[sdiy] really inaccurate zeners

[Neil Johnson]

Hi David,

> Sorry, I mistyped.  I meant 1N4732 (4.7V), not 1N4372 (which is a 3.0V
> zeners -- what genius picked those numbers, anyway?

Lol - yeah it happens: https://www.youtube.com/watch?v=2un9rO2ZF4g

> It's as if people
> intentionally do things with electronic parts to increase confusion, like
> putting the power pins on the wrong sides of quad opamps so they need to be
> oriented backwards relative to every other IC).  The zeners I measured today
> are 1N4730A (3.9V), 1N4731A (4.3V), and 1N4732A (4.7V).

Ah, those are power zeners...

> So, who ever runs a zener diode with 20mA of current?  You could heat your
> home by using your synth as a fireplace.

As power zeners the Izt is 53mA.
https://www.vishay.com/docs/85816/1n4728a.pdf

The table clearly shows that the zener voltage is tested with the
diodes dissipating around 250mW once the device has reached thermal
equilibrium.
The second test at a much lower current (1mA and under) is used to
measure the dynamic resistance, or the slope of the I-V curve.

> Also, there was a 0.5V difference between zeners from the same denomination.
> The datasheet suggests +/-5% tolerance for the A denominations, and I'm
> seeing much looser tolerances than that.

These devices are spec'd at 5% tolerance of the zener voltage AT THE
TEST CURRENT.  At any other current (e.g., much lower) then as there
are no specs you're on your own as it were.  And we also have a
smorgasbord of column headings in the table on page 2 (NOM, TYP, MAX
oh my).

Cheers,
Neil