I'm building a parallel circuit that uses a red LED and a green LED and a 3v power supply. I know that I use a higher Ohm resistor (using 130 Ohm) on the red LED because it has a lower forward voltage than the green LED (using 75 Ohm on that one). I also know that if I used the same 75 Ohm resistor on both, the red LED will glow brighter than the green LED.
How do I talk about what the resistor is doing? Do I say that the red LED needs less current than the green LED to glow as brightly, so it has a stronger resistor to reduce the current? Or do I say that the red LED needs less voltage than the green LED to glow as brightly, and the resistor soaks up voltage because of V=IR? It seems like resistors are always talked about with current, but LEDs are always talked about with voltage. Not sure if there's even really a difference between these two ways of phrasing.
Best Answer
In this case you call the resistor a current limiting resistor, because it's function is limiting current to the diode.
There is no way to compare a red led with a green led, there are several factors at work when you look at an LED including:
1) Human eyes assign diff the brightness of different frequencies differently, here is the response of a human eye, we see green really good.
Source: NPL
2) The LED's have different efficiency curves and have different voltage drops and have different intensities. Red is more luminous than green, but to the eye green has an easier time showing up.
Source: LED magazine
You could say, the red led needs less voltage for the same amount of current because its voltage drop is lower than a green led. But voltage drop and current have little to do with how bright an led is.