Go Down

Topic: LED current limiting resistor (Read 1 time) previous topic - next topic

jerseyguy1996

I was thinking of using one of these:

http://www.mouser.com/ds/1/311/RTB_GFTM_Pb_free-24628.pdf

in a project and I am a bit confused on the correct value resistor to use.  I am powering my project from a 3.7V LiPo battery and a 3.3V regulator.  Page 4 of the PDF linked above shows a forward voltage for the green LED of 3.2V and a forward current of 20mA.  I believe the formula to use would be (3.3 - 3.2)/.02A which would equal a resistor value of 5 Ohms.  This seems really small and I would like to verify that this is correct.  Any assistance would be great!  Thanks!
Arduino Uno;
Mega328

patduino

#1
Dec 25, 2012, 11:19 pm Last Edit: Dec 25, 2012, 11:26 pm by patduino Reason: 1
I think your math is right. You are not trying to knock the voltage down very far.  I see, however, that the forward current for "RED" is 40mA.  Even still, you should be good with a small value resistor, and the wattage is still below the 1/8W range.  Anything should work.  I might even go without...

That said, you should probably go a little higher.  Try it with a 10 or 22 ohm resistor and see if it's bright enough.
There are 10 types of people in the world, those that understand binary, and those that don't.

jerseyguy1996


I think your math is right. You are not trying to knock the voltage down very far.  I see, however, that the forward current for "RED" is 40mA.  Even still, you should be good with a small value resistor, and the wattage is still below the 1/8W range.  Anything should work.  I might even go without...

That said, you should probably go a little higher.  Try it with a 10 or 22 ohm resistor and see if it's bright enough.


Thanks!  I will probably size each resistor specific to the forward voltage and current of each individual LED.  The footprint for it is a bit odd....non-standard pad shapes.  I may have follow up eagle questions to get the pad shape just right.
Arduino Uno;
Mega328

dc42

The problem is that your proposed supply voltage (3.3V) is too close to the nominal LED forward voltage (3.2V), so a simple series resistor won't give you good current control. I suggest you either run the whole project from 3.7V instead of 3.3V, or use constant current regulators fed from the 3.7V supply to drive the LEDs. Unfortunately, the standard 2-transistor constant current circuit needs around 1V minimum headroom and you only have about 0.5V, so you will need a more complicated constant current circuit, e.g. an op amp driving a mosfet.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

jerseyguy1996


The problem is that your proposed supply voltage (3.3V) is too close to the nominal LED forward voltage (3.2V), so a simple series resistor won't give you good current control. I suggest you either run the whole project from 3.7V instead of 3.3V, or use constant current regulators fed from the 3.7V supply to drive the LEDs. Unfortunately, the standard 2-transistor constant current circuit needs around 1V minimum headroom and you only have about 0.5V, so you will need a more complicated constant current circuit, e.g. an op amp driving a mosfet.


That sounds like a lot of trouble.  I may just find a more suitable LED.  I want to run the project from 3.3V because all of the peripherals are only 3.3V tolerant.
Arduino Uno;
Mega328

Go Up