Author Topic: EEVblog #908 - Zener Diodes  (Read 19253 times)

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Offline EEVblog

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EEVblog #908 - Zener Diodes
« on: August 06, 2016, 06:28:27 am »
Fundamentals Friday.
A tutorial on Zener Diodes.
Avalanche breakdown, zener breakdown, zener effect, knee voltage, power dissipation, differences from regular diodes, calculating dropper resistor value, voltage regulation, clipping, clamping, transient surge overvoltage protection, characteristic curve, forward and reverse characteristics.

 

Offline KerryW

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Re: EEVblog #908 - Zener Diodes
« Reply #1 on: August 06, 2016, 02:51:48 pm »
Very nice introduction to Zener diodes, but you left out a BIG "trap for young players": Temperature compensated Zeners.

Temperature compensated Zeners have the same symbol as "regular" Zeners, but they DO NOT conduct in the forward direction!  So, for your AC clamp, you would need to put the Temperature compensated Zeners in parallel, not in series.  And in your protection circuit, they would not protect against negative voltage.  You would need a parallel diode for that.

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Offline gemby

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Re: EEVblog #908 - Zener Diodes
« Reply #2 on: August 06, 2016, 07:21:18 pm »
I would also like to se some theory around zener and transistor regulators, for example: 

and


And how those regulators cope with modern lienar regulators in all spects, power discipation, speed, accuracy, power drop and so on.
« Last Edit: August 06, 2016, 07:29:24 pm by gemby »
 

Offline Wilksey

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Re: EEVblog #908 - Zener Diodes
« Reply #3 on: August 06, 2016, 09:15:48 pm »
If I had a 3v Zener acting as a protection device to a uC input, should I expect to see a voltage drop?

I had a voltage divider taking 16V down to 3V, I put this into a uC input, which worked fine, I thought about adding a ZD to protect if the input to the divider went above 16V but I was seeing a 0.3V drop with the ZD in circuit.  I didn't have a series limiting resistor, could that have caused the drop? It was used for V Monitoring rather than driving a current.
 

Offline Galaxyrise

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Re: EEVblog #908 - Zener Diodes
« Reply #4 on: August 06, 2016, 10:25:12 pm »
If I had a 3v Zener acting as a protection device to a uC input, should I expect to see a voltage drop?

I had a voltage divider taking 16V down to 3V, I put this into a uC input, which worked fine, I thought about adding a ZD to protect if the input to the divider went above 16V but I was seeing a 0.3V drop with the ZD in circuit.  I didn't have a series limiting resistor, could that have caused the drop? It was used for V Monitoring rather than driving a current.

Diodes never block current 100%; they have a "reverse leakage" that goes up as voltage increases.  Just under the breakdown voltage, that 3V zener is maybe conducting a couple hundred uA or something. Imagine putting a 15k resistor in parallel with that leg of the divider: the divider voltage goes down as a result. If you had used a 6V zener, you probably wouldn't be able to measure the shift--the diode will have been conducting some nA instead.

If you had included another resistor in series with the zener, the effect would have been reduced but not eliminated.
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Offline Macbeth

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Re: EEVblog #908 - Zener Diodes
« Reply #5 on: August 07, 2016, 12:51:04 am »
Bloody Zeners and their ACL reverse breakdown voltages right at the knee...  >:D
 

Offline Wilksey

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Re: EEVblog #908 - Zener Diodes
« Reply #6 on: August 07, 2016, 01:55:19 am »
If I had a 3v Zener acting as a protection device to a uC input, should I expect to see a voltage drop?

I had a voltage divider taking 16V down to 3V, I put this into a uC input, which worked fine, I thought about adding a ZD to protect if the input to the divider went above 16V but I was seeing a 0.3V drop with the ZD in circuit.  I didn't have a series limiting resistor, could that have caused the drop? It was used for V Monitoring rather than driving a current.

Diodes never block current 100%; they have a "reverse leakage" that goes up as voltage increases.  Just under the breakdown voltage, that 3V zener is maybe conducting a couple hundred uA or something. Imagine putting a 15k resistor in parallel with that leg of the divider: the divider voltage goes down as a result. If you had used a 6V zener, you probably wouldn't be able to measure the shift--the diode will have been conducting some nA instead.

If you had included another resistor in series with the zener, the effect would have been reduced but not eliminated.

So, you are saying that the ZD was causing some parasitic resistance to the output causing another divide?
 

