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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.

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How do I talk about what the resistor is doing?

In this case you call the resistor a current limiting resistor, because it's function is limiting current to the diode.

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?

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. enter image description here 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.
enter image description here Source: LED magazine

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?

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.

enter image description here Source: LEDnique

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  • \$\begingroup\$ If current has little to do with how bright an LED is, why do I want to use a current-limiting resistor? I limit the current to the LED because... why? What effect is the current having on the LED that I need to limit? \$\endgroup\$ – pdmorrill Aug 3 '17 at 17:13
  • \$\begingroup\$ It has little to do with the relative brightness between LED's, of course current limiting changes the brightness of an LED, you can't put the same resistor on two LEDS and expect the same brightness. The other reason why you current limit, is to protect the LED from a supply that has a much higher voltage than the LED's current. If you put more than 3V across a red LED without a resistor it would probably destroy it. \$\endgroup\$ – Voltage Spike Aug 3 '17 at 18:04
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I would say that the resistors used in series with LEDs control the current through the LEDs, not the voltage across the LED.

The voltage across an LED is determined primarily by the LED chemistry and colour, and varys only slightly with currenet.

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You stay stuff like "The Red LED D1 has a forward voltage drop of 2.2 Volts and resistor R1 limits the LED current to 10 mA"

LED series resistors serve the purpose of limiting current. LEDs have a "forward voltage" specification at some bias current. This means when some specified amount of current is flowing through the LED (usually called If or forward current), you will read some specified voltage if you put your meter across the LED (usually called "forward voltage").

Take a look at the table here: http://dangerousprototypes.com/docs/File:LED_FWV.jpg It comes from this page: http://dangerousprototypes.com/docs/Basic_Light_Emitting_Diode_guide

It says the "typical" red LED has Vf = 2.2 Volts at 20 mA. So if you have say a 5 Volt supply, and 2.2V is across the LED, this means the resistor would have 5V - 2.2V = 2.8 Volts across it.

So let's say we want 10 mA instead of 20 mA. We can assume that with reduced current the voltage across the LED is still close to 2.2V (because that's how diodes work).

So you can figure out the resistor value you need using Ohms Law: 2.8 Volts / 10 mA = 280 Ohms to limit the LED current to 10 mA current.

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LED's are just like Zener diodes and the current can be easily be controlled by a CC limiter or a CV source with the voltage difference limited by the series R.

If= (Vdd-Vf)/Rs , If = forward current rated

The light output is determined chip size , Pd rating , Lumen Rating, colour, efficacy , lens magnification or beam width.

Many modern 5mm LEDs are .>1000x brighter in the last 15 years from efficacy growth and chip size. 10000-30000 mcd 30 deg parts make old 350 mcd look pretty dim

But when blue , green , white are used 3.3V is better than 3.0V since Vi @If ranges around 3.1V so Rs is small depending of batch and specs eg 100 ohms for Rs may not be 0.2V drop which is 20 mA

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