BD678 Pinout, Equivalent, Applications, Datasheet

The BD678 is a high-gain PNP Darlington power transistor in a TO-126 package. This guide explains the BD678 pinout, datasheet, and working principle for amplifiers, regulators, and control circuits.

Introduction to BD678 PNP Transistor

The BD678 is a high-gain PNP Darlington power transistor designed for medium-power amplification and switching applications. It belongs to the BD67x transistor family, which also includes the complementary BD677 (NPN).

Encased in the TO-126 package, BD678 offers excellent heat dissipation and compact design, making it suitable for audio amplifiers, relay drivers, and voltage regulators.

Being a Darlington pair transistor, the BD678 internally combines two PNP transistors to achieve very high current gain, allowing control of larger loads with minimal base current. This feature makes it highly effective in power control, industrial automation, and consumer electronics where efficiency and reliability are critical.

BD678 PNP Transistor

Pinout of BD678

Understanding the BD678 Pinout Configuration

The BD678 follows the same standard pin layout as other BD67x devices. Knowing the correct pin configuration ensures proper installation and avoids circuit damage.

Pin Configuration of BD678 Pinout

Key Features of BD678 Transistor

• PNP Darlington configuration for high current amplification
• Capable of switching medium-power loads efficiently
• High DC gain and stable thermal operation
• Low base drive current required for high output control
• Complementary to BD677 (NPN) transistor
• Suitable for linear and switching circuit applications
• Compact and durable TO-126 plastic package

BD678 Transistor Datasheet and Specifications

• Collector-Emitter Voltage (Vceo): 60 V
• Collector-Base Voltage (Vcbo): 60 V
• Emitter-Base Voltage (Vebo): 5 V
• Collector Current (Ic): 4 A
• Power Dissipation (Ptot): 40 W
• DC Current Gain (hFE): 750 minimum
• Transition Frequency (fT): 3 MHz typical
• Junction Temperature (Tj max): 150 °C
• Package Type: TO-126
• Polarity: PNP Darlington

According to the BD678 datasheet, this transistor offers balanced performance between voltage rating, gain, and current handling. It’s widely used in complementary amplifier circuits and control systems where precise signal control and power efficiency are essential.

Working Principle of BD678 PNP Transistor

The BD678 PNP transistor functions using a Darlington pair configuration, where two PNP transistors are internally connected for high gain amplification. The emitter of the first transistor drives the base of the second, resulting in extremely high combined gain (β ≈ β₁ × β₂).

When a small negative voltage is applied to the base relative to the emitter, the transistor turns ON, allowing a large current to flow from emitter to collector.

This high-gain design makes BD678 ideal for applications where a small input current must control a large output load, such as in amplifiers, relay controls, and power stages of regulators or drivers.

More Circuit Layouts

Typical Applications of BD678

• Audio amplifier driver and output stages
• Relay and solenoid control circuits
• Voltage and current regulation circuits
• Motor driver and controller systems
• Power control modules and switching supplies
• Complementary push-pull stages with BD677
• Industrial automation and load switching systems

NPN Complementary Transistor

The NPN complementary transistor of BD678 is BD677.
Both are designed as a complementary Darlington pair set, ideal for push-pull amplifier designs. When combined, BD678 handles the negative half-cycle while BD677 manages the positive half-cycle, providing smooth and efficient AC amplification with minimal distortion.

Equivalent Transistors and Alternatives

If BD678 is not available, the following equivalent or near-equivalent PNP Darlington transistors can be used depending on voltage and package requirements.

Equivalent Transistors:

• BD680 (higher voltage version)
• TIP127 (PNP Darlington, TO-220 package)
• MJE802 (PNP Darlington equivalent)
• 2N6036 (PNP Darlington transistor alternative)

Before substitution, always compare the datasheet values for Vceo, Ic, and hFE to ensure compatibility and safe operation.

Comparison BD678 vs BD677 Complementary Transistors

The BD678 (PNP) and BD677 (NPN) are complementary Darlington transistors intended for symmetrical amplifier and switching circuits.

The BD678 conducts when its base is driven negatively relative to its emitter, while the BD677 conducts with a positive base drive. Together, they form efficient push-pull configurations, minimizing distortion in audio and signal processing systems.

Both have the same voltage rating (45 V), current capacity (4 A), power dissipation (40 W), and gain (≥750), making them perfect partners in dual-supply or AC signal environments.

Frequently Asked Questions (FAQ)

What is the BD678 transistor used for?

BD678 is used in medium-power applications such as amplifiers, voltage regulators, relay drivers, and control systems.

What type of transistor is BD678?

BD678 is a PNP Darlington power transistor in a TO-126 package.

What is the BD678 pin configuration?

Facing the flat side, the pins from left to right are Emitter (1), Collector (2), Base (3).

What is the complementary transistor for BD678?

The complementary device for BD678 is the BD677 (NPN) Darlington transistor.

Can I replace BD678 with TIP127?

Yes, TIP127 can serve as a replacement, but it uses a TO-220 package with higher power handling, so adjust the heat sink accordingly.

Conclusion

The BD678 PNP Darlington transistor is a reliable and efficient choice for medium-power switching and amplification tasks. With high gain, low base drive requirements, and a compact TO-126 package, it’s suitable for a wide range of audio, control, and automation circuits.

When paired with its NPN complement BD677, it forms an ideal push-pull amplifier configuration that ensures linear performance and energy efficiency in complex circuit designs.

Datasheet & Pinout of BD678 NPN Transistor

Click the following Button to download the datasheet of BD678 Transistor :

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