Franc Zabkar writes:

If you hadn't identified the component as a dual zener, I would have
guessed that it was a PNP transistor with an ECB (123) pinout.

Yah, it does look sort of like a transistor, both visually and on meter.
However, I have reasons to believe its actually some kind of multiple-
zener-diode contraption. More on this below....

In fact I can't recall seeing a dual diode package where the tab
wasn't either the common anode or common cathode. But then my memory
isn't the best anymore ...


I'm guessing pin 2 is common anode, but the two zeners are very different
breakdown voltages (like 1V and 5V or something like that), so that the
zener from pin 2 to pin 1 conducts both ways on a diode meter, but the
zener from pin 2 to pin 3 doesn't.

Can you help us out with a little circuit tracing? The circuitry in
that area is typically very simple. In fact, we may be able to work
out the zener voltage(s) by knowing the output voltage of the supply
plus the values of any feedback resistors in potential dividers, etc.


Many of today's switching power supplies use a transformer
switched by a transistor or even directly by a PWM controller IC.
The transistor or controller gets feedback from the transformer via
an optocoupler (P181 is almost invariably used), with the LED
cathode of the optocoupler fed to ground through a back-biased
zener diode. Sort of like this appnote for the 2A265 PWM controller:
http://www.well.com/~lonewolf/appnote.gif

The zener diode, in practice, is typically pins 2,3 of the mystery
"3B" device I'm trying to identify.

For example, one of the circuits this part appears in is the Hyundai N100
motor controller. The power supply board on that looks like this:
http://www.well.com/~lonewolf/schematic.gif

Note that if you envision a zener diode in the "3B" device with
cathode on pin 3 and anode on pin 2, it becomes much like the
appnote I link-to above. (As to what pin 1 is connected to
internally, I have no idea yet.)

Could we see a close-up photo?


My camera, even set to "macro", can't give a good image of an SMD.
The diagram I linked to in my OP gives a good idea of what it looks like,
though: http://www.well.com/~lonewolf/3B.jpg

Oh, and I did find the package name for that shape: its "SO- 89".
Here's what the SO-89 package looks like:
http://cdn.sigma.octopart.com/889872...B1132T100Q.jpg
(Turns out, the tab and pin 2 are all one piece. Can't see that from top side,
but on removing the part, I can see that it's so.)

So now its a matter of finding a SO-89 device with a zener diode from
pin 2 to pin 3, and (????) from pin 2 to pin 1.

I've seen this "3B" gizmo on boards from 2 different manufacturers, so
I'm assuming its actually a common part in switching power supplies.
So I'm surprised I haven't been able to track it down yet.

--
Still puzzled,
Robbie Hatley
Hatley [dot] Software [at] gmail [dot] com