The 2N5551 is a high-voltage NPN bipolar junction transistor (BJT) designed for low-power amplification and switching applications. Its high collector-emitter voltage rating makes it suitable for circuits that operate at higher voltages while requiring relatively low current.

Supplied in a compact TO-92 through-hole package, the 2N5551 is commonly used in audio and signal amplifiers, high-voltage driver circuits, telecommunications equipment, switching circuits, and general electronic projects. Its complementary PNP transistor is the 2N5401.

Pin Configuration

A commonly used TO-92 pin configuration is:

Pin 1 – Emitter: NPN emitter terminal.
Pin 2 – Base: Controls transistor conduction.
Pin 3 – Collector: NPN collector terminal.

Important: TO-92 pin arrangements can vary between manufacturers. Always verify the datasheet for the exact 2N5551 supplied before installation or PCB assembly.

Technical Specifications

• Transistor Type: NPN Bipolar Junction Transistor (BJT)
• Package: TO-92
• Collector-Emitter Voltage (VCEO): 160V Max.
• Collector-Base Voltage (VCBO): 180V Max.
• Emitter-Base Voltage (VEBO): 6V Max.
• Continuous Collector Current (IC): Up to 600mA Max.
• DC Current Gain (hFE): Approx. 80 at IC = 10mA under specified test conditions
• Transition Frequency (fT): Approx. 100MHz
• Mounting Type: Through-Hole
• Complementary PNP Transistor: 2N5401
• Typical Use: High-Voltage Switching and Signal Amplification

Using the 2N5551 as an Amplifier

The 2N5551 is well suited for small-signal and low-power amplification. In its active region, a relatively small change in base current controls a larger collector current.

The DC current gain can be expressed as:

hFE = IC / IB

where:

IC = Collector current
IB = Base current

The emitter current is related to the collector and base currents by:

IE = IC + IB

Actual current gain is not fixed and varies with collector current, collector-emitter voltage, temperature, and individual transistor characteristics.

Using the 2N5551 as a Switch

The 2N5551 can also be used as an electronic switch in applications requiring a higher voltage rating than many standard general-purpose NPN transistors.

When used as a switch, it normally operates between the cut-off and saturation regions.

A suitable base resistor should be used to limit base current. When controlled by a microcontroller or logic circuit, the transistor's voltage, current, power dissipation, and interface requirements should be considered carefully.

Complementary Pair with 2N5401

The 2N5551 NPN and 2N5401 PNP transistors form a commonly used complementary high-voltage transistor pair.

They can be used together in amplifier stages, complementary driver circuits, signal processing circuits, switching circuits, audio electronics, and other designs requiring matched NPN and PNP device types.

Applications

The 2N5551 is commonly used in low-power amplifiers, audio amplifiers, small-signal amplification, high-voltage switching circuits, current amplifiers, signal boosters, driver circuits, telecommunications equipment, complementary amplifier stages, and general-purpose electronic projects.

It may also be used as part of suitable Darlington transistor configurations.

Important Notes

The 600mA collector current is an absolute maximum rating, not necessarily a recommended continuous operating current. Safe operating current depends on collector-emitter voltage, power dissipation, temperature, thermal conditions, and the specific manufacturer's safe operating area.

The 6V specification refers to the maximum emitter-base reverse voltage (VEBO). It is not the normal forward base-emitter operating voltage. During normal conduction, a silicon NPN transistor typically has a forward base-emitter voltage around 0.6-0.7V, depending on operating conditions.

When switching inductive loads such as relays, solenoids, or motors, appropriate transient protection such as a flyback diode should be used.

Always verify the electrical ratings and pin configuration against the datasheet for the exact manufacturer supplied.

Package Includes

1 × 2N5551 NPN Transistor