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Topic: Which resistor for a white LED 3,6V / 75mA ??? (Read 3105 times)previous topic - next topic

1010101

Feb 25, 2009, 01:49 pm
Hi all,

I need to use 5 white LEDs connected to an Arduino Duemilanove

I'm absolutely new to electronics and have no idea which resistor value I should apply to each LED. Looks like 200 ohms or 270 ohms is fine for the LEDs provided with the starter kit but for the LEDs I intent to buy, I'm not sure. See info below.

? LED Ã˜ 5 mm
? 50cd@ 3,6V / 75mA
? Maxi: 50cd @4,2V

If someone could tell me how to calculate that value that would be great.

Thanks,
J

Grumpy_Mike

#1
Feb 25, 2009, 02:04 pm
5V supply
3.6 volts across the LED
therefore 5 - 3.6 = 1.4 V across the resistor.

If this resistor has to carry 75 mA then it needs to be:-
R = E/I = 1.4 / 0.075 = 18 Ohms.

Two problems:-
1) You can't get that current out of the Arduino's output pin so you will have to use a transistor to drive it.
2) White LEDs are best driven not with a resistor but with a constant current drive. But you might get away with just one LED per transistor.

anachrocomputer

#2
Feb 25, 2009, 04:09 pm
Quote
I need to use 5 white LEDs

You'll need to wire up a separate resistor in series with each of those five white LEDs, i.e. five LEDs, five resistors, 18 Ohms each.  You can't connect all five LEDs in series (not enough voltage) nor can you connect them in parallel with a single resistor (forward voltages won't match).

If you want to control the LEDs indepenently, then you'll need five separate Arduino output pins.

1010101

#3
Feb 25, 2009, 04:45 pm

My intention was to use this sample by just changing the resistors accordingly to the LED characteristics.

http://arduino.cc/en/Tutorial/Loop

Will it work with this kind of LED and 18 ohms resistors only if I use 5 output pins or do I need a transistor?

retrolefty

#4
Feb 25, 2009, 05:00 pm
Quote
Will it work with this kind of LED and 18 ohms resistors only if I use 5 output pins or do I need a transistor?

You will need 5 transistors. The reason is an Arduino output pin can only source or sink a certain amount of current before it will burn out. 75ma is too much current for a output pin.

Lefty

1010101

#5
Feb 25, 2009, 05:06 pm
Okay, thanks.
Last n00b question: what kind of transistor?

1010101

#6
Feb 25, 2009, 05:19 pm
My conclusion.

So, if I use this kind of LED (18 cd @ 3,1V / 20mA - Maxi : 22cd @ 3,6V), I don't need a transistor (below the 40 mA) and can only use 95 ohms resistors.

Am I correct?

retrolefty

#7
Feb 25, 2009, 05:21 pm
Quote
[Am I correct?/quote]

Yep, now you got it

Lefty

1010101

#8
Feb 25, 2009, 05:32 pm
Thanks to all of you guys for your reactivity.
I learned something today

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