Description
The TIP42C is a medium-power PNP power transistor frequently used for industrial, automotive, and general-purpose switching and amplification.
Important Technical Clarification: While you referred to it as a Darlington, the TIP42C is a standard (single-junction) PNP power transistor, not a Darlington pair. A true PNP Darlington equivalent would be the TIP127.
Key Specifications
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Transistor Type: PNP
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Collector-Emitter Voltage ($V_{CEO}$): 100 V
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Collector Current ($I_C$): 6 A (Continuous)
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Power Dissipation ($P_D$): 65 W (when properly heatsinked)
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DC Current Gain ($h_{FE}$): 15 to 75 ($I_C = 0.3 \text{A}$)
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Package Type: TO-220
Pinout Configuration
When looking at the front of the TO-220 package (the metal tab is at the top) with the pins pointing down, the pins from left to right are:
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Base
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Collector
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Emitter
Comparison: TIP42C vs. Common Small-Signal Transistors
Unlike the BC557 or 2N3906, the TIP42C is designed to handle high current and heat, making it suitable for driving power-hungry components.
| Feature | TIP42C (Power) | BC557 (Small-Signal) |
| Max $I_C$ | 6 A | 0.1 A |
| Max Power ($P_D$) | 65 W | 0.5 W |
| Package | TO-220 (needs heat sink) | TO-92 (no heat sink) |
Common Applications
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Motor Drivers: Used in H-Bridge circuits to control the direction and speed of DC motors.
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Audio Power Amplifiers: Acts as the output stage transistor for medium-power audio systems (often paired with the TIP41C NPN).
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Power Regulation: Used in linear voltage regulators or high-current switching circuits.
Essential Design Requirements
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Heatsinking: Because this transistor is rated for 65W, it will overheat and fail almost instantly if used at high power without a sufficient aluminum heatsink. Always mount it with thermal grease and a mounting kit.
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Driving the Base: As a standard power transistor (not a Darlington), it has a relatively low $h_{FE}$ (gain). To switch 6A, you need to provide a significant amount of base current. You will likely need to use a smaller transistor (like a BC557 or 2N3906) as a “driver” to provide enough current to the TIP42C’s base.
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Safe Operating Area (SOA): Ensure you are operating within the manufacturer’s SOA curves, especially when switching inductive loads, to prevent secondary breakdown of the silicon.
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