Darlington Pairs

Sometimes you want to control a really large amount of current; more than a single transistor's gain can provide. I.e. the output current has to be more than the input current multiplied by the transistor's gain.

In this case we can combine two transistors in what is called a Darlingtion connection. In this configuration the gain of the pair is very slightly more than the product of the gain on the individual transistors and can be 1000 or more. Specifically, its the product of the gains plus the sum of the gains, but the product is usually high enough that the sum can be ignored for calculations.

At its core, a Darlington pair is simply two transistors cascaded together:

components_322px-Darlington_pair_diagram.png
CC BY-SA 3.0 by wikipedia user Michael9422

A common Darlington pair is the TIP120 which is a great choice for the examples later in this guide. It can handle up to 5A continuously (with bursts of 8A) and up to 60V across the collector and emitter. It has a gain of ~1000.

components_tip120.png
From the TIP120 datasheet: http://www.adafruit.com/datasheets/TIP120.pdf

A TIP120 is just the circuit above packaged in a convenient 3-pin package (base, collector, and emitter). Since it's meant for power switching/amplification it comes with a metal backplate to which a larger heatsink can be attached.

TIP120 Power Darlington Transistors - 3 pack

PRODUCT ID: 976
Transistors are powerful little electronic switches, and when our little NPN transistors aren't power enough for your project, we have been known to use these beefy TIP120 Darlington...
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The "drive" connections of the CRICKIT are, in fact, darlington drivers. They are all on a single IC, called a ULN2803, with 7 or 8 Darlington's each with the following design:

components_uln2003A-driver.png
From the ULN2003 datasheet

This chip is very convenient to use as it provides everything needed: all the biasing resistors, and all the protection diodes needed for inductive loads such as electromagnets, solenoids, and relays.

The CRICKIT uses 4 of these drivers, though the chip contains 7. There are a variety of versions of this design, some with 8 drivers in a slightly larger chip.

ULN2803: 8 Channel Darlington Driver (Solenoid/Unipolar Stepper)

PRODUCT ID: 970
Bring in some muscle to your output pins with 8 mighty Darlingtons! This DIP chip contains 8 drivers that can sink 500mA from a 50V supply and has kickback diodes included inside for...
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This guide was first published on Sep 12, 2018. It was last updated on Sep 12, 2018. This page (Darlington Pairs) was last updated on Dec 05, 2019.