Not me Mike. When using zeners to clamp AC signals I've only ever
biassed-up the zener and used diodes to isolate any below-clamp AC
voltage from the zener.
Good Morning, Tony. Thanks for the reply. I was thinking of some of
the marvellous posts you and Win supplied long ago. A little
searching found some of them.
Here's one from Win's post of May 7, 2002:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
from my post of 2Aug1997
/| 1N5245B at 50uA
200mV FS / |
/| / | /|
/| / | /| / | /| / | /|
/ | /| / | / | / | /| / | / |/ | /| _____
| / |/ | /| / |/| / |/ | / |/ | / |/ |
|/ | / |/ |/|/ |/ | /|/ |
|/ |/ |
/
scope trace, continuous events, 10us FS 14.41 V
average value
The trace above is from a zener diode that was a bit more noisy than
most. A type of relaxation oscillation is clearly seen, with the
diode's 285pF self capacitance charging from the 50uA current, until
a discharge event is triggered, which happens randomly with an
increasing probability as the average voltage is reached and
exceeded. The discharge step looks nearly instantaneous, stopping at
random voltages at or below the average voltage.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Win calcuates a diode capacitance of 285pF from the charging slope.
It was difficult to find a capacitance plot for the 1N5245B, which
is a 15V zener. Vishay has one at
http://www.ortodoxism.ro/datasheets/vishay/85588.pdf
Figure 6 shows the diode capacitance vs Zener voltage at 2V reverse
bias. Reading from the graph, the capacitance for the 1N5245B is
about 25pF. From Jeorge's post, the capacitance decreases with
increasing reverse bias, as one would expect.
However, Win's post seems to indicate the capacitance is much higher
in Zener mode.
The thread is at
http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/9f7d8166d41f94bb/94a90c1a0c4d1938
Very interesting and well worth reading.
Another thread that Win refers to is the 1997 thread on Zener Diode
Oscillation. I can't seem to figure out how to get the start of the
thread, but google provided this link that enters partway through:
http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/399b6a0bf6cfa4f5/99f63edb6894429f
Very well worth reading.
Regards,
Mike Monett