Zener Diode

Look at a data sheet.

What are you trying to do with this Zener. If you exceed it's max power rating, you will release smoke.

Since you are asking about maximum supply voltage, you either are trying to make a maximum voltage clamp, or a surge arrestor.

Typically you need a current limit resistor, to prevent exceeding the maximum power rating of this.

Spec. say's this has minimum zener voltage of 11.4V, and maximum of 12.7V.

20watts (heat sink mount) @75 deg C.

First post that understands how a zener is used... GA

You usually put a resistor in series with a Zener diode to limit current. The Zener diode is actually a highly doped diode that breaks down in the reverse direction at a specific voltage, and this voltage is used as a reference voltage in a circuit.

A zener diode needs to be part of a schematic. It keeps the voltage over the diode in the range of the specs.

The dissipation (power) is important. Let's say your zener is 12 Watts (this is not verified for your example but to calculate easy only) then the max current through the diode can be 1 Ampere DC.

The max supply voltage is without limits but depends on the value of a series resistor that needs to dissipate the power generated at that current. As long as the current remains within the limit.

Because I am lazy today as example you can apply 24 Volts when you put a 12 Ohms resistor in series that can dissipate 12 Watts of heat.

Your voltage reference however will not perform well. The idea is to use the zener as a reference where the current will be split between you load in parallel on the diode and the zener itself.

The maximum current through the zener is its maximum dissipation wattage divided by its operating voltage. The resistor upstream must be selected so that the current is no more than this value, bearing in mind the difference between the supply voltage and the zener voltage; ideally it wants to be a lot less subject to design in the circuit downstream of the connection onto the zener.

This question just makes me cringe.

One should never put any zener diode forward or reverse biased across any supply voltage. This is a waste of time, money and sometimes silicon because the diode either never conducts or conducts only once before damaging either itself or the supply. (If your lucky the protection circuitry in the supply works.) To properly use a zener diode there must be some current limiting component in series with the diode. There are so very many zener diode example circuits and tutorials on the web.

The reason I am asking this question is we have Active Level Crossing LED Flashing lights from a reputed company which has this zener. Now the circuit diagram in the flashing light asks us to supply 24 volts to the zener which is a 12 Volts zener which then is connected to the LED panel. Now our belief is that if 24 Volts has to be supplied to the zener then the zener BZY93C24R should be used which is a 24 Volts zener. There is no resistor in series with the zener which makes us believe is what 'frying up' the zener and the lights stop flashing. Which becomes a safety issue!

Are we correct in our belief?

Thanks for your responses.

As you describe the circuit to us, this is a bad design. Even with a 24V zener diode in place. This is so bad of a design that I doubt what you describe to be what a company intended to be used. I suspect that a power resistor is expected in the design between your 24V supply and this power zener. Somewhere this information got lost in translation to you.

This is the circuit design the unit has. Not sure if the resistor does anything there or for that matter the zener itself may not be required.

The LED is likely a LED package that includes something (resistor) to limit the current. More than likely the zener is there to drop the 24V to an acceptable 12V for the LED package.

This is not the circuit you described and makes more sense. How much current is that LED package rated to handle? A twenty watt, one ampere zener diode now looks like overkill, now.

Thats right, it is a LED package and you are right in saying that it pulls down the voltage to 12 Volts from 24 volts. The LED package requires 10 V @1 Amp as it is a 10 Watts LED package. But then why should the zener fail. Unless something in the circuit is of lower quality.

I understand things can and will fail but if more than one equipment from a reputed brand start failing at the same time then there is a problem and in this case it becomes a safety issue.

Now we are getting somewhere but possibly no where you will like. First, when you have a problem and you do not understand what is happening explain everything you do understand. This will save everyone time. One of the CR4 FAQ states this much more eloquently than I can before my morning coffee. Obviously nobody reads the FAQ.

I'm back to this being a poor use of an expensive power zener. If this was my circuit, I would design a small DC-DC buck converter supply with an input reverse polarity blocking diode to produce the 10V 1A that this LED package requests. This will draw about 11 watts from the 24V [unregulated?] supply. My next choice would be a linear regulator but it will not be close to efficient. Putting anything in series to linearly step down the voltage will always mean that 24W will be drawn from the supply with 10W used by the lamp and 14W by the series component. The simple fix is to replace the zener with a $4, 15 ohm 25W power resistor.

To get back to your implied question, you have two non-linear power devices wired in series; the zener and the multiple(?) LED consuming 10W. [Presumably other parts exist in the LED package, too.] The voltage drop across a non-linear device will change with their junction temperature. Additionally any 60/50 Hz noise pick up will put surge currents into these non-linear devices. I suspect these ripple pickups are what damages the zener but there is so much still unknown here and I am out of time here.

If the leds are in parallel (like a ladder), one shorted led (or connection to it) will pull enough current to blow the Zener. The same will happen with a series (chain) of leds if you manage to short the two ends of the chain, which can easily happen when connecting everything up in the field.

Either a current limiting resistor needs to cope with this extreme (in which case it is now too big for normal operation) OR use a fuse.

If shorting is likely to happen alot, you can limit the current via a current limiter (configure a votlage regulator, op-amp or BJT OR get an off-the-shelf solution, e.g. dc-dc converter / led driver).

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"Life Is A Process Of Illumination." jb

The zener voltage is approximately 12 volts+/-, rated 1 amp (with adequate cooling)

You state the LED lamp is 10V, 1 amp. At 12 volt what does it consume?

The Zener is marginally rated , since you are using it at 100% of its rating under ideal conditions. If I were to use a zener for this application I would prefer a 2 amp, 14 volt rating. For long life only use 50% of the device power rating.

You would be better off using a 12 volt power supply. A simple "wall wart" would probably work fine.

The arrangement you sho is not very informative. Possibly, the zener diode is rated to suit the LED. What is the LED rating?

If the 24v and 0v terminals are indeed supply, the variable resistor appears to be across the and It isn't clear what function it serves.

More information would be useful. What does the resistor control? Is the LED merely a power indicator?

I think its a mistake, because a LED does not oppose current flow much when the applied voltage is well above the voltage needed to just light the LED.

Which is why in reality, they always need some form of resistance to current flow, a simple resistor is often used to be safe....I have never seen a LED, with nothing in series to drop voltage....

But the LED in question is a 10 volt LED according to the OP (series resistor built in I suspect), which means as long as the voltage is at a max of 10 volts DC or even slightly less, all is great, LED lights....

But if the applied voltage is significantly above the 10 volts, 12 volts for example as here, a far higher current will flow, probably over and above the maximum dissipation for the Zener I suspect. e.g. Zener burns out. Maybe not immediately....

That is my personal take with the limited infos supplied.

The other resistor is most likely an MOV(metal oxide varistor) surge /overvoltage suppressor

Did you ask the manufacturer? Do that first!

1) Instantaneous Maximum Voltage is defined for peak charge and duration - may be 15kV for 1us but ask the manufacturer as it is for ESD protection purpose only.

2) Forward voltage is similar to that of diode so look for current and saturation voltage of the junction which may limit to 1V or of that order

3) Reverse breakdown voltage is what you have and you have to limit the maximum current through some resistor and watch out for ESD damage limit as that may not be repetitive.

I think this much information is good enough free advise.