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<p>Hi Dave</p>
<p>It could be an effect of saturation and capacitances in the power
type zeners.</p>
<p>Have you tried reverse biased 2N3904 NPN transistor junctions for
a 5-6 volt-ish zener?</p>
<p>These work like low power, high speed zeners. These have been a
go-to for me in high speed clipping circuits.</p>
<p>My 2 bits</p>
<p>- Oren<br>
</p>
<div class="moz-cite-prefix">On 11/29/20 11:20 PM, David G Dixon
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:66A0A33CF070458DB475DD7D092556E6@david78c70950b">
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<div><font size="2" face="Arial"><span class="311575304-30112020">Hello
Friends,</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020"></span></font> </div>
<div><font size="2" face="Arial"><span class="311575304-30112020">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.</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020"></span></font> </div>
<div><font size="2" face="Arial"><span class="311575304-30112020">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.</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020"></span></font> </div>
<div><font size="2" face="Arial"><span class="311575304-30112020">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.)</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020"></span></font> </div>
<div><font size="2" face="Arial"><span class="311575304-30112020">Has
anyone else experienced similar inconsistent behavior with
zener diodes? How likely is it that these 4733s are
actually mislabeled 5W zeners?</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020"></span></font> </div>
<div><font size="2" face="Arial"><span class="311575304-30112020">Cheers,</span></font></div>
<div><font size="2" face="Arial"><span class="311575304-30112020">Dave
Dixon</span></font></div>
<br>
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