[sdiy] really inaccurate zeners

[Neil Johnson]

Hi David,

> The amplitude control is achieved in each core with a single zener diode inside a "zener bridge" consisting of four 1N4148s.  Since the 4148s drop approximately 0.5V, a 3.9V zener (1N4370) should give a total drop of roughly 5V, which should guarantee waveforms which are 10Vpp.

Umm, I thought the 1N4370 was a 2.4V device?  All the datasheets I can
find confirm this.

> One was about right, but the other was way low.  I replaced the zener of the low one, and it got worse.  So then I decided to actually measure some zeners.  After much grumbling, the bottom line is that I had to delve into my bag of 1N4372s (4.7V) before I found two zeners that read almost exactly 3.9V.  They were all very low relative to their nominal values.  (I tested them by putting a 3.3k resistor between +15V and the cathode, and connecting the anode to ground.  I use 3.3k resistors on the comparator in my VCO core.)

Umm, and 1N4372 are supposed to be 3.0V.

You may want to recheck the part numbers?

> So now I have decided that I have to measure each and every zener diode, and separate them into plastic bags based on their actual voltages rather than their denominations.  In this way, after a certain amount of tedium, I will have a collection of precision zener diodes (within +/- 0.05V).  However, I shouldn't have to do this.  I would expect zeners to be off by 0.2 (or, at most, 0.3V) from their nominal values, but to cross over one or two (or even three) denominations?  That is unacceptable.  What are these zener diode manufacturers doing wrong?

Well, zeners - like all electronic components - have tolerances.  In
your case, assuming you have 1N4372 diodes, without any suffix they
have a 10% tolerance.  You can get tighter tolerance parts but you pay
more for them (they sort them on the production line).

Also, as Tom pointed out, the advertised voltage is at a stated
current (Izt - Zener test current). For the 1N437x series this is
20mA.  At lower test currents you will measure a lower reverse
voltage.  For example, if you dig out the Motorola datasheet on these
parts:

https://datasheetspdf.com/pdf-file/1388706/Motorola/1N4370/1

on page 6 you can see how much the zener voltage varies with current
(answer: quite a lot!)

Neil