Zener Barriers

Try Wikipedia: Zener Diode

Zener barrier is always placed in non hazardous area. The supply to barrier may be as per specifications but need not be very controlled one.

Zener barrier limits the current / voltage / wattage passing to hazardous area to safe limits. Even if something fails, like, if the input power to zener barrier fails from its normal supply voltage of say 24 V DC and supply voltage becomes 230 V, the fuse and zeners lmit the current/ voltage/wattage going to hazardous area to safe limit.

The barriers may be designed for single fault condition or double fault condition etc, so that even if one of the zener fails, other one limits the i, V, w

Read this article

http://www.iaei.org/subscriber/magazine/02_f/babiarz.htm

Rgds

Roland

Thanks Roland. The website was very informative

Thanks Roland:-

For other casual readers this is the first picture from the article referenced:-

Congratulations Randall, you can cut and paste !! Once again well done, you deserve your "Guru" title.

However, sorry no GA for you though.

I had no idea what a Zener barrier was, on a normal sort of day I would have read the thread, but, would not have followed the link and read enough to get a rough idea what it was all about. Today I had a little more time, and, now anyone curious can glance through the thread and get a quick idea.

This diagram is to my mind wrong, the resistor is on the wrong side of the Zeners. I am 99% sure that it should look like this.

In reality, in the original diagram, the Zeners would both blow as soon as power is applied, as the voltage dropping resistor could not drop the voltage correctely for the attached load device.

Remember that basically for any voltage above the set voltage, the zener is a short circuit.......

Hmmmm I guess I still don't know what a zener barrier is. I sort of thought that in the original picture I copied you would use a Zener of slightly higher voltage than the "working" (both input and hazardous area) voltage: that way the Zeners are just their as protection devices.

Now: I'll have to go and read more of the article, I suppose.

Andy, if you look at the diagram (even though it is drawn backwards from normal conventions), the "safe area" is shown to be at the left end of the original diagram, and the current limiting resistor is in the proper position in the circuit.

If it had been labeled differently, I might have agreed, but I have looked at it again and the input voltage is from the left hand side and the device being fed is on the right hand side. The fuse is on the left, if the zener is "meaty" enough, it will blow the fuse instantly.....

I stay with my correction. It will not work as shown....

You might like to look at this diagram, I am certain that it supports exactly what I wrote:-

I love such discussions!!

I concur. I hastily looked at the original circuit, but I failed to notice that the left side was labeled, "Input Voltage," and instead looked at only the label, "Safe Area."

If the source is capable of supplying enough current, then the zeners will fail miserably.

Maybe someday I will learn to pay more attention before I speak (write). On the other hand, I'm getting old, so I'm probably wrong about that too . . .

Hey, no problem, you weren't rude or anything, I enjoyed the discussion......have a great day.

This is another cut and paste from the article linked by Roland at post #3

"The fuse will blow when the diode conducts."

Because the outside source can include lightening or arcing that may deliver high currents from Voltages that would be little attennuated by any reasonable resistor, the voltage across the Zeners* is expected to increase beyond what is allowable within the "hazardous" (or protected) area. The resistor is in the position originally shown to reduce the energy that the overvoltage can deliver into the protected area. But you will often find a varistor used as a first limiter, with a resistor between it and a Zener on the protected side of the resistor.

P.S. And yes, I have read the additional notes - they are good for normal operation of Zeners to provide a fixed output Voltage, but protection against surges is a somewhat different requirement

*Though these days it's more common to use metal-oxide varistors for first-line protection, because they can absorb higher energy/volume before dying

I think the diagram is OK as is.

The input voltage is presumed to be regulated to 24 vdc, so that under normal conditions the zeners never come into play. The resistor limits current on the output side (the hazardous area) to no more than the regulatory maximum. If there is a fault or spike from the input side, the zeners conduct, blowing the fuse if the fault duration is long enough. The zeners are in parallel for redundancy: either one can cause the fuse to blow. Positioning the resistor as you have would defeat the protection, by making the fuse less likely to blow in an overvoltage condition.

Not shown is the power supply itself, which is also fuse-protected on its input side.

Agreed that the resistor is needed on the hazardous/protected side - and not only under input fault conditions, as even the current from 24-Volts needs to be limited to prevent overheating in the event of a short circuit.

