[sdiy] Zener diode knee sharpness differences?

Neil Johnson neil.johnson71 at gmail.com
Tue Jun 24 11:48:46 CEST 2014


Hi,

Mattias Rickardsson wrote:
> 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?

It is not unexpected.  As you say, the zener diodes are characterised
at a current of 5mA.  That is the minimum current that the
manufacturer guarantees correct operation.  Below that level and
performance is not guaranteed, so they can behave exactly as you are
finding.

Neil
--
http://www.njohnson.co.uk



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