Tried searching it up on the internet and not too much came up on my specific question :/ Anyhow does a Zener Diode reset when the power is removed like a Polyfuse or does it permanently conduct like... well a normal fuse?

Thanks!

 

Uhm.. what? A zener diode does not need to reset, it is not a thyristor or anything like that.It has no memory, and the relation between voltage ad current is just a simple curve.
Imagine a standard diode instead of the zener, just connected backwards so the current can flow. Does it need any kind of resetting?

 

As long as you have not physically damaged the device by excessive voltage, current, heat, then there is no "reset" involved. If you apply a certain voltage across it, you will get the corresponding current through it (at a given temperature) and, similarly, if you put a certain current through it you will get the corresponding voltage across it (again, at a given temperature).

 

Uhm.. what? A zener diode does not need to reset, it is not a thyristor or anything like that.It has no memory, and the relation between voltage ad current is just a simple curve.
Imagine a standard diode instead of the zener, just connected backwards so the current can flow. Does it need any kind of resetting?

Here let me elaborate a little more as I didn't in the original question. I'm building a circuit where I'm using the zener diode as a overvoltage protection sorta thing. (Zener is placed so that when the voltage goes past its threshold it shorts to ground). I'm not sure if the diode would conduct normally again after it goes past its "Zener voltage"

 

Here's a small diagram of my circuit for those who are more visual. I do believe this is how you do overvoltage protection using Zener diodes.

Diagram Link

 

The design is not appropriate for overvoltage protection. What you probably looking for is a crowbar circuit to short the supply when an over-voltage occurs. The circuit you have will likely over-load the zener during over-voltage.

 

Here's a small diagram of my circuit for those who are more visual. I do believe this is how you do overvoltage protection using Zener diodes.

Diagram Link

Nope, not unless the source of the input signals has a high output impedance. As you have shown it, the expectation is that the zener diodes will clamp their signal lines at the zener voltage, but this requires that they sink whatever current is needed in order to make this happen. In general, zener diodes are not intended for high currents (there are exceptions, but they tend to be much more expensive). So if you were just a small amount over the zener voltage with an input signal you might get hundreds of milliamps or perhaps even several amps of current through the diode (and, at that current, the clamping voltage would be elevated quite a bit, too) that will likely destroy the device quickly, usually by letting the magic smoke out resulting it in acting like an open circuit from that point on.

 

Please upload your circuit to the forum next time, thank you.

 

Here's a small diagram of my circuit for those who are more visual. I do believe this is how you do overvoltage protection using Zener diodes.

What you want to use are TVS diodes. Like high power zeners, they have a tolerance that you need to consider.

Specs for a 5V TVS, note that the minimum breakdown voltage is 6V:


If you require more precise voltage protection, you may want to design crowbar circuits.

 

Zener diodes do not suddenly short to ground. They simply start conducting lots of current while holding the voltage relatively steady.
If you do not include a current limiting resistor you are likely going to blow the zener diode. Then there is no "reset" because it's dead as a door nail.

 

Please upload your circuit to the forum next time, thank you.

View attachment 124995

Is there a way to upload an image directly to a forum post on mobile? I tried using the img tag but it requires a link from an external site. I'll see if I can do it a different way next time, Apologies.

 

The design is not appropriate for overvoltage protection. What you probably looking for is a crowbar circuit to short the supply when an over-voltage occurs. The circuit you have will likely over-load the zener during over-voltage.

Although a crowbar circuit is probabally the best option for overvoltage, building 3 of those for each line of the power supply is a lot of effort so I may just leave the line unprotected altogether. I appreciate the input everyone gave on me blowing up Zener diodes left and right with my terrible method, but I am still learning

 

Is there a way to upload an image directly to a forum post on mobile? I tried using the img tag but it requires a link from an external site. I'll see if I can do it a different way next time, Apologies.

I don't know about from a mobile device specifically, but look at the bottom of the Reply dialog and (on a desktop machine) you will see a link to Upload a File.

 

Although a crowbar circuit is probabally the best option for overvoltage, building 3 of those for each line of the power supply is a lot of effort so I may just leave the line unprotected altogether.

What is the nature of the power source that creates a need for over voltage protection?

 

Is there a way to upload an image directly to a forum post on mobile? I tried using the img tag but it requires a link from an external site. I'll see if I can do it a different way next time, Apologies.

WBahn already explained for you how to upload the file, if you want yo upload the files from cellphone, you should find a compress app for your phone system, before you upload it then you can compress them to a clear resolution as 800x600 or 1024x768 jpg.

 

But please try to avoid uploading files that are more then 400 to 600 pixels wide unless you really need the detail. Usually 300 to 500 works very nicely.

 

With what little I know about Zeners, I like to think of them like a glass of water. The bigger the glass the more water it holds. However, when you overfill the glass the excess spills. Think of the water as voltage. The glass (zener) can hold so much back. But when you exceed the rim of the glass (the zener breakdown voltage) the excess is wasted.

Zener's don't reset. As long as you don't blow them up they do what they're suppose to do. Excess voltage is shunted to ground. When the voltage drops below the zener threshold it no longer conducts. So (in theory) a 12 volt zener will not conduct to ground unless the incoming voltage exceeds 12 volts. Lets assume your supply is pushing 12.5 volts. The 12 volt zener will conduct 0.5 volts to ground leaving the 12 volts alone.

If you're familiar with pressure safety valves, the valve does not open until the set pressure is exceeded. Once it is the excess pressure is released. When the pressure drops back within the set limit the valve closes.

A zener is a dam. When water in the dam exceeds the top of the dam the excess is wasted. Think of California with that dam that was over-topped. Excess water went down the spillway. However, so much water went over that its ability to handle that was damaged. So exceeding the zener limit is - um - limited. Go too far (high or too much power) and you'll blow the zener.

 

So (in theory) a 12 volt zener will not conduct to ground unless the incoming voltage exceeds 12 volts. Lets assume your supply is pushing 12.5 volts. The 12 volt zener will conduct 0.5 volts to ground leaving the 12 volts alone.

That is misleading to a newcomer. The zener diode does not conduct 0.5V to ground.
The zener is going to start conducting until the voltage across the zener diode stabilizes to the zener voltage, which in this case is 12V. The excess 0.5V must be taken up by any external resistance in the circuit.

We have to understand that there is a difference between an ideal zener diode and a real world zener diode.

Let us assume for the moment that this is an ideal zener diode with a zener voltage of 12V.

Now we connect this across a car battery that is at 12.5V. The zener diode will start to conduct and attempt to pull the voltage down to 12V.
Bam! The zener is blown because the internal resistance and the total circuit resistance is almost zero ohms. The current through the zener diode is in excess of 100A and cannot handle that.

A real world zener does not have a sharp turn on voltage. That is, the resistance doesn't suddenly switch from infinity to zero just after 12V. Hence the current will gradually increase as the voltage climbs up beyond 12V. Let us assume this 12V zener diode is rated to handle 1A. This means that with an excess 0.5V @1A, the zener might survive if the total circuit resistance is 0.5Ω or greater. If the total circuit resistance is less than 0.5Ω then bam! your zener is fried.

In order to protect the zener diode you have to insert resistance in series in the circuit to limit the maximum current that the zener diode will ever have to conduct.

 

With what little I know about Zeners,

That is misleading to a newcomer.

As I said; "With what little I know about Zener's - - -" . Thanks for teaching me something on this subject. I suspected there would be a need for a resistor, and I hope I didn't take anyone down a road of smoke. Thanks Mr. Chips.

 

I suspect the TS is thinking of a circuit that does the same as the circuit below.

Crowbar Circuits