What is c945 transistor? The Ultimate Guide

The C945 transistor, often designated as 2SC945, is a widely used bipolar junction transistor (BJT) known for its reliability in low-power applications. This article provides an in-depth analysis of its specifications, applications, and usage considerations.

1. Overview and Manufacturer Information

• Part Number: C945 (abbreviated from 2SC945, following Japanese JIS naming conventions).
• Type: NPN BJT (Bipolar Junction Transistor).
• Manufacturers: Originally produced by companies like Toshiba, now available from multiple semiconductor manufacturers (e.g., Fairchild, ON Semiconductor).

2. Key Specifications

• Collector-Base Voltage (VCBO): 60V.
• Collector-Emitter Voltage (VCEO): 50V.
• Emitter-Base Voltage (VEBO): 5V.
• Continuous Collector Current (IC): 150mA (max).
• Power Dissipation (Ptot): 250mW (at 25°C).
• DC Current Gain (hFE): 70–700 (varies with operating conditions and manufacturer).
• Transition Frequency (fT): 250MHz (typical for small-signal amplification).
• Package: TO-92 (common through-hole package).

3. Pin Configuration (TO-92 Package)

When viewing the flat side with pins downward:

1. Emitter (E).
2. Base (B).
3. Collector (C).

Note: Always verify pinout with the datasheet, as variations exist between manufacturers.

4. Applications

The C945 excels in low-power scenarios:

• Amplification: Audio preamps, RF stages, and signal processing.
• Switching: Relay drivers, LED drivers, and digital logic circuits.
• Oscillators: Used in radio and timing circuits.
• Consumer Electronics: Found in TVs, radios, and toys due to its cost-effectiveness.

5. Equivalent and Substitute Transistors

If unavailable, consider:

• 2N3904: Similar VCEO (40V) and IC (200mA).
• BC547: Comparable voltage ratings (45V VCEO) and gain.
• 2N2222: Higher current capability (800mA) but lower gain.
• KSC945: Direct equivalent with identical specs.

Caution: Verify substitutions match voltage, current, and gain requirements.

6. Thermal and Operational Considerations

• Temperature Range: -55°C to +150°C.
• Derating: Reduce power dissipation above 25°C (e.g., 2mW/°C decrease).
• Heat Management: No heatsink needed for low-power use; avoid prolonged operation near max ratings.

7. Common Usage Mistakes

• Incorrect Pinout: Reversing emitter and collector damages the transistor.
• Overloading: Exceeding 150mA collector current or 50V VCEO risks failure.
• Insufficient Base Current: In switching, ensure IB ≥ IC/hFE to saturate the transistor.

8. Testing the C945

Use a multimeter in diode mode:

1. Base-Emitter (B-E): Forward bias ~0.6V, reverse open.
2. Base-Collector (B-C): Forward bias ~0.6V, reverse open.
3. Emitter-Collector (E-C): Open in both directions.

9. Historical Context and Availability

• Introduced in the 1970s, the C945 became a staple in Japanese electronics.
• Still in production but less common in modern SMD-dominated designs; available through distributors like Digi-Key and Mouser.

10. Example Circuit

Simple LED Driver:

• Components: C945, 1kΩ resistor (base), LED, 5V supply.
• Operation: A microcontroller signal (3.3V/5V) to the base switches the LED via the collector-emitter path.

11. Datasheet Reference

• Always consult the manufacturer’s datasheet for precise specs (e.g., Toshiba 2SC945 Datasheet).

Conclusion

The C945 transistor remains a versatile, cost-effective solution for low-power amplification and switching. Its enduring presence in electronics underscores its utility, though designers should verify substitutes and adhere to operational limits for optimal performance.