[sdiy] Zener diode knee sharpness differences?

Mattias Rickardsson mr at analogue.org
Tue Jun 24 11:29:30 CEST 2014


Hi,

I'm using SMT zener diodes from Diodes Inc, and I have noticed that
some of them have a zener knee that is significantly less sharp. The
zener voltage at the 5 mA test current in the datasheet is correct -
but before that zener voltage is reached, the knee has a more rounded
shape.

As a result of this, when these rounded-knee zeners are used as a
resonance limiter in a VCF, acting well below the 5 mA test current,
they limit the resonance to a much lower signal level, and the filter
acts and sounds very different.

Is it expected that the zener knee can vary a lot in its shape from
unit to unit, or should it be seen as a defect?

The units tested have the same date code and markings on them. The
many ones that do have a sharper knee seem very identical to each
other. Another detail is that these SMT zeners are dual - and when
found, the rounded-knee problem seems to occur in both diodes in a
package.

/mr



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