TIP120 and TIP125
Plate 1

Tutorial TIP120-TIP125 Power Darlington Transistors

by Lewis Loflin



In this tutorial we will continue to explore basic transistor theory by designing and building our own Darlington transistor switch. This will be done with separate transistors as opposed to a single 3-pin unit. We will have a negative common and the control voltage will be 5-volt TTL level.

Darlington transistors are built from two or more bipolar transistors and thus are current operated devices. They have a DC current gain (hfe) often over 1000.

Instead of separate transistors both are combined in a single case and may include other components such as noise suppressor diodes and speedup resistors.

In plate 1 illustrates the packaging and internal circuitry of the TIP120 and TIP125. They are known as a complimentary pair having the same electrical specifications but opposite semiconductor polarity. They are rated at 60V, 5A, and a DC gain (hfe) of 1000.

4N29-4N33 Darlington Transisto opto-coupler.
Plate 2

Because of their very high current gain Darling transistor amplifiers are used to construct sensitive opto-coupler photo transistors such as those in the 4N29.

In the case of the ULN2003A Darlington transistor array we have 7 Darlington transistor amplifiers in a single 16-pin DIP package.

See Tutorial ULN2003A Darlington Transistor Array.

Basic Darlington transistor switching circuit.
Plate 3

What is a Darlington Transistor?

A Darlington uses at least two bipolar transistors where the collectors are tied together, the emitter of smaller transistor is tied to the base of larger transistor, while circuit connections are made to the emitter from the larger transistor and the base of the smaller transistor is the input.

This is done to obtain higher power gain than a single transistor can provide. The current gain is the product of the hfe of each individual transistor, while most of the current is carried by the larger transistor.

Basic TIP120 Darlington transistor switching circuit.
Plate 4

Illustrated in plate 4 is the typical connections to an Arduino or other micro-controller for a TIP120 NPN Darlington transistor. With a hfe of 1000 very little current is needed from the micro-controller to turn the motor on-off.

While in this case 1mA input my suffice to drive the motor at 1A extra current should be used (within limits) to assure complete cut on of the TIP120.

Basic Darlington transistor switching circuit connections.
Plate 5

Plate 5 illustrates how the electrical connections differ between the TIP120 and TIP125 with a negative ground system. Note the high voltage present on the base of Q1.

Basic TIP125 Darlington transistor switching circuit.
Plate 6

Plate 6 illustrates the connections for using the TIP125 PNP Darlington transistor with a 5-volt micro-controller. Because of it's opposite electrical polarity from the TIP120 an extra NPN low power driver must be used. This is because the voltage on the base is greater than 5-volts and will damage any 5-volt micro-controller.

Q2's gain of 100 can be multiplied by the gain of Q1 for a total hfe of 100,000. 2.2K is a good value to assure proper switching and limiting the Ib of the TIP120.

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