The 1N4730AUR is a 3.9 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 3.9 V breakdown means the actual Zener voltage for any given unit sits between 3.705 V and 4.095 V — tight enough to clamp a 3.3 V logic rail or provide a stable reference for a low-voltage bias circuit without an external trim. The 9 Ohm maximum Zener impedance (Zzt) at the test current means the regulation slope is shallow — a 10 mA change in current shifts the output voltage by roughly 90 mV, which is acceptable for a shunt regulator or protection clamp but not for a precision reference without a buffer.
Leakage and forward drop — the second-order specs that matter
Reverse leakage is specified at 50 µA maximum at 1 V reverse bias — well below the breakdown knee, so this is the leakage before the diode starts conducting. At 25°C the actual leakage is typically much lower, but the 50 µA ceiling is the number to budget for a high-impedance node that sits below the Zener voltage. Forward voltage is 1.2 V maximum at 200 mA forward current. If the diode is used as a general-purpose rectifier or flyback clamp in the forward direction, the 1.2 V drop at 200 mA means the power dissipation in forward conduction adds to the thermal budget — the 1 W total power limit applies to both forward and reverse operation combined.