[sdiy] really inaccurate zeners

David G Dixon dixon at mail.ubc.ca
Tue Jun 30 22:02:04 CEST 2020


Hello SDIY Superfriends,
 
Yesterday, I finished up my first bought-and-paid-for Sketchy Labs 5U
Frequency Shifter, and upon testing it, found that the dual-core TZFM VCO
was generating sine and cosine waves with very different amplitudes.
 
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.
 
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.)
 
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?
 
Cheers,
Dave Dixon
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