LED Current Draw

Using Arduino General Electronics
1

So, i'm trying to figure out how much current my LEDs are actually drawing. I'm trying to power 20 LEDs from an Arduino Nano, 1 LED per pin. They are standard white, 3.3v, 20mA LEDs. I've found a ton of LED calculators online, but they are meant to show you what resistor you need, not to tell you how much current draw with a given resistor value. For example: One calculator i put in the voltage drop (3.3v), # of LEDs (1), desired current draw (1mA *lowest i could put), and it said i would need a 2.2k resistor.

So, here's where i'm getting lost, or maybe not? With the white LEDs, on my projects, they are VERY bright, so i've been using 5.1k resistors, which seem to dim them like 50%. Still bright enough to see with the lights on, but not blinding in the dark. So, if the above example is true, then does that mean with a 5.1k resistor that the LED is only drawing ~0.4mA? and i should have no problem driving my 20 LEDs? or for that matter, i could drive several LEDs in parallel per pin if i needed to.

Thank you.

2

LEDs in parallel must each have their own resistor.

Time to buy a DVM, measure the voltage across the LED to get the actual forward voltage drop.

To get current, measure the voltage across your series dropping resistor then divide by the resistance.

(5v - 3.3v)/5100k=333μA=.333mA

A single pin can drive 20mA safely.

.333mA is quite small, example .333mA*20=6.66mA

One LED per pin is well within the safety margin.

Or measure the current in the LED series circuit, use the DVM amperage range.

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3

As a quick-and-dirty check, 5V/5K is 1mA per LED.

...That would be "worst case" with no voltage across the LED so with 20 LEDs the actual total current will ALWAYS be LESS than 20mA.

4

Thank you larryD and DVDdoug! That helps a LOT!

LarryD, as a follow-up question to your Parallel diagram, what happens if you have just one resistor for all 3 LEDs? (shown by the Red Bar)

LEDs.png

5

LEDs.png

You need one series resistor per LED as in the diagram I posted.

If you have 3 white LEDs connected to a NANO output pin, you would need three 5.1k resistors, one for each LED.

Since all white LEDs don’t necessarily have 3.3v forward voltage drop, you ‘cannot’ use just one series resistor.

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6

Be aware that there are a few other limits

20mA per pin (mentioned)
100mA per port
200mA through Vcc or Gnd of the processor.

7

Hello,

You can use a driver, like the MOSFET, and then you can use all the power of the LEDs

8

rtek1000:
You can use a driver, like the MOSFET, and then you can use all the power of the LEDs

As the OP has said that using a 1K resistor is too bright why on earth would he want to put 20mA through it?

9

because it goes to 11!!!

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Arduino_Jarod:
as a follow-up question to your Parallel diagram, what happens if you have just one resistor for all 3 LEDs?

It might work, but they could have quite different levels of brightness! Worst case only one will light up and all the current will go through only this one LED.

In your case probably nothing bad will happen because of your low desired current. But if you had chosen a resistor for for example 100mA for 5 LEDs in parallel, and because of slight variations in forward voltage, one LED might conduct all 100 mA and burn!

11

Grumpy_Mike:
As the OP has said that using a 1K resistor is too bright why on earth would he want to put 20mA through it?

Well, instead of getting calculate resistor value, would not it be possible to use PWM?

So, a pulse with very small width, but with high current, would certainly need a driver.

12

When you are in a hole stop digging.

13

If you want to know way more about LEDs and how to use them, check out my Blog:

http://www.sinistercircuits.com//Blog/LEDs_LightemWithoutSmokinem/index.php