[sdiy] really inaccurate zeners

[David G Dixon]

So, Neil, what you are basically telling me is that zeners are useless
components for synth circuits.  I see that the 1N7XX zeners are 1/2W while
the 1N47XX zeners are 1W, and that their test current is uniformly 20mA no
matter the voltage.  You're telling me that this makes a huge difference,
but I've never used or even seen a 1N7XX zener.  All the ones at my local
shop are 1N47XX, as are all the zeners I've ever scavenged from my school
electronics shop or electronic stores.  Those 1/2W ones must not be very
common, so for all intents and purposes, I'm going to assume that the only
zeners are the 1N47XX (1W) ones.

As far as testing, I never quite got to the test current for any of them.
The highest current I measured was 53mA on the 1N4728, and the test current
is 76mA.  I was close for the 1N4730s, with highest currents of 52mA and
test currents of 64mA.  The thing is, at those currents, everything was
getting stinking hot, so I plugged in the resistor, let the reading settle
and then pulled the resistor quickly.  I also realized that I needed to
start with the 20k resistor and work my way down so I didn't start with a
hot zener.  If zeners can only be relied upon to behave in a predictable
fashion at their test currents, then they simply cannot be relied upon, full
stop.  I will never design a circuit where the components are running too
hot to touch.

The thing is, I've been using zeners in mostly two places (as a feedback
limiter in filters to make sine waves, and in a bridge as a voltage limiter
in VCOs) for years, quite successfully, and this is the first time that I've
had a problem.  I think I just got into a bad batch of zeners from my local
shop.

And yes, it would be nice if all zeners were as well behaved as the 5.1V
ones, but sometimes you want a different voltage.  I guess that's why they
make so many different ones.

So, at least I've learned a lot about zener diodes, and I hope (and trust)
that I'm not the only one on SDIY who is not a stone-cold expert on every
aspect of zener diodes.  I've got a lot of electronics textbooks, and none
of them have told me that I need to run 20 (or 53) mA through a zener to use
it effectively.

Bottom line: I'm going to measure every zener at the current at which I
expect to use it (about 2 mA), and separate them into bin bags on that
basis.

 

-----Original Message-----
From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
Sent: Wednesday, July 01, 2020 2:08 AM
To: David G Dixon
Cc: Synth-diy at synth-diy.org
Subject: Re: [sdiy] really inaccurate zeners

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