Offline timb

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Re: EEVblog #908 - Zener Diodes
« Reply #7 on: August 07, 2016, 02:31:46 am »
I would also like to se some theory around zener and transistor regulators, for example: 

and


And how those regulators cope with modern lienar regulators in all spects, power discipation, speed, accuracy, power drop and so on.

They can be good basic regulators that work much, much better than a Zener alone, in all aspects (plus they can dissipate orders of magnitude less quiescent current). All they require is a single additional transistor to implement, so cost is minimal.

Just make sure to take into account the transistor's VBE drop. For example, if you want a 5V output, you need a 5.6V Zener (since VBE is roughly 600mV).



That said, you can get linear regulators in SOT-23 packages that will do the same job, for the same price (or less) with better regulation, speed, and only uA of quiescent current, so there's not many places you'd need to use a Zener+Transistor regulator these days.
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Offline FrankBuss

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Re: EEVblog #908 - Zener Diodes
« Reply #8 on: August 07, 2016, 10:22:51 am »
Some interesting facts if you want to build an analog noise generator with diodes:

https://en.wikipedia.org/wiki/Noise_generator

So looks like when using the avalanche effect, the noise frequency spectrum is flat (white noise) from 1 Hz to 100 kHz for some diodes:

http://www.edn.com/design/analog/4420926/White-noise-source-flat-from-1Hz-to-100kHz

BTW, with a FPGA once I implemented a white noise generator with Rule 30. The noise looks good (if you are far away from the Nyquist frequency) , but needs more LEs than a LFSR.
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Offline b_force

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Re: EEVblog #908 - Zener Diodes
« Reply #9 on: August 07, 2016, 11:51:35 am »
Zeners are pretty noisy anyway. That's why it's not a very good idea to use them in a regulator.
It's fine for circuits which are not very sensitive off course.
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Offline Whales

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Re: EEVblog #908 - Zener Diodes
« Reply #10 on: August 07, 2016, 12:34:10 pm »
I recently had to reverse-engineer my soldering iron and it used zeners and large resistors absolutely everywhere.  Perhaps it's cheaper for TH stuff than vregs?

Offline Tom45

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Re: EEVblog #908 - Zener Diodes
« Reply #11 on: August 07, 2016, 04:55:09 pm »
When I was a EE student in the 60s Zener diodes were relatively new, about a decade old. So when Dave said Dr. Zener came up with the zener effect long before any of us were born, I was skeptical.

But a little research came up with a date of 1934 for his paper, 11 years before I was born. So it was around 20 years before the theory in his paper surfaced in a useful device.

Also, Dr. Zener pronounced his name ZEE ner, not ZEN ner.
 

Offline zapta

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Re: EEVblog #908 - Zener Diodes
« Reply #12 on: August 07, 2016, 05:39:55 pm »
Very good video. I would like to see more like this.

The I/V diagrams of Zener diodes should be flipped in the horizontal direction IMO. That is, the Zener affect should be on the positive voltage. This is more consistent with their use case. That is, a 4V Zener diode that is at its Zener region has '4V, not '-4V'. Yes, I know, it's a matter of convention.

BTW, I always assumed that the Zener symbol was selected because it resembles 'Z' (and the Schottky symbol for 'S').
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Offline gwideman

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Re: EEVblog #908 - Zener Diodes
« Reply #13 on: August 07, 2016, 10:26:10 pm »
Fundamentals Friday.
A tutorial on Zener Diodes.

Correction for 29:00 in the video. Summary: Not dynamic resistance, but rather RC effect, with Zener showing appreciable C.

Dave discusses using a 5.1V Zener diode as a signal clamp. He demos what amounts to a voltage divider having a 1k input resistor, and then 5.1V Zener to ground.  The scope shows 7V square wave input and ~5.1V output signal, as expected.  Zooming in to 50ns timebase, the scope shows leading edge of output waveform with slow rise time.



(And then Dave repeats the demo with 100ohm || 1k, so R = 90 ohm, and shows much faster but still slowed rise time. )

Dave attributes this to the dynamic resistance. I'm fairly certain the effect is due to the RC effect of the input resistor with the capacitance of the Zener (and scope probe, although that's set to 10X, so minimal).  Calculating: The rise time from around 1V to 5V (so about 63% :-) ) was 150ns, which with R = 1k gives a capacitance of about 150pF.  That aligns well with data in, for example: https://www.onsemi.com/pub/Collateral/HBD854-D.PDF page 25.



