect which presents a much sharper knee. In any case, plotting with a (less practical) linear IFscale will illustrate the knee more clearly.The 6.2 V one is perhaps the best behaved. Look at the first three data points at low current: nearly vertical plot. This is why these are nearly exclusively used in Zener voltage references. They are usually combined with a forward biased Si diode in series (either internally or externally) to cancel the temperature coefficient. This is why Zener references (LM399, LTZ1000, etc.) tend to be around 6.8 to 7 V range.Some of your lower voltage ones are clearly not what they claim to be. I'd expect the nominal voltage to appear somewhere around 1 mA (give or take an order of magnitude).

author: magic
author_rank: Super Contributor
timestamp: July 09, 2020, 01:30:30 pm
content: On a related note, I bought a kit of ceramic disc capacitors from an auction site the other day and almost all were over 20% off

author: paulossant
author_rank: Newbie
timestamp: July 09, 2020, 02:05:08 pm
content: The log scale was to get a better look at the supposedly knee. The 4V7 and 5V1 are as expected, but the 3V3 is absolutely awful.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 09, 2020, 02:32:06 pm
content: I have tested now one of my 3V9 Zeners - it is different from TS (clearly better).It has about 2.0 V ar 0.01 mA; 2.55 V (+-) at the 0.1 mA; then 3.3 V at 1.0 mA; and 4.1 V at 10 mA. It is ok.(It is easy to test because I have "1 kOhm resistor at output" mode in my bench supply, so I can see Zener or LED voltage drop right onto device panel meter).Tested one of my 3V3 Zeners:1.60VDC at 0.01 mA; about 2.05VDC at 0.1 mA; 2.7VDC at 1.0 mA; and 3.56 V at 10.0 mA.My Zeners are not from Ebay but from shops in my city (they usually sell normal, I mean good, parts).

author: madires
author_rank: Super Contributor
timestamp: July 09, 2020, 02:41:38 pm
content: My personal rule of thumb is to drive low-voltage low-wattage Zeners with 3mA for best performance.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 09, 2020, 03:25:17 pm
content: You may want to purchase a few devices from reputable vendors, and test under the exact same conditions and make A-B comparisons.After all, these are Ebay components. There may be a good number of functional but borderline components.

author: mikerj
author_rank: Super Contributor
timestamp: July 09, 2020, 03:42:14 pm
content: Quote from: NANDBlog on July 09, 2020, 01:23:42 pmIndeed, I really don't see much point in the very low voltage parts; they make terrible voltage references and are little use for clamping signals.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 03:58:56 pm
content: Technically, the lower-voltage parts are true "Zener diodes", and have the "soft" V-I curve you measured.The higher-voltage parts (above roughly 5 or 6 V) are "avalanche diodes" and have "harder" curves.See:https://circuitglobe.com/difference-between-avalanche-and-zener-breakdown.html

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 09, 2020, 07:58:12 pm
content: As suggested above, I'm sure low voltage zeners are good for something, but I'm not sure what it is. Further, friends don't let friends buy electronic parts from the big auction site because they tend to be of questionable pedigree and quality.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 08:35:16 pm
content: At those low voltages, you have no choice but "true Zeners", unless you use band-gap devices such as the TL431 as shunt regulators.

author: muvideo
author_rank: Frequent Contributor
timestamp: July 10, 2020, 07:25:22 am
content: Quote from: TimFox on July 09, 2020, 08:35:16 pmSome forward biased LEDs are not that bad, Red gives about 2V, Blue about 2.8-3V,but their curve and internal resistance is not in datasheet usually and has to be tested.

author: exe
author_rank: Unknown
timestamp: July 10, 2020, 07:52:34 am
content: very interesting, I'd expect the best zenners to be 5.6V and 6.2V. May be 6.8V if including a diode that should do temperature compensation.

author: srb1954
author_rank: Super Contributor
timestamp: July 10, 2020, 12:46:34 pm
content: Testing with 100mA Zener current is going to cause significant self-heating in the diodes, particularly if they are small packages. This will affect the shape of the curve due to the temperature coefficient of the Zener voltage - positive for voltages above 6V and negative for voltages below 5V, approximately zero between 5V and 6V.If you really need to test the Zener voltage at high currents e.g. for clamping circuits you need to use a pulsed current technique to determine the true voltage vs current curve.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 01:02:51 pm
content: Zeners around 6 V have the best tempco. 6.8 V reference Zeners are a 6.2 V Zener and a forward-biased PN junction in series, so that their tempcos cancel at a specified current. There are similar devices with two or three PN junctions to get higher reference voltages.

