Re: zener diode

Michael (NoSpam@xxxxxxxxx) writes:
Allan Adler wrote:

I have what I believe is a zener diode. One end is red and the other end
is blue. What kind of zener diode is this and which end is which?
--
Ignorantly,
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.


I don't recognize the color markings you describe but I've been inactive for
*a*long* time ... and at no time did I know everything. ;-)

Use a resistance meter to measure "front/back" resistance ratio. This tells you
which end is anode, which is cathode. Be sure you know the actual polarity (is
red positive? negative?) of your particular meter. My venerable old Simpson
269 puts positive on the red lead but I have seen meters that put positive on
the black lead. Also, take care not to use a resistance scale that could allow
a damaging amount of current to go to the diode.

Once you know which end of the diode is cathode, hook it up to a variable DC
source, forward biased, and through a series resistor (to limit the current).
Put a voltage meter across the diode and, starting at zero (0) volts, slowly
increase the voltage on your DC source while watching the voltmeter. When/if
you perceive a "knee", that is, the voltage increase across the diode is sharply
non-linear with respect to the increase in current through it, then you have
exceeded the breakdown, or "zener", voltage. Of course, if your zener happens
to be, e.g. an 80v one and your DC power source maxes out at less than 80v then
the diode won't "break down" and you won't see a knee; you'll have to use higher
voltage.

But then you still won't know the current capacity of the zener.

And realistically, the hobbyist has very little use for zeners nowadays.
Pretty much anytime you need a specific voltage, a three terminal regulator
will do the job. At the very least, if you need one you pay the way under
a dollar and get one to the exact specs you need.

But then the specs don't really need to be known in this case. How it's
used in the circuit will give a good indication of what kind of diode it is.
Or, it won't be really relevant to understanding the circuit.

A zener diode, or any type of diode, will not give clues to help define
a multi-pin IC, once the detail of the IC becomes relevant the function
of the unknown diode becomes relatively clear.

Michael

.

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