Re: zener diode
- From: Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx>
- Date: 04 Dec 2007 15:47:57 -0500
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?
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.
I don't have any test equipment at the moment, not even a resistance meter.
I'll buy one if necessary. Right now I'm trying to do as much as I can
using only literature and what I can directly observe. Also, the diode
is still installed on a PCB and I don't want to remove it to measure it.
Having it connected to the other components on the board would complicate
the measurements, but they would probably still be good enough to decide
which end is the anode.
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.
This diode is in a mouse. If I get a resistance meter, I'll probably get
it from Radio Shack, since I have no other local places to purchase from.
Radio Shack used to carry IC's and other components but the local one says
they don't carry them anymore. If I have to purchase online, I wouldn't do
it from Radio Shack. At least they still have solderless breadboards.
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.
The blue end connects to pin 1 of the unidentified EICI127400 IC, which
I believe is a quadrature encoder. It also connects to the blue wire of
the mouse cable, which goes directly to the mouse port. I don't happen to
know the pinout of the plug, but the six pins are arranged as follows,
the notch being up at the top:
o o
o o
o o
The red end of the diode connects directly to the green wire of the mouse
cable and therefore to another pin of the mouse plug. I don't know yet which
pins correspond to which wires, but I figure that the voltages going into
two pins of a mouse port can't be too high. My guess is that some batteries
in series would suffice as a voltage source. But hopefully no measurements
will be necessary.
I did a google search for "color code zener diode" but didn't find anything
that related to a red-blue marking. Searching for "zener diode red blue"
leads mostly to hits about LEDs.
--
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.
.
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