zener diode

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. 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.

 

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.

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.

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.

 

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

 

Search the web for home brew zener tester, the usual setup is a small
circuit to supply a limited current with a decent voltage overhead, as long
as the supplied current is not too high so as to damage the device you can
measure the device volt drop each way round - this will give you some clues
as to what it is and which way round it goes.

If you have a DMM, have a look whether it has a diode check function, this
reads the forward volt drop of a single diode junction, its worth measuring
a few sample diodes that you can identify to get the idea - most diodes read
somewhere around 0.7V but different types vary slightly, fast soft recovery
tend to read slightly lower while zeners often fill the top end of the rang.
If it reads just under 0.2V don't be too hasty to bin it - it could be a
Germanium or Shottky-barrier diode, both of which have their uses -
Germanium are becoming rare and S-B are a little more expensive.

 

One reason to expect that it should be possible to figure this all out
without using any test equipment is that commercially available components
need to be documented. So, somewhere there ought to be some literature that
describes this diode.

I used the monocle to take a closer look at the diode. It is some kind of
surface mount device. Earlier, I had thought it was a glass diode with wires
at each end. Closer examination shows that the ends are more like disks which
hold the diode the way a roll of paper towels is held by its holder. I don't
see any letters or other markings on it, just the colors. Maybe there is a
different system for identifying surface mount zener diodes, one that includes
putting colors on the two halves?

I looked at
http://www.marsport.org.uk/smd/mainframe.htm
and found some information about color coding of surface mount diodes, namely
what they call SOD-123 package diodes. The page shows a table with
"Cathode Band" in one column and "Devices" in the second column. The
colors mentioned are red, yellow, green, purple, blue, white. Following the
table is a remark that devices with a color code might also have an
alphanumeric code, which might only be a date code. In my case, all I
see are the colors.

Now, my diode has two colors and their table gives the impression that the
authors are only interested in one color, namely the color of the "cathode
band". Taking another look at the diode, I see that, in the half with the
blue color, the color is solid and opaque, filling the half very well. In the
half with the red color, the color seems less solid and I can probably
convince myself that it is more like a band or that it is red with a white
band in the middle. Now that I look again at the blue part, it is not all blue.
Just before the junction with the red, there is a distinct purple band. Maybe
the purple is made of the red and the blue and the whole purpose of the red
and the blue is to make this purple band where they come together. In other
words, maybe the cathode band is purple, which would uniquely identify this
diode as being a BB730, according to this table.

I'm not absolutely sure that there is not a little bit of blue between the
thin purple band and the red half, but I think there is no doubt that the
purple band is on the blue side of the diode. If these observations hold
water, then that seems to settle the question of identifying both the
diode and its polarity.

Next I need a data sheet. OK, I'm at
http://www.datasheetarchive.com/search.php?t=0&q=bb730
and downloaded the data sheet. It is all black and white. It's hard to tell
whether this is really my device.

According to the data sheet, it is a variable capacitance tuner diode
(whatever that is). They seem to be assuming that the voltage across
it will be 28 V, which doesn't seem credible in the case of a mouse,
with both leads of the diode connected to pins of the mouse port.

The data sheet seems to show the band as being close to the end of the
diode, whereas the purple I observed is in the middle. But it is pretty
definitely on the blue side.

 

I decided to take an even closer look at the diode using a pocket microscope
with magnification between 30x and 70x. It helped a little. What I had
thought, using the monocle, was a purple band is not really there. There
are two very thin red bands on the blue part. One is right near the red
part. The other is near the other end of the blue part. The portion of the
blue between that second red line and the actual end of the tube looks like
a slightly lighter shade of blue. By way of contrast, I could see no color
but red in the red half of the diode.

Between the red and the blue part of the diode, there was a bit of an empty
space which contained what looked like a dark red pencil eraser near the axis
of the diode and also what looks like a glass wire joining the red and blue
portions. I'm assuming that the "red eraser" is really part of the red stuff.

After all this scrutiny of the diode, now I'm wondering what these red and
blue substances are and what the "glass wire" is and how this configuration
of these elements cause the device to function as a diode. So, maybe it is
time to read a little about fabrication of diodes. Searching for
fabrication diodes
leads to a lot of stuff about fabricating them on silicon wafers, but not
about the manufacture of diodes as discrete components. Searching for
manufacture diodes
leads to patents, manufacturers and vendors. So, what does one search for?

 

Well, that was fun and will lead to future projects. But meanwhile,
I was musing on the fact that the two leads of the second zener diode DZ2
(the red-blue one) each connect to wires that go directly to pins on
the mouse port. I don't actually know which wire is connected to which
pin, so I can't use that information to determine the polarity of the
diode from the pinout of the PS/2 mouse port, but it does potentially
tell me the voltage difference between the two ends of DZ2, just not
the polarity. Then I started wondering whether the other zener diode DZ1
on the PCB is also connected to mouse port pins in this way and reached
for my 7x monocle. That was the right thing to wonder about, since it
turns out that the two leads of DZ1 are connected to the very same mouse
port pins as the two leads of DZ2. Furthermore, unlike DZ2, the markings
on the PCB for DZ1 show very clearly what is the intended polarity of DZ1.
Therefore, I now know the polarity of DZ2 in terms of the known polarity of
DZ1, and I potentially know the voltage across each diode in terms of the
voltage differences between pins of the mouse port.

So, that's progress and, so far, I haven't had to use any test equipment.

 

re-evaluate your first belief and expand the component choices.

it may be a 'resettable " fuse to limit over voltgaes.

it may be a capacitor, glass encased ones are used often in euro devices and
your descripttion mimics them

evaluate the peripheral circuityry by critical physical examination, make a
pencil schematic of what it looks like

Boston.

 

Are they ? It's news to me !

Graham

 

I've often found ceramic chip caps encapsulated in tiny glass tubes.

 

80's tech. If you ever go into an Apollo workstation (card-and-backplane
technology from the 80s, built up from scads and scads of 74xx series
chips) you'll find those little guys absolutely *EVERYWHERE* - there's
one or more connected to practically every chip on every board.

 

I can confidently say I've never seen one other than as a result of a Usenet
discussion.

And certainly it's not some European obsession.

Graham

 

There is no doubt that it is a zener diode. It is clearly labeled as such
on the printed circuit board.

 

How so ?

The red and blue colours provide no help whatever however.

Graham

 

I didn't take any notice of where the boards were made - I just got on with
harvesting the parts I wanted - loads of 74F chips.

 

is it text as a "zener" or a pictograph like a zener symbol?

is it like a Z with a crossbar thru it?

come forth with the info, we are intrigued...

 

HP had a factory/assy plant in UK for a while and i beleive they outsourced
more than we are aware of.

 

The component has a label DZ2 next to it.

 

OK, that sounds fairly compelling.

The pcb legend will normally have some way of indicating polarity. Typically some
thicker print at one end which denotes the cathode.

Graham