But inhibiting current through the fuse is not be a problem, and the majority of practical safety barriers not only have two (resistor-separated) Zener/MOV stages, but also place a small resistor between the safe-side Zener and the energy source; I have been led to understand that its main purpose is to allow the equipments to continue working in the event of modest external faults, but I don't know this from personal analysis or experience.

From here-on in it only gets worse. My experience here is about 40 years outdated, but deep mines did not allow diecast enclosures, and any plating on iron tools or boxes was strictly controlled - this to minimise sparking should you drop anything.

I hate to say this, but it would appear that Zeners are not your strong point.......

You only need an over voltage for a split second and BOTH Zeners will burn.

Remember a high power zener can only handle around 5 watts (that was the highest power Zener that I could find with a quick search on the internet, I personally have only used Zeners with a few hundred milliwatts....!).....at 24 volts, that is a little over 200mA..........even a few AAA cells connected to make 24 volts can supply more amps than that.......

I will try and explain how a zener is most often used for you:-

A resistor is calculated that will drop the supply voltage to almost the voltage needed, the zener of the correct voltage is connected to allow it to drop the voltage (as in both the diagrams I supplied), by drawing a small current, within its capabilities, through the dropping resistor to regulate the voltage more precisely......

Its a cheap way to stabilize a voltage, though very inefficient at the same time......the extra power is just wasted as heat in the resistor....

It is possible to "amplify" its effect with a power transistor, but to save the transistor, you must still have a series resistor of similar in circuit. Its only really Ohms law!!

I hope this helps, but if not use the link I placed in my last post, that should explain it well enough for anybody.....

This has to be one of the best put downs I have read. I bet Andy has a pain in his head with the slapping you just gave him. A great response, its a pity I cannot give you a GA.

Having worked in this area for 15years and more I have to conclude that Andy has gotten his hands on a very bad book - he is making absolutely no sense (apart from to himself). In addition it is clear that Andy has no practical experience in this area.

Rgds

Mark

Again, use the spray under both arms and between the legs:-

Andy perhaps if you spent more time studying Zener Barrier technology instead of trying to defend your limited (read zero) knowledge on this subject then perhaps I might be able to assist your educational needs - because you need education in a big way.

You are clearly wrong and its just a pity that you don't admit it instead of attaching stupid pictures to your response.

Regards

Mark

Roland tried to access your link to the iaei. org article on zener barriers but unfortunately drew a blank can you please help.

You will see that I am a very new member but hungry for knowledge.

Regards,Mikee .........

I agree with Bill, Blink, and Physicist. Sorry, Andy, you've been outvoted.

Its not the first time that I have been outvoted! Probably not the last either....

I still do not believe that this circuit is "safe"......Zeners are simply too sensitive when "over driven" and a fuse takes a finite time to blow, even "fastblo", once the zeners are blown, the full voltage would appear at the so called safe area.

If I was designing such a circuit, I would have an SCR/Triac as a "Crowbar" circuit, triggered by a zener, to make sure 150% sure that the fuse would blow and no over voltage could get to the load!!!!

Its maybe a bit old fashioned, but the temperature inertia of the Crowbar is (should I say can be if correctly designed) far, far longer even than even "Slowblo" fuses.......

Then there is no need to have the resistor in where I put it before, the circuit would appear more like the original design. It would make the Crowbar action faster and the resistor would not be stressed in any way at all.....

I am not saying that the original circuit will not work under "perfect" conditions, it might. But I never trust items made by humans to be perfect and the health risks could be substantial if something unlucky happens.....

I have to admit that I have always been an "overly" safe person in most things I do, especially where human life comes into question. I am also used to being "alone" iwith regard to this point.....

Being out voted by more of my peers does not make me necessarily wrong! Been there, done that and got the T-Shirt! I will defer to the vote of course.....

You Guys have won the battle, but not the war!!!!

Have a great day in spite of me.....

"Overly safe" in one area sometimes leaves you open to creating new risks in others; in this case...

As non- function of intrinsically safe circuits can have its own safety implications, it would be unwise to use a barrier structure that remained non-functional following a temporary disturbance; that means that crowbars are not such a good idea.

