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### Topic: Confusing about maximal current per Pin (Read 1 time)previous topic - next topic

#### marcinburz

##### Feb 05, 2011, 03:51 pm
Hi

As You know: "Absolute Maximum Ratings DC Current per I/O Pin = 40mA".
(Atmel, doc8161.pdf)

"If you've got experience with electronics, you may notice that the LED
doesn't have a resistor in series with it. This is because the amount of
current coming out of the output pin of the Arduino is low enough that
it won't damage the LED. "

So, I made some measurements (Arduino Uno, pin 8 and 9, AMPROBE 33XR-A):

little red LED = 70 mA,
red power LED = 57 mA,
white power LED = 68 mA

I tested my multimeter with 10k ohm and 220 ohm to verify it measures
current flawless.

My question: is there really no need for current limiting resistor?
Or is my multimeter fooling me?

Best Regards

Confusing about maximal current per Pin

#### marcinburz

#1
##### Feb 05, 2011, 05:08 pm
Thank You Richard, it is absolutely clear now.

#### retrolefty

#2
##### Feb 05, 2011, 06:09 pm

Thank You Richard, it is absolutely clear now.

To make it clear for other begineers, an Arduino output pin has a rated absolute maximum current rating of 40ma. However it does not have an internal means to limit the output current to that rating. If the external component (led, motor, lamp, etc) has a resistance low enough to try and draw more then that amount of current, per ohms law, the output pin will keep providing more and more current until it burns out.

Lefty

#### Grumpy_Mike

#3
##### Feb 05, 2011, 08:27 pm
Quote
This is because the amount of
current coming out of the output pin of the Arduino is low enough that
it won't damage the LED.

This is absolute twaddle the writer should be ashamed of himself and so should the administrators of this board for letting this total nonsense stay on this board. I have written to the administrators in the past about this but they seem blissfully unaware or unconcerned about people damaging their arduinos by doing this.

The true tale is here:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

#### marcinburz

#4
##### Feb 05, 2011, 09:34 pm
Grumpy_Mike and Retrolefty. Thanks for additional explenations.

In my experiment, I noticed current had stabilized at circa 70 mA.
I hope no internal structures have been damaged. The pin 9 and
8 work as usual. I promise to use current limiting resistor on pins.

Regards

#### retrolefty

#5
##### Feb 06, 2011, 12:26 am
Quote
I promise to use current limiting resistor on pins.

Our work here is done. Mission accomplished.

#### DojoDave

#6
##### Feb 06, 2011, 11:35 pm
the page:

has been updated to include the text "use a resistor", the drawing is not updated but it is in the todo list

Thanks for the comment, and for the critique

/d

#### Grumpy_Mike

#7
##### Feb 06, 2011, 11:48 pm
Thanks David, this has been a long running bug bare, glad to see it has been tackled.
Thanks again.

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