2N3904 NPN Transistor (40V 0.2A)

Overview: 2N3904 NPN Transistor

The 2N3904 NPN transistor is a popular general-purpose bipolar junction transistor (BJT) engineered for low-power switching and small-signal amplification. With a 40V collector-emitter rating and 200 mA collector current capability, it is a dependable choice for microcontroller interfaces, LED drivers, sensor conditioning, and learning projects.

Key Features

• General-purpose NPN transistor suitable for switching and amplification
• Collector-emitter voltage (VCEO): 40 V
• Maximum collector current (IC): 200 mA
• Low power dissipation: up to 625 mW
• Fast switching speed for digital circuits and signal processing
• TO-92 through-hole package for easy breadboard and PCB use

Specifications

• Type: NPN bipolar junction transistor
• VCEO (Collector-Emitter Voltage): 40 V
• IC (Collector Current): 200 mA
• Power Dissipation: up to 625 mW (device dependent on mounting and ambient temperature)
• Package: TO-92, leads: Emitter, Base, Collector (pinout may vary by manufacturer)
• Typical applications: switching, amplification, buffering

Applications

• Low-power switching circuits (LEDs, buzzers, small relays with appropriate drivers)
• Microcontroller output expansion and interfacing
• Small signal audio amplification and preamplifiers
• Sensor interfaces and signal conditioning
• Prototyping, hobby electronics, and educational projects

Design and Usage Tips

Using the 2N3904 as a Switch

• For saturated switching, choose a base resistor so the base current Ib is roughly IC/10 to IC/20 depending on required saturation and transistor hFE.
• Example base resistor calculation: Rb = (Vcontrol – Vbe) / Ib, where Vbe is typically about 0.7 V and Ib = IC / hFE(for saturation use a conservative hFE of 10).
• Use a flyback diode with inductive loads like relays or solenoids to protect the transistor from voltage spikes.

Amplifier Considerations

• For small signal amplification, bias the transistor in its active region with proper resistor networks and coupling capacitors.
• Be aware that hFE varies with collector current and temperature. Check the specific datasheet for gain curves.

Packaging and Handling

The 2N3904 comes in a TO-92 through-hole package that is convenient for hand soldering, breadboarding, and PCB mounting. Observe standard ESD precautions when handling small-signal transistors. Store in a dry environment to prevent corrosion of leads.

Why Choose the 2N3904

• Proven, widely available, and cost effective for hobbyists and professionals
• Versatile performance for both analog and digital low-power applications
• Easy to integrate into prototypes and educational circuits

Ordering and Notes

Commonly sold individually or in multi-packs. Check the manufacturer datasheet for detailed electrical characteristics and absolute maximum ratings before finalizing designs.

Note: Images are for illustration purposes only.