Swapping LED's to different colors - Page 1
Forum section: electronics / beginners

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 07, 2012, 05:27:54 pm
content: I have a keyboard that is backlit with green LEDs only. I want to swap out some of the green LEDs to red, blue, yellow, and white. Since different LED colors (wavelengths) require different voltages, is it possible to have assorted colors on the same board? Do I need to add a resistor for every new LED I swap in? The current green LEDs do not have resistors on them.

author: david77
author_rank: Frequent Contributor
timestamp: November 07, 2012, 06:21:32 pm
content: The LED's are probably connected in series or some kind of matrix with a current limiting resistor or maybe fed by a constant current supply.You may be able to swap green LED's with white or blue. These colours often have very similar specs.I'd go in there with a DMM and measure the voltage across one of the original LED's. I'd expect it to be somewhere between 3...3,5V for a typical green LED.To be on the safe side you could also desolder one side of one LED and measure the current flowing, probably between 5...20mA.If so, it is possible to substitute a white or blue one with similar specs.Reds and yellows are different, they usually have Vf of 2...2,5V.

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 07, 2012, 09:32:15 pm
content: Thanks david, I'll try that with a multimeter. So what would happen if I added a yellow or red LED in there too? I'm assuming going over the rated voltage would fry these red and yellow LED?

author: Psi
author_rank: Super Contributor
timestamp: November 07, 2012, 10:41:15 pm
content: Yellow and red will work, but they require less voltage so will draw more current than a green led with the same resistor.That may not be an issue, it depends on the resistor and how much current flows. It may still be within their specs.If the keyboard has a brightness control for the lighting make sure you measure the voltage when its set to max.If it's PWM your DMM will only give a valid reading with the brightness at 100% duty cycle.

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 08, 2012, 08:46:15 am
content: Ok here is a hypothetical measurement, so please check if I have this concept correct:I use a DMM and measure the green LED to have a voltage of 3.5v and current supplied to them is 20mA. That means there is a resistance in the matrix somewhere that's 175ohm. Now I change some green LEDs to some red LEDs that is rated at 2.0v. I can't change the current since there is already 20mA applied. That means I'll need a 100ohm resistor, but how can I fix this problem if there is already a 175ohm resistor in there? I'll need to remove 75ohm somehow.Please let me know how to get around this problem, and check if my math is correct too. Thanks!EDIT:The omega symbol for ohms shows up as a "?", so I'll just replace it with "ohm"

author: TriodeTiger
author_rank: Regular Contributor
timestamp: November 08, 2012, 09:01:28 am
content: Morecurrent will flow with a lower voltage LED!R = (Vsupply - Vforward) / Iled(5? - 3.5) = 1.5 / 0.02 = 75 Ohms(5? - 2.5) = 2.5 / 0.02 = 125 OhmsWhy? Because the LED will consume virtually no current until its breakdown, and so the (supply - Vf) will create a massive current if it is over if you're supplying more than Vf (which is typical, so almost always the case) and there is no resistor, and the LED must require a current limiting resistor or constant current source.In the end your LEDs may be using a far less than 20mA which is often the maximum, and +/- 30% current may not be that noticeable. You'd have to try swapping them (unless the current is really 20mA and the red LED may be driven above that, which it can, but at reduced life)Alexander. (and lets hope my 1am ramble is coherent)

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 08, 2012, 09:38:04 am
content: Thanks for clarifying, MmCoffee.I didn't know the current from the same source can vary. Was the 5V supply just an arbitrary number you picked? Or is that value from the USB power source?

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 09, 2012, 06:10:55 am
content: Ok so if the green LED is 3.5v, draws 20mA, then there is a 75ohm resistor in the matrix. If I want to swap out with a red LED that's rated at 2.0v, also 20mA, then it would need 150ohms, but since there is already 75ohms (from the matrix), then I would need to add an additional 75ohms to the one red LED for a total of 150ohms?

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2012, 07:44:11 am
content: Yes, if you add a 75ohm resistor in series with the new 2V red LED the whole thing will have 3.5V across it and behave the same as the original 3.5V green led.(Note: This is only true for a 5V circuit)

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 09, 2012, 08:04:56 am
content: Yeah it should be 5v because the USB cable only supplies 5v, +/- 5%