The STTH3R04 is a 400 V, 3 A fast-recovery rectifier from STMicroelectronics, built with standard planar technology and housed in a through-hole DO-201AD axial package. Its 35 ns reverse recovery time puts it in the class used for high-frequency rectification in switch-mode power supplies, PFC stages, and flyback converters where the diode's turn-off speed directly affects switching losses and EMI. The 1.5 V forward drop at 3 A is a typical figure for this voltage and speed class — expect it to run warm in continuous 3 A service, so derate the current or add a heatsink if the ambient climbs above 85 °C.
ST officially lists the STTH3R04 as Obsolete. That means no further production runs, no last-time-buy window still open. The 400 V / 3 A / 35 ns trr combination is common enough that a pin-compatible replacement from the same STTH3R family may exist — but no official successor is recorded.
DO-201AD — field-swap friendly
The DO-201AD axial package is about as field-serviceable as a diode gets. Two solder points, no polarity confusion if the cathode band is still visible — you can swap it on site with a basic iron and a solder sucker. No hot-air station, no reflow profile. The through-hole leads also tolerate a bit of mechanical stress during extraction, which matters when the board is already installed in a panel or a drive cabinet.