[sdiy] Zener issues (again)

[Oren Leavitt]

Hi Dave

It could be an effect of saturation and capacitances in the power type 
zeners.

Have you tried reverse biased 2N3904 NPN transistor junctions for a 5-6 
volt-ish zener?

These work like low power, high speed zeners. These have been a go-to 
for me in high speed clipping circuits.

My 2 bits

- Oren

On 11/29/20 11:20 PM, David G Dixon wrote:
> Hello Friends,
> You may recall a few years ago that I built an interpolating scanner 
> with a trapezoid generator for controlling linear VCAs.  The 
> trapezoids were generated with a kind of folding circuit that uses 5.1 
> zener diodes to limit the voltage in a couple of places.  As the 
> incoming CV rises linearly, the circuit output rises sharply from a 
> little less than 0V to 5V, stays there briefly, and then falls sharply 
> back to a little less than 0V as the CV continues to rise.
> So, I built the original circuit with 1N4733 5.1V 1W zener diodes, and 
> it worked perfectly.  Just the other day, I built a very similar 
> circuit for someone, and again used 1N4733s.  However, in this 
> circuit, instead of getting nice well defined trapezoids that peak and 
> ride at 5V, I'm getting narrow peaks that don't quite rise to 5V 
> before falling again.
> I have checked and re-checked (and re-checked, and re-checked) the 
> layout against the schematic, and it is correct.  I went to my 
> simulation, and nothing I did changed the general robustness of the 
> circuit.  However, then I tried replacing the simulated 1N4733 diodes 
> with simulated 1N5338, a 5.1V 5W zener, and -- lo and behold -- the 
> simulation generated wimpy little narrow peaks up to about 4V instead 
> of trapezoids up to 5V.  So, I can only conclude that the 1N4733s that 
> I bought (from Small Bear) are actually 1N5338s in disguise.  I 
> haven't yet desoldered the old zeners and tried them in the new 
> circuit, but I'm going to shortly. I'd bet that this will fix my 
> circuit.  (And, before you ask, I decreased the current-limiting 
> resistors from 4.7k in the original circuit to 3.3k in the new 
> circuit, to no avail -- also, the simulation shows no effect of 
> decreasing these resistors.)
> Has anyone else experienced similar inconsistent behavior with zener 
> diodes?  How likely is it that these 4733s are actually mislabeled 5W 
> zeners?
> Cheers,
> Dave Dixon
>
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