BD139 and BD140: pinout, ratings, a 5 V pin switch, heat, and the push-pull pair

• The gain is high only in the middle of the range. At 150 mA onsemi guarantees 40 to 250; at 5 mA and at 0.5 A the floor drops to 25.
• The BD140 comes in two gain grades, BD14010 and BD14016. Sellers often print the grade as a suffix (BD140-10, BD140-16).
• ST's BD139 sheet would have been the second source, but it timed out from our server, so this page uses onsemi's figures only.

Say a 5 V microcontroller pin has to switch a 150 mA load on a 12 V rail: a small fan, a relay coil or a short LED strip.

• Base current: with the minimum gain of 40, 150 mA needs at least 3.8 mA. Double it for a firm switch: 7.5 mA.
• Base resistor: take the worst-case base-emitter drop of 1 V, so R ≤ (5 − 1) ÷ 7.5 mA = 533 Ω. The next standard value down is 470 Ω, which gives at least 8.5 mA of base current, a forced gain of about 18.
• That is inside what an ATmega328P pin is specified for (20 mA; see the ATmega328P page).

The trouble starts near the top of the BD139's current range. At 0.5 A the gain floor is 25, so the base needs at least 20 mA, and onsemi specifies the 0.5 V saturation limit with 50 mA of base drive. No microcontroller pin supplies that. Above a couple of hundred milliamps, drive the BD139 from a small transistor such as a BC547, or use a logic-level MOSFET.

Switched fully on, the BD139 stays cool: at 0.5 A and 0.5 V it makes at most 0.25 W. Used as a linear pass element it runs hot. Say it drops 12 V to 5 V at 0.3 A inside a box at 40 °C:

• It turns (12 − 5) × 0.3 = 2.1 W into heat.
• In free air onsemi allows 1.25 W at 25 °C, less 10 mW for each degree above, so 1.1 W at 40 °C. Without a heatsink, 100 °C/W would put the junction at 250 °C, far past the 150 °C limit.
• With a heatsink, the whole path from junction to air may total (150 − 40) ÷ 2.1 = 52.4 °C/W. The junction-to-case part takes 10 °C/W, so the heatsink and its insulating pad together must stay under 42 °C/W. A small clip-on TO-126 or TO-220 sink is usually in that range; check its own data.

The BD140 is the PNP mirror: −80 V, −1.5 A (−3 A pulsed), the same 12.5 W and 1.25 W ratings, and the same E-C-B pin order. Its collector-base rating is 80 V where the BD139 has 100 V. onsemi has stamped its BD136/D sheet DISCONTINUED, not for new design; the BD139G sheet carries no such mark.

A BD139 on the low side and a BD140 on the high side make a push-pull output stage for audio or a motor. Say it runs on ±15 V into an 8 Ω speaker:

• A full 15 V peak would need 1.88 A, past the 1.5 A rating. Keep the peak under 12 V (1.5 A × 8 Ω), which is about 9 W of sine output.
• The transistor that is off sees up to 30 V, well inside 80 V.
• In a class-B stage the worst case for each transistor is about Vcc² ÷ (π² × R) = 2.85 W, more than the 1.25 W free-air rating. Both need a heatsink.

• Check the pin order before soldering. Hold the part with the printed face toward you and the leads down: E, C, B from the left. A meter on diode test reads a junction from B to C and B to E (red probe on B for the BD139, black on B for the BD140).
• Fit a flyback diode across a relay coil or motor. The diode comparison says which one.
• Put a resistor of about 10 kΩ from base to emitter when the drive can float, so the transistor stays off while a microcontroller boots.
• For anything that needs more than about 1.25 W, plan the heatsink first: the transistor is small, so the limit is usually heat rather than current.

We'd use a BD139 for loads from about 100 mA to 1 A where a TO-92 part like the BC547 is too small and a TO-220 is more than needed, with a BC547 in front of it above a couple of hundred milliamps. For a new push-pull design we'd check the BD140 is still made by the maker we buy from, since onsemi has discontinued its own.

The BJT as a switch lesson works through base resistors from the start, transistor packages and heat covers the heatsink sum, and the push-pull output stage explains the BD139/BD140 pair. For a bigger part, see the TIP120.