MPSA42High-voltage NPN transistor

MPSA42 is a NPN bipolar transistor with a 300 V collector-emitter absolute rating and a 500 mA collector-current limit. These are separate stress limits. Dissipation and the operating point constrain how much voltage and current it can handle together. This reference selects Diotec MPSA42BK in the straight-lead TO-92 package: emitter 1, base 2 and collector 3 from the flat front. The separately rated MMBTA42 SMD alternative uses base 1, emitter 2 and collector 3. The pinout, physical package and footprint follow the selected part. The illustrated circuit deliberately uses only 5 V and a small LED load to explain base drive. Its calculated readings do not demonstrate high-voltage operation. High-voltage designs need their own clearance, insulation, transient and thermal review; the footprint alone does not provide that rating.