Medium-power NPN for through-hole designs
The BD139 is an NPN epitaxial planar transistor rated for 80 V collector-emitter breakdown and 1.5 A continuous collector current, with a maximum power dissipation of 1.25 W in the TO-126-3 package. It is a general-purpose switching and linear amplifier transistor — common in relay drivers, solenoid drivers, linear regulators, and audio output stages where a through-hole part with a bolt-down tab is preferred over an SMD alternative.
The 80 V collector-emitter breakdown voltage means the BD139 can switch or regulate rails up to about 48 V with margin — a 24 V or 48 V industrial supply, a 36 V battery string, or a 12 V automotive rail are all within its safe operating area. The 1.5 A continuous collector current rating supports loads like a medium-sized relay coil (typically 100–300 mA), a small DC motor (500 mA–1 A stall), or the pass element in a 12 V linear regulator delivering up to 1 A. Saturation voltage is 500 mV maximum at 500 mA collector current with 50 mA base drive — a typical switching loss figure for this class of transistor; for lower-loss switching at similar current, look at a Darlington or a MOSFET.
Thermal budget in a TO-126 tabbed package
Maximum junction temperature is 150°C, and the 1.25 W power dissipation is specified with the device in free air — bolting the tab to a heatsink or chassis panel can roughly double the usable dissipation, depending on the thermal interface and airflow. The TO-126-3 package (also known as SOT-32-3) has a metal tab that is electrically connected to the collector — the heatsink must be electrically isolated or the collector rail must be at chassis ground potential.
No official successor or second-source is listed on the record.