12 V shunt regulation with 1 W headroom
The 1N4742PE3/TR12 is a 12 V Zener diode rated for 1 W continuous power dissipation in the standard DO-204AL (DO-41) axial-lead package. The 12 V nominal voltage and 1 W power ceiling define its shunt-regulation envelope: at 12 V the diode can sink roughly 83 mA before hitting the power limit, so the series resistor must be sized to keep the diode current below that threshold across the full input-voltage and load-current range. Tolerance is ±10%, meaning the actual Zener voltage of any given unit falls between 10.8 V and 13.2 V. For precision rails this spread may require a tighter bin; for generic overvoltage clamping or crude reference it is usually acceptable.
Dynamic impedance and leakage — the real-world regulation story
Maximum dynamic impedance (Zzt) is 9 Ohms. This figure governs how much the Zener voltage shifts when the diode current changes: a 10 mA swing through the diode shifts the output by up to 90 mV. For a supply rail that needs tighter regulation, a lower-impedance Zener or a shunt reference IC would be a better fit. Reverse leakage is specified at 5 µA maximum at 9.1 V, well below the Zener knee. This confirms the diode stays off until the applied voltage approaches the nominal 12 V threshold, important for battery-powered circuits where standby current matters. Forward voltage is 1.2 V maximum at 200 mA, a typical silicon junction drop.
Package, tape-and-reel, and board-fit
The DO-204AL (DO-41) axial package is the through-hole workhorse for 1 W Zener diodes. Lead spacing is standard 0.300-inch (7.62 mm) hole pitch on a PCB; the body diameter is approximately 2.7 mm. The TR12 suffix indicates tape-and-reel packaging — 12 mm tape width, suitable for automated insertion.