[Later edit] It occurs to me that where "dynamic resistance" does come into play is at the top of the rise where the output transitions from rise to horizontal at ~5.1V.

Dynamic resistance is really just a way of characterizing the slope of the I vs V curve, modeling it as an ideal Zener (either on or off with a knee at say 5.1V) in series with a resistance... but then taking into account that the curve above the knee voltage is not straight as a resistance would provide. Instead it's curved, corresponding to somewhat gradual turn-on of the Zener, hence "dynamic" resistance.

In the demo, at the rising edge, the output voltage rises, slowed by RC, until it reaches near 5.1V. At 5.1V, with an ideal Zener, the output voltage would abruptly transition to a horizontal line. But it does not -- it gently curves from rising to flat. That's due to the Zener's only gradual transition from high resistance (below 5.1V) to low resistance (above 5.1V).  Quite noticeable in the 90 ohm example:




So, the "dynamic resistance" is indeed displayed on the scope. However it's not causing the most conspicuous feature (slow rise time) but rather the non-sharp transition from rise to flat.
Graham
« Last Edit: August 08, 2016, 09:27:11 am by gwideman »
 
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Offline lordnoxx

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Re: EEVblog #908 - Zener Diodes
« Reply #14 on: August 07, 2016, 11:31:18 pm »
Hi Dave,
thanks for that good educational video.
The graph from the component tester showed quite similar slopes of the
curve in the forward and in the reverse operation region. So why not using a couple of standard diodes in series and forward biased to have lets say a 5.4V (9*0.6V) voltage reference (or clamping voltage) instead of using only one 5.1V (lets neglect the difference of 0.3V here) Zener? OK sure it is more expensive using nine 1N4148 but hey....from a pure technical point of few? Why not!?!?
I also can think of some single transistor based current sources that use Zener diodes as references but also use LEDs or standard
diodes which are forward biased.
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Offline gwideman

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Re: EEVblog #908 - Zener Diodes
« Reply #15 on: August 08, 2016, 01:14:10 am »
So why not use [...]  nine 1N4148 [in series, forward biased]

The extent to which a diode does not sharply turn on at a particular voltage (I vs V is not vertical at 0.6V say) is represented by its series resistance. (And that line being curved corresponds to "dynamic resistance".)

If you connect nine diodes in series you get nine times the series resistance, so nine times less vertical an I vs V line, so to speak.  So the regulation capability of nine 1N4148's in series at (say) 5.4V will be nine times poorer than a single 1N4148 at 0.6V. (As judged by deltaV/deltaI or reciprocal.)
 

Offline Tom45

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Re: EEVblog #908 - Zener Diodes
« Reply #16 on: August 08, 2016, 03:13:41 am »
In Australia I always heard and assumed it was ZENner probably because of the British way we say Zed not Zee. I shall endevour to say ZEEner from now on. Although I shall continue to accept ZEN-er quite calmly too.

I suppose then also I should always capitalise the name Zener. Something I am notoriously sloppy about. Ohms Amperes Hertz and so on.

Until Dave's video I had no idea there was a person named Zener. A name like that sounds like it could have been created by a marketing committee.

For those interested, you can read Clarence Zener's obituary at:
http://www.nytimes.com/1993/07/06/obituaries/clarence-m-zener-87-physicist-and-professor-at-carnegie-mellon.html
 

Offline b_force

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Re: EEVblog #908 - Zener Diodes
« Reply #17 on: August 08, 2016, 06:29:24 am »
Also, Dr. Zener pronounced his name ZEE ner, not ZEN ner.

In Australia I always heard and assumed it was ZENner probably because of the British way we say Zed not Zee. I shall endevour to say ZEEner from now on. Although I shall continue to accept ZEN-er quite calmly too.

I suppose then also I should always capitalise the name Zener. Something I am notoriously sloppy about. Ohms Amperes Hertz and so on.
Jeez, if we are going to fall over pronunciation, than we can make a huge list about how English speakers say things totally wrong and vice versa.
Voltage is for example all totally 'wrong' in English. :-//
In general there are a lot of original French words in English, but not pronounced on a French way.

On top of that you just have difference in accents, slang and dialect.
(I can tell you that 'Murican English is way different than Kiwi/Aussie and British, even with technical jargon)
« Last Edit: August 08, 2016, 06:47:33 am by b_force »
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Offline David Hess

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Re: EEVblog #908 - Zener Diodes
« Reply #18 on: August 08, 2016, 08:59:23 am »
Also, Dr. Zener pronounced his name ZEE ner, not ZEN ner.

