Why an Arduino Needs a Transistor to Run a Motor

Wire a DC motor straight to an Arduino Uno R3 pin, and you can fry the pin. Here's why we put a transistor in between, and how a small current switches on a big one. On the Uno R3 (ATmega328P), one pin's absolute maximum is 40mA, and 20mA or less is recommended, so stay well below the maximum. Even a small DC motor can draw hundreds of milliamps as it starts up, so don't run it straight from a pin. An NPN transistor has three legs: base, collector and emitter. A small current from base to emitter opens the way for a big current to flow from collector to emitter. So the MCU pin only supplies a small base current, and the transistor carries the motor current for it. Always add a resistor on the base (for example 220Ω) to limit the base current. Put a diode across the motor, band toward the motor's + terminal, to catch the voltage kick from its coil when you switch it off. Transistor pinouts vary by part, so check the datasheet. The heat colors are a thermal-style visual. #transistor #arduino #DCmotor #electronics #insideembedded