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Topic: How many LED's on Pin 13 that already powers the on-board Green L-LED? (Read 610 times) previous topic - next topic

Andy_Cool

Hi

The max pin output current on an Arduino is 40 mA.
So I can put in 2 LED's (with appropriate resistors) on a single pin.
(Each LED will source upto 20 mA)

However pin-13 is already connected to the on-board L - LED (green).
Does this mean I cannot connect 2 normal LED's on Pin 13?

Thank you.

fungus


Hi

The max pin output current on an Arduino is 40 mA.
So I can put in 2 LED's (with appropriate resistors) on a single pin.
(Each LED will source upto 20 mA)

However pin-13 is already connected to the on-board L - LED (green).
Does this mean I cannot connect 2 normal LED's on Pin 13?


Yes.

PS: Aiming for exactly 20mA with resistors is a BAD idea. This forum has a loooong history of the reasons why.

Short version:
a) At the "20mA" point on an LED output curve (see datasheet) even a tiny error in voltage can produce a large error in current.
b) The variation in voltage between individual LEDs is enough to make sure that the error in part (a) will happen.

Solution: Either aim for 15mA (which won't be much dimmer) or use a constant-current circuit (eg. LED driver chip)

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

CrossRoads

"Does this mean I cannot connect 2 normal LED's on Pin 13?"
Better answer is Maybe.
Recent Uno's have a buffer (LM358) on D13 that drives the onboard LED.  Older Unos have a 1K resistor to the LED, which consumes ~3mA of current when driving High.
Both early and later have recommended Absolute Max of 40mA both high & low.  However, high voltage drops down as current exceeds 20mA, and low voltage rises up as current exceeds 20mA.
There is also a limit on how much current a port will support, and an overall limit on how much current the Vcc/Avcc & Gnd pins can support. (200mA/pin)  See Section 29 of the datasheet, excerpted below.
Depending on Vf of your LEDs, you might be able to run them at 20mA by putting both in series with a single resistor.

ATmega328P:
DC Current per I/O Pin ................................................ 40.0mA
DC Current VCC and GND Pins................................. 200.0mA

3. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150mA.
2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150mA.
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.

4. Although each I/O port can sink more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
1] The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100 mA.
2] The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100 mA.
3] The sum of all IOL, for ports D0 - D4, RESET should not exceed 100 mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater than the listed test condition.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

retrolefty

Nothing magic about running standard leds at 20ma, run them at 10 to 15ma and you will be fine. I have some 3mm red leds that at even at 3 ma are bright as heck, too bright for just a always on power indicator.

Lefty


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