In Australia I always heard and assumed it was ZENner probably because of the British way we say Zed not Zee. I shall endevour to say ZEEner from now on. Although I shall continue to accept ZEN-er quite calmly too.

Here in the US, I use ZED for Z in amateur radio call signs which gets a second look from people not used to it but ZEE is not quite as distinct.

For US watchers, Dave's videos have a lot of entertainment value just for his Australian English.
 

Offline Brumby

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Re: EEVblog #908 - Zener Diodes
« Reply #19 on: August 08, 2016, 09:02:29 am »
Here in the US, I use ZED for Z in amateur radio call signs which gets a second look from people not used to it but ZEE is not quite as distinct.
:-+

Quote
For US watchers, Dave's videos have a lot of entertainment value just for his Australian English.
I hadn't noticed.....  :D
 

Offline zapta

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Re: EEVblog #908 - Zener Diodes
« Reply #20 on: August 08, 2016, 02:04:55 pm »


Quote
For US watchers, Dave's videos have a lot of entertainment value just for his Australian English.
I hadn't noticed.....  :D

You got used to it.

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Offline Kleinstein

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Re: EEVblog #908 - Zener Diodes
« Reply #21 on: August 08, 2016, 07:39:25 pm »
Many normal diodes in series is not a good replacement. It's not so much the series resistor but the temperature dependence that is rather poor. However for rather low voltages like 2 V, something like 2 or 3 silicon diodes or an LED might be an alternative. Some of the low voltage zener diodes are actually forward biased diodes not real zener diodes.   
 

Offline g0hjq

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Re: EEVblog #908 - Zener Diodes
« Reply #22 on: August 08, 2016, 10:36:01 pm »
Thanks Dave,

It's great to see fundamentals Friday back. My favourite segment of your blog, and definitely the most educational.

 :-+
 

Offline VK3DRB

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Re: EEVblog #908 - Zener Diodes
« Reply #23 on: August 09, 2016, 01:14:39 pm »
An alternative protection input is using two schottky diodes (often in the one SOT-23 package for example). D1: anode at ground, cathode on signal line; D2: anode on signal line and cathode on PSU rail. The advantage is positive transients are clipped to 0.3V or so above rail and the negative transients are clipped to 0.3V or so below ground. And they are extremely fast in switching compared to  zeners which have a greater junction capacitance. Zener breakdown voltages are not that accurate at lower voltages in particular where as with two schottkies that is not a problem for low voltage supplies on some microcontrollers.

 

Offline rrinker

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Re: EEVblog #908 - Zener Diodes
« Reply #24 on: August 09, 2016, 01:26:56 pm »
 Interesting you should mention that, I JUST saw that same suggestion, pair of schottkeys in SOT-23 format soldered to the traces on an existing circuit (in this case, the secondary terminals of a current sense transformer) to clamp the voltage developed across the 1K resistor which is also across the secondary terminals under higher than expected current flow in the primary. As designed, it generally worked fine, but higher currents cause the resistor to greatly exceed its power capacity. As in 2 watts peak power in a 1/4W resistor. Oops. No real room to put a big sandbar power resistor, so someone came up with the schottkey clamping solution.

 

Offline timb

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Re: EEVblog #908 - Zener Diodes
« Reply #25 on: August 10, 2016, 12:26:38 am »
Actually, it's possible to use the Zener's junction capacitance to your advantage! All you need a fast diode in series with the Zener.

So, let's say you've got signal coming in that you need to clip (or limit) to 5Vpp. In this case, I'm using a 1MHz@10Vpp signal. We're using a 4.7V Zener.

Here's what the straight up Zener clipping circuit looks like:



Ouch, that GPIO.Vi waveform looks awful. However, because of the Zener capacitance, we expected that. So, how can we fix it? Add a diode:



Now look at that GPIO.Vi waveform, it looks *much* better! So, what exactly is going on here? Well, the first time the input goes over ~5V (4.7V Zener + 0.3V Diode), the Zener starts to conduct, then the square wave drops to 0V and Zener stops conducting, however it stays biased by the junction capacitance (that is, it holds the voltage up to just under the conduction point).

You can also use a single Zener (pre-biased from your VCC rail even) to protect multiple pins:



Either implementation works well and will allow the Zener to react within a nanosecond!



