[sdiy] really inaccurate zeners

[David G Dixon]

Thanks for the good advice, Neil.  Here are my general thoughts:

1) Since I've only ever seen 1N47XX zeners in the flesh, and I wasn't aware
that there were any particular problems with those particular zeners, I had
never thought to look for anything different.  I'm not an EE, just a
hyperactive hobbyist -- I missed the lecture on the 243 different kinds of
zeners.  Now that I know, maybe I'll try some different ones.

2) If the test current of a certain 3.3V zener is 76mA, but this is 20 times
as much current as would ever reasonably be put through the diode, then what
exactly is the point?  (Dave Dixon: I want to use a zener where it will take
about 2mA and still limit the voltage to something in the ballpark of the
nominal value.  SDIY Experts: Zeners only work properly when they pass as
much current as is needed to power a small city.  Dave Dixon: Oh.  Shit.)

3) It turns out that I can quite happily handle 12Vpp waveforms from my dual
core oscillator in my frequency shifter application (the multipliers will
still work perfectly), so I think I'll just use 1N4733 5.1V zeners, which
I've discovered can be safely relied upon to limit the voltage to something
very close to 5V even at pretty low currents.  The sine shaper I'm using
will still generate 10Vpp sine waves from 12Vpp triangles, and since it's
mostly the sines I'm interested in, it's all good.  Problem solved, and I
have a big ol' bag of 1N4733s.

Cheers,
Dave

 

-----Original Message-----
From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
Sent: Wednesday, July 01, 2020 2:57 PM
To: David G Dixon
Cc: Synth-diy at synth-diy.org
Subject: Re: [sdiy] really inaccurate zeners

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