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Topic: Reason for not using resistor to LED (Read 1 time) previous topic - next topic

cloudystrife

Hi,

I have a question to ask about my project.
Question: Why is there no resistor to LED?

Connection of LED as attached picture.

Thanks

eddiea6987

I could print the Arduino logo on a box of cereal and sell it as "Arduin-O's"

Osgeld

people think the resistor does nothing cause it does not cause a instant catastrophic situation, its a really bad practice and it totally unreliable depending on something as simple as temperature, and feeding direct 5 volts to any LED would overload the arduino pins along with the LED itself causing a slow but certain death over any serious amount of time
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

cjdelphi


people think the resistor does nothing cause it does not cause a instant catastrophic situation, its a really bad practice and it totally unreliable depending on something as simple as temperature, and feeding direct 5 volts to any LED would overload the arduino pins along with the LED itself causing a slow but certain death over any serious amount of time


I'd like to see evidence of that, where a person or persons has claimed (under anything but CONTROLLED conditions, stable temperature, stable voltage, stable everything) any other thing?..... I'd like to see who (with evidence)

Osgeld

its a generalized statement, if people didnt think that they would pair up resistors with their LED's

and yet since it does not instantly go smoke boom bang it must be fine, hence why its commonly seen

(please get over yourself, I was not referring to you)
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

cjdelphi

Quite frankly, the reason FOR using resistors here is to protect the Atmega chip.

guix

That's what he meant in his first post: put resistor to protect your arduino :)

SurferTim

So the resistor only protects the Arduino? Do you feel it is ok to connect those LEDs directly to a 5v power supply directly with no resistor, and they will survive?

PaulRB

There is a situation where no resistor is required: many ICs designed specifically to drive leds have "constant current" circuits within them. In many cases, the required current is set with a single resistor to drive many leds (like max7219) and in some cases they don't (like saa1064). Arduino outputs don't have constant current circuits.

Paul

guix


So the resistor only protects the Arduino? Do you feel it is ok to connect those LEDs directly to a 5v power supply directly with no resistor, and they will survive?


No it also protect the LED of course

cjdelphi

#10
Aug 10, 2013, 10:32 am Last Edit: Aug 10, 2013, 10:33 am by cjdelphi Reason: 1

So the resistor only protects the Arduino? Do you feel it is ok to connect those LEDs directly to a 5v power supply directly with no resistor, and they will survive?



You answer your own question....

If the most it can handle is 40ma before damage potentially starts to take place, and that LED is drawing > 40ma (to damage it) then there's probably a good chance that the LED to might be damaged hence you WOULD need a current resisting limitor....

if however the Arduino could output a precise voltage which happens to be the correct forward voltage, required for the LED to glow safely within it's tolerance given by the manufacturer/datasheet, the current heat and conditions at that moment in time (to adjust the current (or turning a potentiometer for winter/summer conditions)  were taken into consideration, then yes you could get away without one, but since that's not the case you indeed need a resistor.

(Any more questions?)


SurferTim

Of course I answered my own question! Electronic fails are normally the survival of the fittest. The most durable device will survive the fail and the other won't.
Quote
if however the Arduino could output a precise voltage which happens to be the correct forward for the LED to glow safely within it's tolerance given by the manufacturer/datasheet, the current heat and conditions at that moment in time were taken into consideration, then yes, but since that's not the case you indeed need a resistor.

So what you are saying is that you need a resistor?

You don't need to answer that.

cjdelphi


There is a situation where no resistor is required: many ICs designed specifically to drive leds have "constant current" circuits within them. In many cases, the required current is set with a single resistor to drive many leds (like max7219) and in some cases they don't (like saa1064). Arduino outputs don't have constant current circuits.

Paul


I'm curious how that chip limits the current, is it PWM or is it a case of a voltage divider controlling a Transistor which limits the current to all the outputs?  

See what i'm saying? since each pin out can't possibly have a resistor (since it's user controllable) then the current has to be limited some other way, i might have a look at the datasheet see what i can dig up on it..

cjdelphi


Of course I answered my own question! Electronic fails are normally the survival of the fittest. The most durable device will survive the fail and the other won't.
Quote
if however the Arduino could output a precise voltage which happens to be the correct forward for the LED to glow safely within it's tolerance given by the manufacturer/datasheet, the current heat and conditions at that moment in time were taken into consideration, then yes, but since that's not the case you indeed need a resistor.

So what you are saying is that you need a resistor?

You don't need to answer that.



A resistor is the cheapest, easiest way; but are other ways to limit the current to the LED.

SurferTim

#14
Aug 10, 2013, 10:46 am Last Edit: Aug 10, 2013, 10:49 am by SurferTim Reason: 1
Quote
A resistor is the cheapest, easiest way; but are other ways to limit the current to the LED.

I agree with that. I would not use the max current available on an Arduino pin to do that tho.

edit: Now you are getting to the point of which is stronger. Will the Arduino pin fail first, or the LED?

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