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Offline VK3DRB

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Re: EEVblog #908 - Zener Diodes
« Reply #26 on: August 10, 2016, 02:49:30 am »
Looks good, but you would not want to do that in some cases as you have added more parts to the circuit. More real estate. And more cost, albeit minor. The real estate can easily become an issue if you are protecting multiple lines, like in the case of an off-board data bus. In volume, a garden variety dual schottky like a BAT54 is roughly the same price as a single zener in a SOT-23 package. Depends largely on the application.

If EMI is an issue with off-board digital lines, sometimes simple series damper resistors can be used to lower the slew rate slightly, and often in so doing reduce overshoot and undershoot instead of using a zener or schottky pair.

What is the simulation package you are using on the iPad? Looks nice!
 

Offline David Hess

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Re: EEVblog #908 - Zener Diodes
« Reply #27 on: August 10, 2016, 06:11:31 am »
Another advantage of using schottky diode clamps is that power dissipation of the forward biased junction is less than that of the reverse biased zener diode making it more likely that the later will fail under overload conditions.  Neither has good leakage though making them unsuitable for high impedance circuits.
 

Offline iromero

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Re: EEVblog #908 - Zener Diodes
« Reply #28 on: August 10, 2016, 08:43:02 am »
The big drawback of using shottky clamps to the power rails is that if your power supply can't sink current, a big enough overload will cause the rail to soar, destroying everything attached to it. Timb's solution with a single clamp shared between several pins is a pretty elegant solution for moderately high overloads, pretty brilliant!
 

Offline VK3DRB

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Re: EEVblog #908 - Zener Diodes
« Reply #29 on: August 10, 2016, 09:36:50 am »
The big drawback of using shottky clamps to the power rails is that if your power supply can't sink current, a big enough overload will cause the rail to soar, destroying everything attached to it. Timb's solution with a single clamp shared between several pins is a pretty elegant solution for moderately high overloads, pretty brilliant!

If your power rail is low impedance and your are using low ESR caps, it should not be a big problem. However it depends too on what you are trying to clamp. The recent revision of IEC-61000-4-2 requires 8kV contact discharge (with a peak current discharge of 30A!) and 15kV air discharge to anything accessible on a product. And that includes any exposed pins and gaps in the plastic casing. It is brutal. A zener diode on a signal line won't stand a chance of protecting against an ESD rise time of 0.8ns. The mitigation is isolation, low ESR caps, TVS diodes or physical barriers etc. Finding the cause of such issues can be painful because you cannot physically see where the discharge paths are within some complicated instruments.
 

Online SilverSolder

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Re: EEVblog #908 - Zener Diodes
« Reply #30 on: August 10, 2016, 12:02:05 pm »
Thank you timb, great suggestion.  For ultimate performance, you could have decoupling capacitors to ground close to the high speed diodes - if the trace to the zener is long, it may impact the performance for fast rise time spikes due to inductance.
 

Offline NANDBlog

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Re: EEVblog #908 - Zener Diodes
« Reply #31 on: August 10, 2016, 12:08:31 pm »
Actually, it's possible to use the Zener's junction capacitance to your advantage! All you need a fast diode in series with the Zener.
Sorry, but no. If you want to protect a GPIO (or anything alike), than the best is to use a TVS diode. It is a zener, with all the tricks they can put into it. They make sub picofarad capacitance devices for high speed interfaces. In one tiny package. Why would you even make it from discrete parts? ICs are invented, even if you "only" integrate diodes (some are even the way you re-invented).
 

Offline timb

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EEVblog #908 - Zener Diodes
« Reply #32 on: August 10, 2016, 01:20:18 pm »
Actually, it's possible to use the Zener's junction capacitance to your advantage! All you need a fast diode in series with the Zener.
Sorry, but no. If you want to protect a GPIO (or anything alike), than the best is to use a TVS diode. It is a zener, with all the tricks they can put into it. They make sub picofarad capacitance devices for high speed interfaces. In one tiny package. Why would you even make it from discrete parts? ICs are invented, even if you "only" integrate diodes (some are even the way you re-invented).

A TVS is designed for ESD or other surge events. My solution is designed for clipping the input level on groups of GPIO (or general purpose digital and analog I/O) that might see repeated long duration over voltage events (like driving a 12Vpp clock into a 5Vpp input by accident). It has the side benefit of mitigating ESD.

