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Author Topic: Can this damage my led?  (Read 1138 times)
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Hi,

I have a simple setup of 2 IR leds like this:
They're both LD271, so 1.3V forward voltage and 130mA forward current. The last led's - goes to one of my digital ports so I can control it (using IRremote.) And the resistor is a 33ohm one, according to my calculations the minimum is 22, but 33 is the smallest I have.
So now I'm worried that maybe one of the leds might get damaged, because it's linked directly to the 5 volts. Is this true?

Note: I'm very new to Arduino and electrionics. I can do the programming, though the wiring is still very alien to me.
« Last Edit: May 02, 2013, 03:28:29 pm by The_Oddler » Logged

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I don't think you connected the grounds, Dave.
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130 forward current.
130 what?
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130 what?

130mA, I'll edit the main post too. The calculator I used wants it in mA, so I didn't think to add it here.
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I don't think you connected the grounds, Dave.
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So now are you buffering this?
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So now are you buffering this?

I'm pretty new to Arduino and electrics and don't really know what you mean. Though I'm going to say no.
I described my exact setup in the first post, there's nothing I left out to make the description shorter. So, 5v -> +first led -> -first led to +second -> -second to 33ohm 5V resistor -> Digital3
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I don't think you connected the grounds, Dave.
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It isn't your LED you should be worrying about damaging.
An Arduino pin can source or sink an absolute maximum of 40mA.
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It isn't your LED you should be worrying about damaging.
An Arduino pin can source or sink an absolute maximum of 40mA.

So this could be damaging my Arduino? :o
According to the calculation it's draining 130 mA, how much does the Arduino give (when using the 5V)? And how could I fix so it doesn't kill my Arduino?

Thanks!

Edit: Perhaps I should specify. Not sure it has anything to do with it, but I'm using an Arduino Uno.
« Last Edit: May 02, 2013, 03:39:10 pm by The_Oddler » Logged

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how much does the Arduino give
See reply #5
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how much does the Arduino give
See reply #5

Ok smiley-razz So basecly no I'm not damaging my leds, nor my Arduino. Though since my setup wants 130mA it's getting less than a third of what it wants.
Reply #5 did suggest there was something else I was damaging. What is it? Or did I read that wrong?

Also, could you help optimize my setup? What should I add so the leds get what they want (without damaging anything) and their range is as long as possible? Or is there a better place to ask this?

Thanks a lot for the help already!
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Ignoring for the moment that you shouldn't be "pulling" 130mA from the Arduino...

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So now I'm worried that maybe one of the leds might get damaged, because it's linked directly to the 5 volts. Is this true?
That's no problem!    In a series circuit, the voltage gets divided among the components  (and the same current flows through all of them).    If your calculations are correct and the LED is operating narmally, the LED will have 1.3V across it. If you measure the voltage across the LED with a multimeter, you should get 1.3V.  That's the only voltage the LED "sees"...  It doesn't "know anything" about 5V.

If you measure the voltage on the other side of the LED referenced to ground, you should measure 3.7V.  But again, the LED doesn't "know" anything about 5V or 3.7V... It just has 1.3V across it.   

smiley-grin Here's another of my bad analogies - It's like you are standing on a ladder...  You don't really care about how high you are above sea level, you only care about how high the ladder is.
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Ok  So basecly no I'm not damaging my leds, nor my Arduino.
Yes, you are potentially damaging your Arduino!

With 1.3V across each LED, that leaves 2.4V across the 33 Ohm resistor = 73mA out of the Arduino and through all 3 series components.
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Ok  So basecly no I'm not damaging my leds, nor my Arduino.
Yes, you are potentially damaging your Arduino!

With 1.3V across each LED, that leaves 2.4V across the 33 Ohm resistor = 73mA out of the Arduino and through all 3 series components.

So the Arduino DOES put out more than 40mA? (AWOL made me think otherwise smiley-razz)
So there's 2.4V going back into the Arduino through the digital port? And this is too much? How much can it handle?
And the led actually wants 130mA, so it's getting too little?
« Last Edit: May 02, 2013, 04:27:40 pm by The_Oddler » Logged

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So the Arduino DOES put out more than 40mA? (AWOL made me think otherwise smiley-razz)

It will put out more than 40 mA.  But you SHOULDN'T do it.  The longer you keep that LED lit, the more damage you are doing to the Arduino pin.  Eventually, that pin WILL stop working or even the whole Arduino.  It could last for an hour or a few days.

What you should be doing:



* Arduino LED Transistor Ckt.jpg (56.82 KB, 479x500 - viewed 87 times.)
« Last Edit: May 02, 2013, 06:16:42 pm by codlink » Logged

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40ma is the most you can draw for a short period.

eg, take a car rev counter, the car *can* reach high.. but what happens if you don't reduce? bang... same for a transistor, it can and will, but it would be like sticking the car in 1 gear and wondering why the engine blows when you put your foot down
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It will put out more than 40 mA.  But you SHOULDN'T do it.  The longer you keep that LED lit, the more damage you are doing to the Arduino pin.  Eventually, that pin WILL stop working or even the whole Arduino.  It could last for an hour or a few days.

What you should be doing:
--image--

Thanks, this helps a lot.
I now know what a transistor is (just read some more about it.) Though I'm still a little fuzzy on the exact values needed (or how I could calculate them.) Could you help with that too a little?

Thanks a lot! And to everyone here, really, thanks! I'm learning a lot here smiley-grin

Edit: Also, the pin I'm damaging is my digital pin, not the 5v one. Right?
« Last Edit: May 03, 2013, 04:29:45 am by The_Oddler » Logged

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