2N3055-MJ2955 Complementary silicon power transistors.
Plate 1 2N3055-Mj2955 Complementary silicon power transistors.

Using Darlington Transistors Driving 2N3055-MJ2955 Power Transistors

by Lewis Loflin



2N3055 (NPN), MJ2955 (PNP): Complementary silicon power transistors are designed for general-purpose switching and amplifier applications. They can carry an impressive 15 Amps of collector current making them ideal for high power switching applications such as motor drivers for use in power inverters.

But they suffer two problems that plague most high-power bipolar transistors - low hfe or DC current gain. That means they can be directly connected to micro-controllers due to high drive currents required and in the case of the MJ2955 high voltage on the base can't connect directly to 5-volt TTL logic.

2N3055 transistor switch must have pre-driver.
Plate 2 2N3055 transistor switch must have pre-driver.

This problem is illustrated in plate 2 with the 2N3055 driving a 10 Amp DC motor. The low hfe of 20 means we require a base-emitter current of at least 500mA - far above the limit of most micro-controller I/O pins limited to 20-40mA. We must have a pre-driver.

TIP120 and TIP125 Complementary Darlington power transistors.
Plate 2

An excellent choice for a pre-drivers for both the 2N3055 and Mj2955 are the TIP120 and TIP125 complementary Darlington power transistors. While rated at 5 Amps collector current they can provide the needed current gain and voltage isolation for the PNP Mj2955. Plate provides the basic data for the TIP120 and TIP125 power Darlington transistors.

2N3055 NPN power transistor with TIP120 pre-driver.
Plate 3

In plate 3 we use a TIP120 NPN Darlington to drive a 2N3055 NPN power transistor. Variations of the above circuits are used to construct H-Bridge motor controls, relay and contactor drivers, etc.

TIP125 Darlington transistor driving a MJ2955.
Plate 4

In plate 4 we have a power TIP120 Darlington driving an even higher power Mj2955 PNP transistor. An additional benefit is isolating the 23 volts on the base of Q2 form the 5-volt TTL logic.

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