Also, keep in mind that low voltage TVS diodes typically don't start clamping until around 7V to 20V, while my circuit can clamp at 5V or 3.3V (possibly lower, though I haven't tried it).

So yes, there is a reason you'd want to do it my way!

In addition, a similar arrangement can also be used to prevent damage from large ESD events and lighting strikes at potentially less cost (and with better protection) than putting a TVS diode on *every* pin. In fact, free_electron used just such a setup to protect the digital inputs of some telecom equipment!

« Last Edit: August 10, 2016, 11:41:10 pm by timb »
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Offline timb

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Re: EEVblog #908 - Zener Diodes
« Reply #33 on: August 10, 2016, 01:30:18 pm »
Thank you timb, great suggestion.  For ultimate performance, you could have decoupling capacitors to ground close to the high speed diodes - if the trace to the zener is long, it may impact the performance for fast rise time spikes due to inductance.

Thanks! It actually works quite well. What I ended up doing was staggering an additional Zener and cap for every 5 pins it protects. I agree that a small cap (100pF or 1nF) at each high speed diode could increase protection. You could also increase the protection resistor's value if needed; you'd want to calculate pin, diode and trace capacitance first and just make sure it didn't affect your signal.

I wish I could take credit for coming up with the idea, but I'm sure others have done it long before I thought of it. (I've come across the concept several times since, as you can see by the page from the ON Semi app note I posted earlier and in free_electron's circuit above.)
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Offline VK3DRB

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Re: EEVblog #908 - Zener Diodes
« Reply #34 on: August 10, 2016, 11:14:13 pm »
Your statement about no other connection between system ground and safety ground other than ferrite L2 is important. Creepage and clearance must also be considered between the system and safety ground. If an ESD strike hits the system ground but not earth, it may find an alternative path to frame ground due to the presence of ferrite L2 if these clearances are not maintained.

It would be interesting to know how petrol bowsers assure there is no electrostatic discharge from a car to the fuel nozzle as the fuel pump is connected, igniting the fumes. You cannot guarantee insulation of the metal nozzle, and earthing of the nozzle would be bad. I don't know how it is done.
 

Offline David Hess

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Re: EEVblog #908 - Zener Diodes
« Reply #35 on: August 11, 2016, 12:05:43 am »
The big drawback of using shottky clamps to the power rails is that if your power supply can't sink current, a big enough overload will cause the rail to soar, destroying everything attached to it. Timb's solution with a single clamp shared between several pins is a pretty elegant solution for moderately high overloads, pretty brilliant!

I have never actually encountered damage beyond the diode clamp itself although I have seen a couple of damaged diode clamps.  I have seen far more shorted zener diodes and a few shorted transient surge suppressors which were used where an SCR crowbar would have been more appropriate; of course the shorted transient surge suppressors did do their job by ... shorting ... and maybe that was the intention; they acted as reverse fuses by permanently closing.

If the impedance of the power supply rail is going to be too high which would be pretty unusual, then a separate rail for the clamp can be made with a transistor which will sink current to ground.  I have occasionally run across this in old instrumentation where the clamp voltage differed from ground or the supply voltage.
 

Offline GK

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Re: EEVblog #908 - Zener Diodes
« Reply #36 on: August 11, 2016, 05:17:50 am »

If the impedance of the power supply rail is going to be too high which would be pretty unusual,


Not that unusual. If the current injected into the supply rail via the protection diode exceeds the current drawn by the circuitry powered from said rail then the supply voltage will typically/likely rise. A typical linear regulator, for example, will not sink the additional current and thus maintain regulation of the rail voltage if it is not designed to operate in both quadrants. Consider a positive regulator with an internal NPN emitter-follower series pass device. It can only source current, not sink it, so it will have no control over the rail potential under such a fault condition.

Bulk bypass/filter capacitance on the rail will only protect against surges, but are effectively open circuit to DC. So imagine that you accidentally connect your protected (diode-clamped to the supply rail) signal input to an external source of DC power of higher voltage.

So much for that protection scheme then. I've lost count of the amount of crap I've repaired over the years that was killed in part by ill-considered diode-to-the-rail "protection".
« Last Edit: August 11, 2016, 05:27:06 am by GK »
Bzzzzt. No longer care, over this forum shit.........ZZzzzzzzzzzzzzzzz
 

Online thm_w

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Re: EEVblog #908 - Zener Diodes
« Reply #37 on: August 11, 2016, 07:06:43 pm »
It would be interesting to know how petrol bowsers assure there is no electrostatic discharge from a car to the fuel nozzle as the fuel pump is connected, igniting the fumes. You cannot guarantee insulation of the metal nozzle, and earthing of the nozzle would be bad. I don't know how it is done.

