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BD139 NPN Medium Power Transistor

BD139 NPN Medium Power Transistor

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SpecificationDetails
Transistor TypeNPN (Bipolar Junction Transistor)
Package TypeSOT-32 / TO-126
Max Collector Current ($I_C$)1.5 Amps
Max Collector-Emitter Voltage ($V_{CEO}$)80 Volts
Max Collector-Base Voltage ($V_{CBO}$)80 Volts
DC Current Gain ($h_{FE}$)40 to 250
Max Power Dissipation ($P_{tot}$)12.5 Watts (with Heat Sink)
Operating Temperature-55°C to +150°C
                                 Price

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Barcode: 1012012

Product Description

The BD139 is an industry-standard NPN medium power transistor designed for audio amplification and switching applications. Housed in the TO-126 (SOT-32) plastic package, it is specifically engineered to handle significantly higher currents than small-signal transistors like the BC548. With a 1.5A collector current rating and an 80V breakdown voltage, the BD139 is a versatile choice for driving DC motors, high-power LEDs, and relay coils. In the world of high-fidelity audio, it is frequently used as a driver stage or in the output stage of low-power amplifiers due to its excellent linearity and gain characteristics.

Key Features

  • Medium Power Capability: Can drive loads up to 1.5A, bridging the gap between small-signal and high-power transistors.

  • High Collector-Emitter Voltage: Rated at 80V, making it suitable for circuits operating on higher voltage rails.

  • TO-126 Mounting: The flat package includes a mounting hole for easy attachment to a heat sink for improved thermal management.

  • Excellent Complementary Pair: Pairs perfectly with the BD140 (PNP) for push-pull amplifier designs.

  • Robust Construction: Designed for reliability in industrial controls and consumer electronics.

  • Wide Gain Range: Provides stable amplification across a broad range of collector currents.

Technical Details

  • Pin 1: Emitter

  • Pin 2: Collector

  • Pin 3: Base

  • Mounting Hole: Compatible with M3 screws.

Prototyping and Engineering Tips from Robogears

  • Pinout Awareness: CRITICAL: The BD139 pinout is different from the common TO-92 package. Looking at the front (the side with the part number) with pins pointing down, the sequence is Emitter (1), Collector (2), Base (3). Always double-check your PCB footprint to avoid reversing the Emitter and Base.

  • Thermal Management: While the BD139 is rated for 12.5W, it can only reach this limit if attached to an appropriate Heat Sink. Without a heat sink, the plastic package can only dissipate about 1.25W. If the transistor feels hot to the touch during operation, you must add cooling to prevent thermal runaway.

  • Inductive Load Protection: When using the BD139 to switch motors or solenoids, the back-EMF spike can exceed the 80V rating. Always place a Flyback Diode (like a 1N4007 or 1N5408) across the inductive load to protect the collector-emitter junction.

  • Base Drive Calculation: To ensure the transistor fully switches "ON" (reaches saturation) for a 1A load, you typically need a base current of about 50mA to 100mA (assuming a forced beta of 10 or 20). Ensure your microcontroller can provide this, or use a small signal transistor (like a BC547) in a Darlington configuration to drive the BD139.

  • Audio Biasing: When used in audio amplifiers, the BD139’s VBE​ (Base-Emitter voltage) will drop as it gets hot. To prevent thermal distortion or damage, engineers often mount a sensing diode or another transistor on the same heat sink to compensate for this temperature-related voltage shift.

Package Includes

  • 1 x BD139 NPN Medium Power Transistor

Disclaimer: Product images are shown for illustrative purposes only. The specific brand markings (ST, ON Semi, etc.) or the "gain group" suffix (e.g., BD139-10 or BD139-16) may vary between manufacturing batches, but all technical specifications and 1.5A/80V performance standards remain as described.

Safety Warning: Power transistors can reach temperatures exceeding 100°C; avoid touching the component or the heat sink during high-load operation. Always verify the circuit isolation if using a metal heat sink, as the Collector (Pin 2) is often electrically connected to the metal mounting tab.

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