Metal tab (heat sink) · Ø 3.7 mm holeEpoxy bodyPins B-C-E · 2.54 mm spacing

Black TO-220 body, metal tab with a hole, “TIP120” text on the front
TO-220 body ≈ 10 × 15 mm, metal tab with a hole

Transistors

Darlington (TIP120)

In simple terms

What is this?

A valve made of two transistors stacked one after the other; a very small current switches on a large one.

What is it for?

Drives powerful loads such as motors and solenoids from a small pin signal.

Where do you see it?

On motor driver boards and in robot projects.

How do you spot it?

A black body with a metal tab and three legs; the order is base, collector, emitter.

Caution:

A volt or two is lost across it and it heats up; at high current it needs a heat sink.

Technical definition

A structure in which two NPN transistors are connected back to back and packed in one body; the gains multiply, so a very small base current controls a large collector current. The TIP120 also has a protection diode inside. The price is a high voltage drop of 1–2 V when conducting.

VCEO60 V

IC5 A

hFE≥ 1000

Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 3

1Base (B)With the marked face toward you, legs at the bottom, 1 from the left.B

2Collector (C)Middle leg; the metal tab is also connected to the collectorC

3Emitter (E)Right leg; goes to groundE

Features typical

VCEO: 60 V
IC: 5 A (peak 8 A)
hFE: ≥ 1000 (3 A, 3 V)
VCE(sat): ≤ 2 V (3 A)
VBE(on): ≤ 2.5 V
Power: 65 W (case 25 °C, with heatsink)
Built-in: Reverse diode between C-E
Example part: TIP120 (TO-220)

Common mistakes

High current without a heatsinkAt 3 A, about 2 V drops across it, which makes 6 W of heat. A TO-220 without a heatsink quickly overheats with this heat.

Forgetting the base resistorBase-emitter voltage is higher than for one transistor, but current still needs limiting. Fit a resistor such as 1 kΩ to the base.

Using it on a low-voltage loadOn a 5 V load, a 1–2 V loss is a large share. A logic-level MOSFET is more efficient at low voltage.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Driving a DC motorSolenoid and lockHigh-current LED groupsSimple stepper motor driver