[sdiy] really inaccurate zeners

[Ian Fritz]

I always use 1N52xx 1/2W. (Mouser has 10s of thousands in stock.) 
Curious as to what other folks like.

On 7/1/2020 12:50 PM, David G Dixon wrote:
> 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
> 
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