Comments about RadioShack TIP120 NPN-D Transistor:

This Darlington is both sensitive and powerful: A rare combination. For typical parts, the current gain is somewhere between 1,000 and 2,500! Compare that to a 2N3055, with a gain of 50.

A TIP120 is versatile. You can use it in power supplies, lamp drivers, stepper motor drivers, and much more.

You can switch on a TIP120 with just a small buffer or preamplifier. Other power transistors often require a pre-drive stage. A TIP120 has a built-in pre-drive stage.

Remember that a Darlington needs at least 1.4 volts for turn-on. This is double what a standard transistor needs.

If you use the TIP120 to switch an inductive load, connect a reversed diode across the coil. (Inductive loads include motors, relays and solenoids.) Otherwise, inductive kick can destroy the Darlington. Also, like transistors, Darlingtons are subject to thermal runaway failures. Negative feedback can prevent damage from thermal runaway.

Unfortunately, the TIP120 is the only Darlington that Radio Shack sells. You can't get the complementary, PNP version of the TIP120 (TIP115). Also, Radio Shack doesn't sell any small-signal Darlingtons such as the MPSA13. Too bad.

Comments about RadioShack TIP120 NPN-D Transistor:

if you need to interface to a relay or device that takes more current than your micro controller can sink, then this is an excellent choice. i use it to interface a basic stamp to a relay that controls an air compressor.