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:
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?