The design of the barrier will depend both on the electrical environment on the "safe" side and on the sensitivity (zone classification) on the protected side. Typically, this type of barrier only carries 20-mA operational current, so it is relatively easy to protect a 100Watt average Zener (such as BZV61) with a series resistor - if designing "ad novem", I would be inclined to use a PTC, probably with an electronic series switch to allow fast recovery, and with the fuse acting only as a back-up for extreme events.

That said, there are proven assemblies that are suitable for most environments (along the lines shown in my earlier link), and I would hesitate to reinvent the wheel - but being overcautious I would be inclined to check that the circuit looks as if it would do what is required under expected fault conditions.

You wrote:-

- but being overcautious I would be inclined to check that the circuit looks as if it would do what is required under expected fault conditions.

My personal take is this:-

- but being overcautious I would be inclined to check that the circuit looks as if it would do what is required under all expected and unexpected fault conditions.

I am EXTREMELY careful, that was how I was taught in the RN Schools. Its too late for me to change now.....

I repeat - excess caution based on an unproved risk which causes you to use different methods from those already established can lead to undesired consequences in related areas. It seems to me that you have completely overlooked the analysis of your suggestion - that it would have placed workers at unnecessary risk by extending periods where safety-critical equipment was non-functional.

And don't expect to out-think practical experience without first examining the evidence (another naval nostrum, I believe*). In particular, this is an area where accident investigation and litigation are rife, and even minor incidents will have been extensively investigated, and (where the cause is uncertain) specifications gold-plated to cover any conceived cause (unless it has not been demonstrated innocent); so adding your own constraints and/or ignoring accepted design practice is liable to result in increased cost at best, and more likely additional hazards.

*If we were to design all naval systems on the basis you apparently propose, warships would probably topple over before leaving the dockyard....
Perhaps I am pulling rank here - but... it's never too late to change.

I think that any warship you designed does stand a strong chance of toppling, mine has no chance of such a simple failure......

You appear to try and to misunderstand everything - to your own advantage of course.

In the process of which you may blind some other simple minds with your comments, but anyone who REALLY understands the problems is not fooled.

I am not fooled......

I was trying to use reductio ad absurdam - ie. assuming that equipment was designed for use in a pre-designed warship on the same basis that you were redesigning the intrinsically safe barrier. My expression of this was evidently clumsy - but it was not intended as a generalised personal insult.

With regards to me "trying to misunderstand" (actually, not my intention, but it does happen), why don't you address my issues, and so help others to really understand the problems as you do?
Specifically:
why your suggestion of foldback protection will not cause problems more serious than any that it might solve; and
why you apparently continue to insist that it is cautious to depart from practice which has been reviewed after every remotely relevant adverse event.
_______________________________________
"A design can be considered 'proven' when the last example has been removed from service without having caused problems."

Give it a rest Andy, you clearly have no idea what you are on about and your misinformation could get some gullible fool into trouble.

Ben

Maybe this will help you, spray under both arms and between the legs.

Stand up on your hind legs and take a name, instead of hiding behind jest guest.

I used to have to intergrate Z barriers into elect systems. Go to www.gemssensors.com, then "products" and look under "solid state and zenner barriers" they have a very good line of these componets and associated information showing wiring schemitics and explaining theory.

Typical Wiring Diagram...from Gems Sensors

Postive dual-channel Zener Barrier with floating leads.

Typical Application Examples

Sensors or Switches may be any non-voltage-producing device. Typical are: flow and level switches, temperature switches (thermostats), pressure switches or passive resistive transducers or transmitters. Below are typical examples.

Example 1: P/N 54805 in a continuous liquid level monitoring system. Note: Terminals 3, 4, 5 and 6 are common and are bonded to the mounting tabs for positive redundant grounding.

Example 2: P/N 54805 in process control system.

To Determine Loop Resistance:

R Loop =

; R Loop = R (supply barrier) + R (return barrier) + R (monitoring barrier)

* must be less than 28 VDC (30 Volt Barriers)

I read through the threads, you ladies get nasty!!

Well here are the real world facts without the back bitting & hair pulling.

That's remarkably similar to Omega's barrier (linked directly in post 17...)
But are you sure that you managed to avoid back-biting?

That makes far more sense than what was previously posted by others. Many thanks.