12 V regulation in a MELF barrel
The 1N4742AUR is a 12 V zener diode from Microchip Technology, rated for 1 W continuous power dissipation in the DO-213AB MELF package. The ±5% tolerance on the nominal voltage means the actual zener knee sits between 11.4 V and 12.6 V at the test current — tight enough for reference or clamp duties where a few percent off the rail matters.
Leakage and impedance — the real-world limits
Reverse leakage is specified at 5 µA maximum with 9.1 V across the junction — that is 2.9 V below the zener knee, so the diode stays nearly open until the rail approaches regulation. The 9 Ohm maximum zener impedance (Zzt) at the test current sets the dynamic resistance: a 10 mA load step shifts the regulation point by roughly 90 mV, which is predictable for a 1 W part in this voltage class.
Temperature range and package
The DO-213AB MELF (metal electrode leadless face) package is a cylindrical surface-mount body with solderable end caps — it reflows on standard SMT lines but requires careful pad geometry to avoid tombstoning during reflow.