TIP120 Series Transistors

Description

A TIP120 Transistor is a pair of NPN (Negative-Positive-Negative) Bipolar Transistors, the most commonly used transistor configuration, which are configured as a Darlington Transistor.

The TIP120 Series Collector is connected to the positive (+ve) supply via the load. NB If the load is inductive i.e. has coil windings such as a relay, solenoid or motor, then it is usual to connect a diode across it to prevent the Back EMF from damaging the transistor. (Cathode to supply). The input is connected to the Base via a limiting resistor (typically 1K). The Emitter is connected to the negative (-ve) supply (0V).

Features and Applications

Uses: As current amplifiers, audio amplifiers and electronic switches, for example, Motor Drive Amplifiers, Relay Drive Amplifiers.

Data: The TIP 120 series transistors are Darlington Pairs i.e. they have 2 transistors in the one package, which gives them the advantage of higher power handling and gain.

• Voltage between Collector and Emitter 6 - 60 Volts max.
• Power handling capacity 60 Watts max.
• Current handling capacity 5 Amps max.
• Amplification (gain) greater than 1000.

Darlington Transistor Switch

The switching action of transistors is improved by combining them in a Darlington Pair configuration.

The "first" transistor acts as a signal amplifier which triggers the "second" transistor with a larger signal than the original input and thus improves its switching action. A base voltage of 1.2V is required to trigger the "pair".

Darlington Pairs are most commonly used in the form of Darlington transistors (eg TIP 120) where the combination is produced as a single package. These transistors are capable of handling high currents (e.g 5A) and are useful for driving motors etc. Multiples of Darlington Pairs, are available as I.C. packages (e.g. ULN 2803 contains 8 Darlington Pairs).

The purpose of the IN4001 Diode shown is to restrict any Back EMF resulting from the Transistor being used to switch an inductive device (e.g. Electric Motor, Solenoid or Relay).

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Note:

• When choosing a transistor it is important to make sure that the gain is high enough for the required use and that the maximum current which it can handle is sufficient to drive the output device.
• If the max Ic value is exceeded, the transistor will probably "burn out".
• I.C. Darlington Pairs can usually handle only small currents typically 550 mA.

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