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Dallas
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I'm a beginner in electronics and Arduino and was hoping to get some clarification on how the output pins on my Arduino Uno work with multiple LEDs.

I had an idea for a project that would consist of a bunch of LEDs (somewhere between 30 to 50 I guess) arranged in a circle and connected to an Arduino that could be worn like a necklace (the Arduino would be used to create a light show of sorts). To simplify the project I was planning on connecting 3-5 LEDs each to 10 of the digital output ports (for a total of 30-50 LEDs, driven in groups of 3-5) instead of trying to use an LED driver (I think that's what the IC is called), but after some research and experimentation I don't think it is realistic to connect and use that many LEDs at once.

I have a ton of questions about electronics and Arduino; it would help me a ton if you could answer any of my questions, which may be horribly misguided to begin with. Also, please explain anything like I'm five years old as I am very confused as it is.

1) Is there a limit to how many LEDs that can be connected to the Arduino? I know this answer will be different for each type of LED, but let's take one of these http://www.sparkfun.com/products/9590 as an example.
2) I read that each output port has a current of 40mA, but with a resistor added I would think the current would be decreased (I=V/R). Is 40mA the limit then, or is there a limit?
3) Does the resistance of a resistor connected in a series with and LED affect the voltage drop of an LED? One website I visited said that the voltage drop is always the same for an LED, but in my physics class (I'm a high school student btw) we learned that the voltage drop across a resistor (R1) connected in a series with another resistor (R2) is related to the resistance of the second resistor (R2). So, assuming the voltage drop of an LED is constant, does this mean that if the voltage drop of an LED is 1V, only 5 can be connected to an Arduino (correct me if the voltage difference is not 5V, I thought it was for some reason)?

Thank you for your help!
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Manchester (England England)
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Is there a limit to how many LEDs that can be connected to the Arduino?
No the arduino can control an unlimited number of LEDs given some external driver chips.

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Is 40mA the limit then,
It is less than that, 40mA is the point where damage starts to be done. It is best to run the outputs lower, say 30mA.

 
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we learned that the voltage drop across a resistor (R1) connected in a series with another resistor (R2) is related to the resistance of the second resistor (R2)
True but that is only because a resistor is a linear device, that is the voltage across it is proportional to the current through it. An LED is a non linear device, this rule does not apply. See:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

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does this mean that if the voltage drop of an LED is 1V, only 5 can be connected to an Arduino
Less than that because you need some volts to drop across the resistor. This only applies to series connected LEDs. In practice the voltage drop is between 1.7V and 3.8V depending on the colour.
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Dallas
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First of all, thanks for telling me that 30mA is the maximum current that should run through through an output pin before I melted something. Now I think I am having trouble understanding what "voltage drop" really is. With two resistors in a series I believe that the voltage drop of one added to the voltage drop of the other equals the source voltage. So, is the voltage different or the same if measured through one resistor vs. measured through both resistors? Any other clarification/helpful sites explaining what voltage in a circuit is or what voltage drop is would be much appreciated.

Also, in response to my first question you said that an unlimited number of LEDs could be connected to the arduino, but then said that you couldn't connect more than 5 1V LEDs to the arduino. Even with external driver chips wouldn't the arduino still not have a high enough voltage to power many LEDs? Or do you need an external power supply as well? I just have a hard time believing that one would need a 50V power supply to power 50 1V LEDs, which is an unrealistic LED voltage to begin with.
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Manchester (England England)
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Connecting LEDs in series to one pin requires a voltage greater than the sum of the voltage drops.
So yes you would need 50 V for 50 1V LEDs in series but not when connected in parallel.
A voltage drop is the voltage across a device.
I think you need to look at the difference between series and parall circuits.
External device or drivers supply the current you need to drive the LEDs. I also thing you are mixing up current and voltage when thinking about driving LEDs.
Yes for an unlimited number of LEDs you need a power supply that can provide an unlimited amount of current. In practice the limit comes with how much current you can supply and how quickly you want to turn the LEDs on and off.
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Dallas
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Ok, I guess I will look into parallel circuits a bit more, thanks for your help!
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