When the LED current is greater than the pin's current.

Using Arduino General Electronics
1

Is it OK to connect a 20 mA LED to a 5 mA pin?

An indicator LED datasheet says it has 20 mA Continuous Forward Current.
A micro-controller datasheet says its pins are 5 mA.
The LED will be powered by the micro-controller pin.
Would connecting the LED to the micro-controller pin harm the pin?
Should the LED have a current limiting resistor?

Thank you.

2

wolfv:
Is it OK to connect a 20 mA LED to a 5 mA pin?

An indicator LED datasheet says it has 20 mA Continuous Forward Current.
A micro-controller datasheet says its pins are 5 mA.
The LED will be powered by the micro-controller pin.
Would connecting the LED to the micro-controller pin harm the pin?
Should the LED have a current limiting resistor?

Thank you.

Which microcontroller? Most AVR chips can supply up to an absolute max of 40mA, and 20mA is a safe level.

If the micro can really only supply 5mA safely, you can't drive the LED with it. You'd need to use a transistor, switched by the micro.

And LEDs always need a current-limiting resistor. That's the law.

3

OldSteve:
Which microcontroller?

Teensy LC has 4 - 20mA pins and 19 - 5mA pins.

4

Teensy LC has 4 - 20mA pins and 19 - 5mA pins.

Then use the 20mA pins, you cannot use the 5mA ones. If you can't use the 20mA pins you'll have to add some hardware.

5

wolfv:
Teensy LC has 4 - 20mA pins and 19 - 5mA pins.

Right, an ARM-Cortex processor. Is 20mA the absolute maximum current, or the safe maximum. If it's the safe maximum, try to do as Graynomad suggests, otherwise use an NPN small-signal transistor with it's base switched by the Teensy. A BC548 or similar would do nicely.

A 2K7 to 4K7 resistor could be used between the output pin and the base of the transistor, then if also powering the LED from 3.3V, the series resistance is calculated: R = ((3.3V-Vce(sat)) - Vf)/0.02 , where Vf is the LED's forward voltage and Vce(sat) is the voltage between the transistor's collector and emitter when it's turned on. (For such a small load, Vce(sat) can really be ignored.)

So if the LED Vf is 2.0V, the series resistance would be: R = (3.3-2.0)/0.02 = 65Ω. A 68Ω resistor would do the trick.

6

Can a current limiting resistor limit the current to 5mA?
A dim LED would be OK.

pin Supply Voltage Vs = 3.3
LED Forward Voltage Vf = 2.2
pin Current If = 5 mA
R = (Vs - Vf) / If = 220 Ohms
7

wolfv:
Can a current limiting resistor limit the current to 5mA?
A dim LED would be OK.

pin Supply Voltage Vs = 3.3

LED Forward Voltage Vf = 2.2
pin Current If = 5 mA
R = (Vs - Vf) / If = 220 Ohms

Yep. As long as 5mA is the safe maximum and not the absolute maximum.

8

OldSteve:
Yep. As long as 5mA is the safe maximum and not the absolute maximum.

What is the difference between "safe maximum" and "absolute maximum"?

9

Safe maximum means can be used continuously at this level
Absolute maximum means any transient greater than this level will cause damage

10

Thanks OldSteve and Jackrae.

11

wolfv:
Can a current limiting resistor limit the current to 5mA?

And note, you should never run a led without current limiting, in it's simplest form a resistor. Not even on the 20mA pins. And modern leds are very bright at 20mA so as an indicator 2mA is probably more then enough.