The Infineon BC 817-40 E6433 is a general-purpose NPN bipolar junction transistor in a SOT-23 surface-mount package. It is rated for a collector current of 500 mA and a collector-emitter breakdown voltage of 45 V, with a DC current gain (hFE) of minimum 250 at 100 mA collector current and 1 V Vce. The transition frequency of 170 MHz supports switching and amplifier circuits up to the low-MHz range. Typical applications include load drivers, relay and solenoid drivers, and linear amplification stages in industrial control, consumer electronics, and telecom equipment where the SOT-23 footprint saves board area.
The 500 mA collector current and 45 V Vce breakdown cover many low-to-medium power switching and linear applications, but the SOT-23 package limits total power dissipation to 330 mW — so continuous current near the 500 mA limit needs a derating check against the thermal resistance of the board. The 170 MHz ft is adequate for audio-frequency amplifiers and moderate-speed switching (e.g., 1–10 MHz clock or PWM lines), but not for RF power stages. The 700 mV saturation voltage at 50 mA base current and 500 mA collector current is typical for a small-signal transistor; it means the device will dissipate about 350 mW when fully saturated at 500 mA — right at the package limit, so thermal margin is thin.
The BC 817-40 E6433 is listed as obsolete. Stock is finite and declining. For a BOM that still specifies this exact order code, the sourcing path is to locate remaining inventory through brokers or independent distributors, verify date-code consistency and marking authenticity, and qualify the lot against the original datasheet. No official replacement part is listed in the available lifecycle data, so a drop-in alternate must be identified by matching the SOT-23 footprint, NPN polarity, and the key parametric limits: 500 mA Ic, 45 V Vce, and hFE grade (the -40 suffix indicates a gain range of 250 to 600 at 100 mA).