The nozzle is earthed in all cases, unless its a drum or portable can:
https://www.reddit.com/r/askscience/comments/1yqbxw/why_dont_you_have_to_ground_cars_before_fueling/
 

Offline Brumby

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Re: EEVblog #908 - Zener Diodes
« Reply #38 on: August 12, 2016, 01:13:38 am »
Also, in regards to fuel trucks refilling service stations - I have made a direct observation of an LPG tanker doing just that.

Before any fuel lines are touched, a separate grounding wire is run from the truck to the tank installation at the servo.  It stays there during the whole process.
 

Offline David Hess

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Re: EEVblog #908 - Zener Diodes
« Reply #39 on: August 13, 2016, 01:34:06 am »

If the impedance of the power supply rail is going to be too high which would be pretty unusual,

Not that unusual. If the current injected into the supply rail via the protection diode exceeds the current drawn by the circuitry powered from said rail then the supply voltage will typically/likely rise. A typical linear regulator, for example, will not sink the additional current and thus maintain regulation of the rail voltage if it is not designed to operate in both quadrants. Consider a positive regulator with an internal NPN emitter-follower series pass device. It can only source current, not sink it, so it will have no control over the rail potential under such a fault condition.

Bulk bypass/filter capacitance on the rail will only protect against surges, but are effectively open circuit to DC. So imagine that you accidentally connect your protected (diode-clamped to the supply rail) signal input to an external source of DC power of higher voltage.

So much for that protection scheme then. I've lost count of the amount of crap I've repaired over the years that was killed in part by ill-considered diode-to-the-rail "protection".

For external interfaces, there should always be a series resistance to limit the current if shunt protection is used.  Otherwise the shunt protection potentially lowers reliability.

Oddly enough, many of the older equipment designs I have studied have power supplies which will handle the situation you describe just fine; they include SCR crowbars on the various supply rails including the logic rail.
 

Offline UncleSam

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Re: EEVblog #908 - Zener Diodes
« Reply #40 on: August 14, 2016, 06:58:57 pm »
Nice video, Dave.

On the circuit showing a zener diode regulating the input voltage to an IC.....wouldn't it make more sense to put the resistor in series with the zener diode, but in such a way that the conducting path to the IC DOES NOT PASS  through the resistor?
 

Offline Fungus

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Re: EEVblog #908 - Zener Diodes
« Reply #41 on: August 14, 2016, 07:07:00 pm »
Nice video, Dave.

On the circuit showing a zener diode regulating the input voltage to an IC.....wouldn't it make more sense to put the resistor in series with the zener diode, but in such a way that the conducting path to the IC DOES NOT PASS  through the resistor?
No.

The idea of the Zener is to protect the IC. The path to the IC must pass through the resistor or the Zener won't be able to do its job.
 

Offline SeanB

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Re: EEVblog #908 - Zener Diodes
« Reply #42 on: August 14, 2016, 07:38:56 pm »
Also, in regards to fuel trucks refilling service stations - I have made a direct observation of an LPG tanker doing just that.

Before any fuel lines are touched, a separate grounding wire is run from the truck to the tank installation at the servo.  It stays there during the whole process.

If you look carefully at fuel tankers you will see as well on many of them a chain, or a carbon fibre bundle, that is connected to the chassis and touches the ground. this reduces the static charge on the vehicle relative to the ground. They still have the stainless steel cable ( a lot more wear resistant than a copper wire, and the little bit of extra resistance is not a worry here) with the phosphor bronze grounding clamp or pin, which is either clamped to a post inside the refuelling post or to the metal surround, before connecting the hose, and which is removed only after the hose is removed.

For cars the hose is conductive, but has a high resistance, so providing current limiting in the construction of a large carbon composition resistor. The inner braid is grounded though, and the entire pump housing is metal, and all parts have a grounding strap or wire, even the panels that cover the control switches, so that they all are bonded to the filling station ground mat.
 

Offline dwayne

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Re: EEVblog #908 - Zener Diodes
« Reply #43 on: August 17, 2016, 11:26:15 pm »
Had a friend who called his car the Zener, since when it broke down it was avalanche breakdown ...
 


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