Maximum voltage through zener diode

[Dannyx]

Good day folks. This might be a silly question, but how much voltage can I pump through a zener diode ? I know the CURRENT should stay below a safe margin to avoid blowing the diode, but what about the voltage ? In the traditional regulator setup where I have a resistor in series with the load and the zener, provided the resistor is large enough to avoid excessive current, is there a maximum "input" voltage to such a circuit ? Again, please excuse me if this is a basic building-block question which I should be familiar with, but hey....the more you learn......

[goontron]

Re: Maximum voltage through zener diode

The power through anything is P=V*I.

Say you have a 10w 1.8v zener diode broken down. To dissipate 10w you would be sinking 2 amps at 5v. Sinking 2v would be 5 amps. sinking 10v would be 1amp. sinking .2amps would be 50v. at 110v you would sink .09 amps for 10w.


Its linear.

Expanding into the equation:

P=V*I, so we have 10=5*I. we need to solve for I. Through some simple high school math: P/V=I, so 10/5=2

So its P/V=I for current, V*I=P for watts, and P/I=V For volts.

For a functional example:

Say we need the amperage of a conventional 150w light bulb. 120v, at 150w gives us 150/120=1.25. 1.25 amps for said bulb. We can now plug said numbers into the other functions to check our work.

150/120=1.25 (Amps)
120*1.25=150 (Watts)
150/1.25=120 (Volts)


Larger numbers are the same as well. Say, 2Kv at 1.5Kw.
1500/2000=0.75 (A)
2000*0.75=1500(W)
1500/0.75=2000(V)

This is basic stuff, yes. Another rule to live by is the 90% rule. When rating non-precision components like resistors and the like, add an extra 10% to the expected max rating, and size accordingly. Say you have a resistor that you want to sink 3/4Watt into. Rate it for 1w. If you have a 10v circuit you plan to add a capacitor to, use a 16v capacitor. If you are making a 9 amp switching circuit use a 10 amp FET. Etc...

This knowledge is holdover from the days of gone with tube radios and whatnot, so that 90% rule may be outdated.

[Dannyx]

Re: Maximum voltage through zener diode

Thanks for the detailed explanation. I was familiar with everything you wrote, but the math usually kills me so I'm glad you added some examples.

[Dannyx]

Re: Maximum voltage through zener diode

To take it one step further and put it into practice (not literally please because someone will get hurt ): if I have 230v from the wall socket, turn that to DC and stuff it into a 5w zener, I'd only be able to draw a maximum of 0.02a from said circuit, correct ? Don't know of what use that would be.....lighting an LED perhaps ?

[budm]

Re: Maximum voltage through zener diode

You do pump the Voltage, you forcing the current through the Zener, the Zener will try to maitain the rated Zener Voltage.
You also need to realize how to use the Zener.
What is the Voltage rating of the 5W ZENER? 5V? 10V, 100V. You know the Wattage, you know the Zener Voltage then you can find what the current rating is.
You also do not want to put the Zener across the power source that has high Voltage than the Zener. I.E. put 6V Zener across 12V power source. Try it an see what happen for yourself.
https://www.digikey.com/en/maker/blo...e17bc61e2640f3

[budm]

Re: Maximum voltage through zener diode

To take it one step further and put it into practice (not literally please because someone will get hurt ): if I have 230v from the wall socket, turn that to DC and stuff it into a 5w zener, I'd only be able to draw a maximum of 0.02a from said circuit, correct ? Don't know of what use that would be.....lighting an LED perhaps ?

Show us the diagram and show us how you come up with 0.02A. As I said before many times, do not skip the basic, look up how Zener works and how it is used.

[goontron]

Re: Maximum voltage through zener diode

That's what it can dissipate before blowing up. Zeners can't limit current. That's a resistors task.

The most common use for Zeners, at least in my field, is voltage limiting. Say you have an expensive 5v powered device, and you have a crappy chinese 5v wall wart that you don't trust to power it. You could use a 5.8v zener in inverse parallel across the incoming from the PSU to clamp any voltage above 5.8v to ground. This is actually done in cheap capacitive dropper PSUs commonly.

[budm]

Re: Maximum voltage through zener diode

That's what it can dissipate before blowing up. Zeners can't limit current. That's a resistors task.

The most common use for Zeners, at least in my field, is voltage limiting. Say you have an expensive 5v powered device, and you have a crappy chinese 5v wall wart that you don't trust to power it. You could use a 5.8v zener in inverse parallel across the incoming from the PSU to clamp any voltage above 5.8v to ground. This is actually done in cheap capacitive dropper PSUs commonly.

