TIP120 transistor explained: switch a motor with an Arduino pin

#What it is

A transistor is a switch worked by a small current. The TIP120 is a strong one: a black block with a metal tab (the TO-220 package) and three legs. A few milliamps into its base let up to 5 A flow from its collector to its emitter — enough for motors, fans and strips of lights.

Why bother? An Arduino UNO pin can give about 20 mA (40 mA is the absolute limit). A small hobby motor wants about 500 mA. That is 25 times too much. The pin controls the transistor; the transistor carries the motor's current straight from the 5 V supply. See it in 3D.

#How it works

The TIP120 is an NPN Darlington: two transistors inside one case, the first one driving the second. Each multiplies current, so together a tiny base current controls a big collector current — the data sheet promises a gain of at least 1000.

It switches on once the base is about 1.4 V above the emitter (two transistors, about 0.7 V each). Below that, the collector–emitter path is closed: no current, the motor stops.

#The maths in the motor project

So 3.6 mA controls 500 mA. In our 3D lab the TIP120 behaves like about 2 Ω when fully on, so it eats roughly 1 V and warms up a little — real TIP120s do the same.

#Pins

Hold it with the printed face towards you and the legs down. From left to right:

The metal tab is joined to the collector — don't let it touch other wires.

#How to wire it to an Arduino

This is the motor project's wiring, all on one breadboard:

1. Push the TIP120 into three strips: B, C, E.
2. A 1 kΩ resistor joins pin 6's strip to the base.
3. The motor's + goes to 5V; its − goes to the collector.
4. The emitter goes to GND.
5. A 1N4001 diode goes across the motor: anode on the collector, band on 5 V. It catches the spike when the motor stops — see what a diode does.

Play in 3DRun the code · press the parts · turn it around

#Example code

The transistor follows pin 6. analogWrite() switches it on and off very fast, and the motor feels the average:

const int MOTOR = 6;   // a ~ pin, through the transistor

void setup() {
  pinMode(MOTOR, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  for (int speed = 0; speed <= 255; speed += 15) {
    analogWrite(MOTOR, speed);
    Serial.print("speed ");
    Serial.println(speed);
    delay(150);
  }
  delay(1500);
  analogWrite(MOTOR, 0);
  Serial.println("stop");
  delay(1500);
}

• const int MOTOR = 6; — pin 6 has a ~, so it can do PWM. A pin without ~ could only switch the motor fully on or off.
• analogWrite(MOTOR, speed) — 0 keeps the transistor off, 255 keeps it on, 128 has it on half the time: about half speed. Learn more in PWM explained.

#Common mistakes

• Motor straight on the pin. 5 V across the pin's own resistance and the 8 Ω motor is about 150 mA — far over the 40 mA limit. In the 3D lab the pin burns. Always go through the transistor.
• No base resistor. The pin pushes too much current into the base. Use 1 kΩ.
• Collector and emitter swapped. The TIP120 does not switch properly. Check the B-C-E order with the printed face towards you.
• No diode. It works — until a spike kills the transistor.

#Check yourself

#Questions people ask

#Can I use a 2N2222 instead of a TIP120?

For a small motor under about 500 mA, a 2N2222 can work, but it runs close to its limit and gets hot. The TIP120 has plenty of margin, which is why beginner kits use it.

#Why does my TIP120 get hot?

When fully on it still drops about 1 V or more, and that times the current is heat: 1 V × 0.5 A is half a watt. That is fine; for several amps, add a heatsink to the tab.

#Is the TIP120 a MOSFET?

No. It is a bipolar Darlington transistor, controlled by current. A logic-level MOSFET is controlled by voltage and wastes less power, but the TIP120 is simple and hard to break.

#Where it is used

The TIP120 drives the fan in the motor project and switches it on in the thermostat. Next, look inside the part it drives: the DC motor.

Last updated 2026-10-29