[sdiy] really inaccurate zeners

Ian Fritz ijfritz at comcast.net
Tue Jun 30 23:14:28 CEST 2020



I believe zener breakdowns are specified at very high currents. Like you need to use a 220R series resistor to measure. And there has always been a large spread in breakdown voltages. I believe you can now buy them with tighter tolerances.

I take advantage of the soft knees to make polynomial-like transfer functions for my chaos circuits, eg x - x^3.

>> On Jun 30, 2020, at 2:04 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> 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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