That is one of the clamping method, when power supply is operating normally and putting out 5V that 5.8V will not be conducting, but if the the Voltage try to go up beyond 5.8V the zener will conduct, and trying to maintain 5.8V but it can only maintain that 5.8V for so long if the current flowing though it not exceed the rating which depend on the Wattage rating of the Zener, most of the time the Zener will just short out (power supply may be putting out more than 10V, so putting 5.8V Zener on that is not going to last long) and crowbar the power supply so the power supply goes into ticking mode (cycle ON and OFF). Fixed many of those power supplies.

[goontron]

Re: Maximum voltage through zener diode

^ yeah, the lone zener is for low power systems (100s of mA).

[Dannyx]

Re: Maximum voltage through zener diode

You also do not want to put the Zener across the power source that has high Voltage than the Zener. I.E. put 6V Zener across 12V power source. Try it an see what happen for yourself.

Sounds like the setup to a joke, so definitely not doing it now
To be fair, I was thinking of a 12v zener on a 230v supply straight off the mains....stupid, I know, but I wasn't actually going to try it - it's just to learn something about zeners in general and WOULD happen.
EDIT: the above setup would of course have the limiting resistor, otherwise it would go boom instantaneously, DUUH...I'm not that dumb

[budm]

Re: Maximum voltage through zener diode

Well you do not need 230V just to do the experiment, just use your DC variable bench power supply that can supply lots of current that has Volt and current meters then hook up may be 3V zener at the output of the power supply and then crank up the Voltage see what happen, easy enough.

[Dannyx]

Re: Maximum voltage through zener diode

Well you do not need 230V just to do the experiment, just use your DC variable bench power supply that can supply lots of current that has Volt and current meters then hook up may be 3V zener at the output of the power supply and then crank up the Voltage see what happen, easy enough.

STILL sounds like a joke, so not gonna do it Ok, jokes aside, I see where you're going, as we've discussed this in another thread of mine where I was struggling to power those routers at 9v off a 12v adapter If the series resistor is high enough, nothing bad would happen - the diode would regulate, but would have very little current left, since the resistor would have to be so high to prevent the diode from blowing that there'd be no current available for the load, unless it's extremely small. Pretty sure this is the result of such an experiment.

[budm]

Re: Maximum voltage through zener diode

What a wasted thread.

[Dannyx]

Re: Maximum voltage through zener diode

Ok, for some reason I haven't seen some of your replies (notifications were a bit late), so that's why my replies probably didn't make sense...
Now then, let's clear the mess up:

Show us the diagram and show us how you come up with 0.02A.

Schematic
Math: supposing the zener is rated for 5w. At 230v, 5w would mean 0.02A (5/230v=0.02, unless I'm somehow wrong). Now if the zener is a 10v one and I just had a piece of wire in place of the resistor, it would begin conducting immediately (since 230 is higher than Vz) and would draw immense current from the supply, since there's nothing stopping it from doing that - POP ! To prevent that, you'd size up the value of the resistor accordingly to keep the maximum current through the diode below 0.02a which would correspond to 5w....I may be wrong of course, so feel free to correct me again, that's the whole idea. It's all in my head, but I'm not good at explaining it back to others

[eccerr0r]

Re: Maximum voltage through zener diode

Yeah, a lot of facepalm and head throbbing conclusions here... head is spinning...

Key points:
1. 5W means 5W max is dissipated by the diode. This indicates max current that can pass through the diode when coupled with the zener voltage. A 10V zener means P=VI=5W=10V*I meaning I=500mA through the zener.
2. Connecting a resistor and zener in series is kind of useless. We need to know the load in parallel with the zener being regulated to really know what we're talking about. The variability of the load will dictate whether or not a zener regulator can be used.
3. If you compute back with that useless example above with 10V zener and 230VDC supply and no load in parallel with the zener, 220V would need to be dropped across the resistor. 220V*0.5 = 110W dissipated in the resistor by itself regardless of its value . THIS is the problem of these zener supplies.

[Dannyx]

Re: Maximum voltage through zener diode

Went over the theory once again on my own HERE...read that article many times before in fact, but those formulas don't sink in I kinda got it now and know how to calculate sh!t

[fester]

Re: Maximum voltage through zener diode

you can use something like this

https://electronics.stackexchange.co...erter-lossless

(the first example) The capacitor is here used for AC as serial resistor but without dispating to much heat because of its capacital reactancy(current is shifted against voltage)

[Dannyx]

Re: Maximum voltage through zener diode

you can use something like this

https://electronics.stackexchange.co...erter-lossless

(the first example) The capacitor is here used for AC as serial resistor but without dispating to much heat because of its capacital reactancy(current is shifted against voltage)

Yeah, I knew about those capacitive droppers, but the point of this discussion wasn't to actually build anything in the end (I'd just use a bloody adapter ). It was just a generic "what if..." question...thanks for your input anyhow