author: paulossant
author_rank: Newbie
timestamp: July 10, 2020, 02:05:37 pm
content: Quote from: srb1954 on July 10, 2020, 12:46:34 pmI have taken the temperature effects into consideration. To more precise, I tested the zener diodes with mine diy constant current load.I saw the temperature dependence affecting the readings, so for currents bigger than 25mA i have done that exact pulse measurements (by turning on and off for about 3-4 seconds, allowing me to take note of the values)my diy constant load was not good for currents below 300uA, therefore the lack of measurements bellow that value, but I can test it with other setup.

author: srb1954
author_rank: Super Contributor
timestamp: July 10, 2020, 02:19:42 pm
content: Applying test currents 3-4 seconds is still too long to avoid temperature effects. You really need to be looking at millisecond length test pulses to avoid excessive junction heating.It is not the overall case temperature that matters but that of the small semiconductor junction in the centre of the diode. These have a very small thermal capacity and don't take long (<100ms) to heat up.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:05:50 pm
content: I noticed how strong was the self-heating effect in 1 W Zeners when I tested 62 V diodes (axial lead) on a Grayhill test clip fixture (plugged directly into a DVM, fed from a current source), and saw how the voltage changed when I pinched the diode between thumb and forefinger.

author: paulossant
author_rank: Newbie
timestamp: July 10, 2020, 03:12:52 pm
content: Quote from: srb1954 on July 10, 2020, 02:19:42 pmTesting both ways should give an interesting comparison. I will arrange a setup, maybe a micro with a one time shot pulse (25ms), acting on a transistor to turn on a voltage source momentarily, and just get a resistor and the zener, with a scope on the zener. Would that be a good way to test?

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:27:37 pm
content: How much resolution on the Zener voltage do you need, and what can you get on your digital oscilloscope? 1% resolution should be easily achievable on an 8-bit scope.

author: srb1954
author_rank: Super Contributor
timestamp: July 11, 2020, 02:06:10 am
content: Yes, but it will be difficult to get accurate measurements due to the limited resolution and quantisation noise of the average scope (I am assuming you are using a digital scope and it is not one of the super-expensive high-resolution models).You could get more accurate measurements with a scope that has a differential comparator input. You use this to offset the nominal Zener voltage from the measurement so you can view and measure small deviations from that offset voltage. AFAIK such capability is not available in modern scopes and you have to revert to an old-fashioned analog scope e.g. Tektronix 7000 series with a 7A13 differential comparator plug-in.The most accurate way to measure the Zener voltage under pulse conditions would be to use a a high-speed digitiser such as found in high-end bench multimeters or data acquisition systems. This could be triggered to take a fast measurement or series of rapid measurements a short time (allowing for the Zener voltage to settle) after the current is applied to the Zener.Alternatively, you could build your own test system using a fast ADC, 14-bit or better, paired with the micro of your choice.

author: exe
author_rank: Unknown
timestamp: July 11, 2020, 07:40:15 am
content: I wonder how much it makes sense to measure zener clamping voltage with pulsing current. This is not a normal operating condition, what's the use of it? I understand why bjts are measured this way: to match them better. But zener... (except currents that would normally destroy them in seconds).

author: srb1954
author_rank: Super Contributor
timestamp: July 11, 2020, 01:37:28 pm
content: Zeners are frequently used as clamping devices to limit transient voltages and protect sensitive circuitry. Compared to other clamping devices e.g. MOVs they have the advantage of responding very quickly and clamping transients to tightly controlled levels. Since in this application the Zener is usually clamping short transient signals it is highly desirable to be able to measure the Zener performance under pulse conditions without the complicating effects of thermally induced drift.It is probably fair to say that the main usage of discrete Zeners these days is for clamping circuits. Previous applications of Zeners as voltage regulators and reference voltages have largely been superseded by better performing and more efficient monolithic voltage regulators and band-gap reference circuits respectively.

author: magic
author_rank: Super Contributor
timestamp: July 11, 2020, 01:50:20 pm
content: Actually, there are dedicated ESD protection devices too, with low leakage, sharp knee, guaranteed clamping voltages at a few amps of current and whatnot.Not even sure if they are (modified?) zeners/avalanche diodes or perhaps some small ICs

author: tszaboo
author_rank: Super Contributor
timestamp: July 11, 2020, 03:47:32 pm
content: Quote from: Conrad Hoffman on July 09, 2020, 07:58:12 pmThey can still be used as a safety voltage limiting (and so energy/power limiting) elements. They have their uses, and irreplaceable in some designs.But dont worry, low voltage TVS diodes are also terrible. Standoff voltage of 3.3V, opens at 7-9 ish. Maybe.OP: You can replace them with a TL431 or a LM358 or similar ICs, if the application allows it.