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The circuit below is composed of an 8.2V Zener diode, an R2 series resistor and a load which is a variable resistor R1:

enter image description here

The load resistance R1 is increasing from 1 Ohm to 2k Ohm.

As the load resistance is kept being increased after a point why is the zener diode exceeds its rated zener voltage 8.2V?

Chupacabras
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GNZ
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  • The nominal voltage is 8.2V but the max zener voltage is actually 8.7V which is close to what your simulation displays? See the datasheet here: http://pdf.datasheetcatalog.com/datasheet2/0/0113hha98du019z9gdc0yzoa9jky.pdf – SRR Jun 05 '18 at 19:08
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    Just a note, "increasing load" = you increase current, so you are decreasing load resistance. – Chupacabras Jun 05 '18 at 19:08
  • Can there be a more step by step analytical explanation? – GNZ Jun 05 '18 at 19:17
  • @Genzo the specification is 7.6 - 8.7 on normal operation and 7.6 - 8.8 on high current (20ma), so you will vary this voltage with the load due you are changing the Z of the Zener. Try to read more here https://electronics.stackexchange.com/questions/142003/protecting-resistor-value-for-zener-diode-in-parallel-with-load?rq=1 – Fernando Baltazar Jun 05 '18 at 21:04

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Technically, increasing the load impedance is decreasing the load, i.e., putting lower demands on the supply. That said...

You can easily get a voltage higher than the Zener voltage by pushing more than the Zener current through them. I figure you have about 34mA going through the zener; the older ones are usually spec'd at 20mA, and newer ones tend to be spec'd at 5-10mA. More current, higher voltage.

Even higher current, you can make it blink...after it heats up enough, there's enough electrons in the conduction band to make it a short. Of course, as a short, it doesn't dissipate much power, so it cools down...and so on, ad infinitum.

Cristobol Polychronopolis
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    Are you implying you can make an oscillator from a Zener diode? That it shorts and recovers thermally? I don't believe I've ever seen that kind of behavior in a Zener. – John D Jun 05 '18 at 20:49
  • I've seen it in the field. The customer reported speaker audio was turning on and off. The cause was too low working voltage on ESD zeners, they'd heat up and short, then cool down and recover on about a 5-10 second cycle. – Cristobol Polychronopolis Jun 06 '18 at 14:59
  • Very interesting. I'm going to have to ask some of our device physics guys to see if they are aware of that phenomenon. I've seen lots of over-stressed zeners and have never seen that behavior before. – John D Jun 06 '18 at 15:06
  • If it helps, I believe the part in question was a VCUT0505. – Cristobol Polychronopolis Jun 06 '18 at 16:11
  • Ah, I know some of the guys over at Vishay, I'll ask them. – John D Jun 06 '18 at 16:13