How much current can an Arduino pin supply?

An Arduino Uno's output pin can supply about 20 mA safely. Its chip, the ATmega328P, is rated for an absolute maximum of 40 mA per pin, and running near that damages it. That's plenty for an LED and nowhere near enough for a motor, a relay coil or a string of lamps.

A pin is a tiny battery

Inside, an output pin is a switch between 5 V and 0 V, sitting behind about 25 Ω of its own resistance. HIGH connects your circuit to 5 V through that 25 Ω, so the current is set by Ohm's law around the loop:

A green LED (2.2 V) through 150 Ω:

That's under 20 mA, so the pin is safe. A red LED through 220 Ω draws about 12.5 mA.

Live experiment

How many LEDs can one pin take?

Pin 9 is HIGH, and you hang red LEDs on it, each behind its own 220 Ω, Pip first. Every LED's current comes through the one pin, so the total climbs: past the safe 20 mA at two, past 40 mA at five. Then let a MOSFET carry the LEDs.

Try it yourself: free lesson

“Digital output” is lesson 2 of 6 in The Core, Electronics World 8. Free with an account. Lessons unlock in order, so you’ll do the 50 before it first.

Every load on the pin adds up

Hang a second LED-and-resistor on the same pin and its current comes through the pin too: two red LEDs at 220 Ω each already pass 20 mA. The pin's 25 Ω also drops more voltage as the current rises, so HIGH sags below 5 V. At 40 mA it's lost a whole volt.

There's a chip-wide limit as well. The ATmega328P allows about 200 mA through all its pins together, so ten LEDs at 20 mA is the most it can drive at once, and the datasheet sets smaller limits for groups of pins.

Bigger loads: let a transistor carry it

For anything over 20 mA, the pin shouldn't supply the current at all. It should switch something that does:

• A logic-level MOSFET: the pin holds its gate at 5 V and supplies almost nothing; the load's current flows from the supply through the MOSFET
• An NPN transistor: the pin feeds its base a few milliamps through a resistor, about 1 kΩ, and the collector carries hundreds
• A relay module or a motor driver for mains loads and motors that run both ways

The code doesn't change: digitalWrite(9, HIGH) still turns the load on.

Sourcing and sinking

A pin can also sink current: wire the LED from 5 V, through its resistor, into the pin, and it lights when the pin goes LOW. The same 20 mA limit applies either way.