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Author Topic: Powering an Arduino Uno  (Read 4661 times)
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It doesn't go through the voltage regulator. We assume that the 5V from USB is in range.

You're the man, Nick! Alas, I spoke to soon.  The questions keep on a comin'!  I just read somewhere that if you have two LEDs wired in series, you'll need to have a power source that can handle double their FW supply.  Is this true? In this case, will 5V not be enough to handle my two 3.3V LEDs in series pairs?

Bonus question: As long as only one LED is on at once, I could handle driving all 14 seperately, right?
« Last Edit: May 23, 2012, 02:06:21 am by Fennel Rye » Logged

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In this case, will 5V not be enough to handle my two 3.3V LEDs in series pairs?
This was my suggestion: give it a try.
I guess you'll see a voltage drop of 2.5 V at each led and a current of less than 20 mA. No resistor required, probably.
Now the question is if the brightness is still ok.
My suggestion with the two leds in series implies they are controlled by one pin only, thus you just need 7 pins.
If the leds are wired opposite each other on your circle, you'd have two lights ( or black holes or light trains ) chasing each other around.
If the leds are next to each other, you'd just have an effective granularity of 7 elements only, but can build one "light train" running around.
(Initially, I imagined the first suggestion, or even fewer bigger groups up to 3groups with 5 leds, giving 5 lights chasing each other faster and faster until they seem to shine continuously at or above 20 Hz, when you switch to PWM output pulsing the whole circle)   
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I guess you'll see a voltage drop of 2.5 V at each led and a current of less than 20 mA. No resistor required, probably.

I can't agree with this point. The resistors with LEDs are required to limit current. You can't omit them if you have a lot of LEDs.

Read this:

http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

In there he says:

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There are several web sites and schematics on the web that suggest you can attach an LED directly to an Arduino output pin with no current limiting resistor. They are wrong, and following them will damage your Arduino.

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Is this true? In this case, will 5V not be enough to handle my two 3.3V LEDs in series pairs?

Do the LEDs really have a 3.3V forward voltage? In that case you are limited to one per pin. I think some external circuitry is starting to look good here.

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Bonus question: As long as only one LED is on at once, I could handle driving all 14 seperately, right?

Yes that should be OK. Still have to keep the current within limits for the pin (eg. 20 mA).

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Do the LEDs really have a 3.3V forward voltage? In that case you are limited to one per pin. I think some external circuitry is starting to look good here.
Having a look at the data sheet is always good.
I just picked any Kingbright UV LED (rated VF typ 3.8 max 4.2 V @20mA), which seems to support Nick's Statement.
I'm only right with my assumption about the resulting current to be less than 20 mA smiley-wink
According to the attached diagram it should be around 0.0x mA @2.5V (which would not harm the Arduino), and the voltage drop would typically vary only by 0.7V with a current variation of 5 to 25 mA.
I fully agree with Nick that "external circuitry is starting to look good here". It increases the possible scenarios a lot.
My approach however is rather experimenting with what you have got already, and experience the encountered limits.   


* UV_LED.png (10.98 KB, 393x332 - viewed 4 times.)
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