# Series connected zener diodes problem

1. Mar 1, 2016

### saurav106

I have attached the image with post.

Answer to question is given as A. I am not getting the explanation to how can one zener diodes be in breakdown and other not. I think if the combined voltage of 150V is applied then only breakdown occurrs in both the diodes simultaneously.

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2. Mar 1, 2016

### Hesch

You are right: The answer is D) as for Vsupplied < 100 V.

D1 and D2 will conduct some small amount of current, sharing the supplied voltage, but the voltages VD1+VD2 < 100V are not defined as break down voltages.

Last edited: Mar 1, 2016
3. Mar 1, 2016

### jim hardy

"Does some arbitrarycurrent I100 exist
where the sum of the diode voltages is 100 volts? "
Surely there does.

The diodes see the same current of course ...
Any given current would be a horizontal line on the graph
and where it crosses each diode's characteristic curve is the voltage for that diode at that current.
so let's take a ruler and dividers to it .
Sliding my red horizontal current line down from zero amps
it appears D1 breaks down at 80 volts well before D2 reaches even 20 volts
so if i lay my horizontal current line so it crosses D2's characteristic curve at 20 volts
i have found one possible I100 .

It seems strange that those two diodes have such different avalanche currents

but those are the curves we were given.

I'd have to pick answer A.

I think this was more an exercise in graph reading than zener diodes ?

old jim

Last edited: Mar 1, 2016