The STD840DN40 is a dual NPN bipolar transistor in a single 8-DIP through-hole package, rated for 400V collector-emitter breakdown and 4A continuous collector current. Maximum power dissipation is 3W, and the junction temperature rating of 150°C (TJ) suits it for enclosed power supplies or motor-drive auxiliaries with adequate heatsinking.
The 400V Vceo sets the off-state blocking voltage — adequate for rectified 240VAC rails (≈340V DC) with derating, but tight for universal-input off-line converters where the bulk rail can exceed 375V. The 4A Ic rating covers moderate-load stages: a 60W flyback primary, a solenoid driver, or a 2A buck regulator switch. Vce(sat) is specified at 1V maximum at 400mA base drive and 2A collector current — a fairly high saturation voltage that increases conduction loss, so the base drive needs to be sized for the 8 minimum DC current gain (hFE) at 2A / 5V. The 250µA collector cutoff maximum is typical for a high-voltage bipolar and acceptable in most switched-mode circuits, but a design with a high-impedance off-state should confirm the leakage doesn't exceed the budget at elevated junction temperature.
The manufacturer lists the STD840DN40 as Obsolete. No successor order code is recorded in the available lifecycle data, so there is no factory drop-in replacement to migrate to.