zener diode question, minimum current?

  Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.... ok that's
fine, I can get a resistor value to limit my current to that... but,
is that the spec I should follow... the 5mA test current?  My concern
is that I don't want to be too low and too close to that.... I don't
know what it's called... knee current, where the zener starts to go
into reverse breakdown... I want to be above that current... not near
it... right? please correct me if my thinking is wrong about this....

thanks

 On Tue, 12 May 2009 08:29:10 -0700 (PDT), panfilero


Zeners above, say, 5 or 6 volts have very sharp knees and can run near
rated voltage down to very low currents, microamps or less. Low
voltage zeners, like in the 3 volt range, are rotten, very soft knees,
not much better than curvy resistors.

Zener datasheets usually have the curves.

But why not test some?

John

 panfilero wrote:

I don't know about the TZX3V0A, but low-voltage zeners with rated breakdown
voltages well below 5V tend to have very rounded knees - no abrupt breakdown
levels. They have high dynamic resistance and their breakdown voltage varies
considerably with current. If you must use them at all, yes, the current
should be kept as high as is practicable.

 
So you think 5mA is too low? this is what I'm looking at
http://www.vishay.com/docs/85614/tzxserie.pdf
I wish it had some kind of reccomended operating point for the
current... I guess that's what I'm looking for... I was assuming since
they tested at 5mA that that was optimal...

thanks

 On Tue, 12 May 2009 10:28:33 -0700 (PDT), panfilero


---
Regardless of the shape of the knee,  Iz is the reverse current through
the Zener which will guarantee that Vz will lie between Vmin and Vmax
and, as such, is the recommended operating point for the current.

For the TZX3v0A, that means that with 5mA of reverse current through the
diode, Vz will lie between 2.8 and 3.0 volts.

 
JF