[sdiy] Zener Diodes

mskala at northcoastsynthesis.com mskala at northcoastsynthesis.com
Wed Dec 15 18:01:52 CET 2021

On Wed, 15 Dec 2021, cheater cheater via Synth-diy wrote:
> taught is that they hold the voltage across them at 3.3V no matter
> what (or some other voltage depending on the diode). So then what
> happens if you take two of those in series, and parallel them with
> another one? What will the voltage be across the two series diodes?
> What will the voltage be towards the middle junction?

The Zener diode works by conducting an amount of current that is basically
exponentially increasing, with a curve that bends sharply up at the stated
Zener voltage.  So if you attach it to a constant-voltage source less than
the Zener voltage, you only get the voltage source's voltage, and very low
current.  The diode is not going to magically set the voltage to 3.3V when
the power supply only has 1V or 2V available.  On the other hand, with a
beefy constant-voltage supply at greater than the Zener voltage and low
impedance, you will actually see more than 3.3V across the diode, at very
high current, in the short interval before it explodes.

Applying that to your example, with two 3.3V diodes in series and a third
in parallel to the pair, you'd expect to see very close to 3.3V across the
entire arrangement, because any current that could try to increase the
overall voltage will be shunted away through the parallel diode until
either the power supply stops trying, or the parallel diode explodes.  On
the other branch you're applying 3.3V to a series combination of two of
these diodes.  You'd expect to see close to 1.15V across each, with very
low current; but that voltage may vary a bit because the diodes don't have
tightly controlled parameters at much less than their rated Zener voltage
and current, so they may not really be functioning identically to each
other even if they are identical type, and it's quite possible one might
end up dropping more than half and the other less.

Matthew Skala
North Coast Synthesis Ltd.

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