[sdiy] really inaccurate zeners

[Neil Johnson]

Hi David,

> So, Neil, what you are basically telling me is that zeners are useless
> components for synth circuits.

Nooo.... don't believe I said that....

> 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.

I've never seen a black swan but I gather they exist.

> 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.

LOL!  I just looked on Digikey, they have about 700 lines in stock
today of 500mW zener diodes in DO-35 package.  I'm more familiar with
the BZX55C and BZX79C series:
https://www.mouser.com/datasheet/2/427/bzx55-1767760.pdf

> 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.

Bench power supply set to constant-current mode?

> 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.

Simple zeners are made for simple crude shunt regulators.  If you
design carefully with them, or are able to accept their crudeness and
imperfections, then they can be used in other applications, but the
simple first-order model doesn't hold up too well in practice.

> 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.

Quite possibly.

> 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.

Indeed.  It *is* possible to make precision zener diodes (LTZ1000 for
example) but those are special - and expensive - beasts.
You might try the 1N821 series, but again you need to feed them with
about 7mA to get the stated voltage.

> 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.

I checked some of the books on my shelf:
- The Circuit Designer's Companion, Tim Williams, Newnes
- Electronic Circuits: Discrete and Integrated, Schilling and Belove,
McGraw Hill
- Art of Electronics, Horowitz and Hill, Cambridge

All three discuss the zener diode in varying details, and the issues
to do with operating current.

> 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.

And depending on which ones they are and the intended application
might also need to factor in the temperature of the diode.

Cheers
Neil