Greeetings, group. Lately I've been repairing motor drive boards
for variable-frequency 3-phase AC motors. (These typically use
a diode bridge to rectify 120VAC to 160VDC, then feed the 160VDC
into a network of 6 IGBTs to generate the variable-frequency AC
to power the motor.)

At least 2 of these motor drive boards (one from Advanced Drive
Technology, one from Hyundai) also have switching power supply
circuits for generating low voltages needed (+5V, +12V, -8V, etc),
and these circuits all use a transistor switching a transformer,
with a feedback circuit using a P181 optocoupler and an unknown
device (dual zener?) labeled just "3B". The "3B" device is
designated "ZD3", so I'm assuming it's a dual zener diode, but
I haven't been able to identify it. It looks like this:

http://www.well.com/~lonewolf/3B.jpg

It's surface-mount, with the tab and all 3 pins soldered to the board.
Pin 2 and the tab are connected together internally, and connect to
ground externally.

With my meter set to "Diode", i'm seeing a diode from p1 to p3
and a diode from p2 to p3. The reading between p1 and p2 is
indeterminant.

Anyone have an idea of what this part is?

--
Puzzled,
Robbie Hatley
hatley [dot] software [at] gmail [dot] com