How Does the Number of LEDs Effect the Resistors Needed? - Page 1
Forum section: electronics / beginners

author: Max Holdcroft
author_rank: Contributor
timestamp: May 31, 2024, 12:05:52 am
content: I'm using 4 OSRAM LRTBR48G-P9Q7-1+R7S5-26+NP-68-R33-ZB RGB LEDs wired in parallel and I need to use resistors to drop the voltage going in. I'm using a voltage of 2.8v from a voltage reg and the forward voltage for the LEDs are 1.95V, 2.7V and 2.7V respectively. I'm wondering how the number of LEDs I wire in parallel relate to the resistors I should use?Thanks,Max

author: Psi
author_rank: Super Contributor
timestamp: May 31, 2024, 12:28:17 am
content: If wiring in parallel each LED needs its own resistor.So the number of LEDs plays no effect on the resistor value. It just sets the number of resistors you need (all of the same value for the same LED color).Technically when people say to wire LEDs in parallel they don't mean that literally, they mean you combine 1 LED and 1 resistor together and then you wire many of those "modules" in parallel with the power supply.Since you only have 2.8v available you are forced to do it this way (1 resistor per each LED).You are also forced to do it this way because your LED has multiple LEDs inside all with a common connection. Since your LED is 'common anode' you are forced to put a resistor on each cathode, (you cant insert it into the anodes because they are already joined inside the chip.)So each LRTBR48G that you use will need 3 resistors (since it contains 3 leds). The resistors might need to be different values for R G and B since you may want to tune the brightness so all on together gives a good white.However you may have a problem with your input voltage of 2.8v when it comes to the Green and Blue LEDs which have a forward voltage of 2.7V.This may work, but it's not ideal. You only have 100mV to play with here and that makes things a bit sensitive, thermally and supply voltage wise.eg as the LED heats up and that heat heats up the nearby resistor it will change the resistors value a bit (as heat normally does) and that will upset whatever current you wanted to flow through the led.Also if the power supply outputs a bit less or a bit more than 2.8V that will play a bigger role in LED brightness than it would if you had more than 100mV to play with.How much i don't know, it may not be an issue depending on your application.A starting point is 85 ohms for the Red and 10 ohms for the Blue and Green.

author: ArdWar
author_rank: Frequent Contributor
timestamp: May 31, 2024, 02:32:43 am
content: Quote from: Psi on May 31, 2024, 12:28:17 amYou don't even need any other component shifting for that. LED can do that on their own simply because they still obey Shockley equation. Operating with so little headroom only accentuate the problem.Say you want to power 2.7 VFLED off 2.8 VCC. You use 10R ballast to set 10mA drive current.LED junction heats up when turned on. At ~60C its forward voltage will drop by 0.1 V. Now you get 20mA drive current, and of course the junction is gettinghotter.

author: pcprogrammer
author_rank: Super Contributor
timestamp: May 31, 2024, 06:02:09 am
content: The available 2.8V of the supply might be to low to get it working properly across a larger range of these RGB LED packages. The datasheet specifies a minimum, typical and maximum forward voltage for the separate LED's in the package. The maximum for the green and the blue is 3.4V. This means that there is change that you get a package that needs 3.4V for the green and blue to work.Due to these possible differences you can't just connect the same color LED's in parallel and like PSI wrote each LED needs it's own current limiting resistor.Furthermore it specifies different currents for the LED's to obtain the same light intensity, so to get a white color different currents are needed. 10mA is the absolute maximum, so that needs to be obeyed. It specifies the intensities for RED at 10mA and GREEN and BLUE at 5mA.In the target application for these kind of LED packages usually constant current controllers are used to drive the separate LED's. These use PWM to control the light intensity.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: May 31, 2024, 06:24:16 am
content: If you can just use one resistor, each LED will have a different brightness. I'm not sure how to explain it technically, but with a single resistor, the internal resistance of the LEDs becomes a factor. They will not all be the same. This is because the LEDs are competing with each other for current. If each LED has a resistor, then the resistor dominates the circuit (current draw). The small difference in the LEDs internal resistance is no longer significant.

author: pcprogrammer
author_rank: Super Contributor
timestamp: May 31, 2024, 08:01:36 am
content: Quote from: JustMeHere on May 31, 2024, 06:24:16 amWith a single resistor for the package, so the three different color LED's in parallel, the blue and green won't light up due to the fact that the red LED limits the voltage to below the forward voltage(s) of the other LED's. They act like zener diodes.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2024, 12:54:59 pm
content: Do not parallel LEDs. Choose supply voltage so that is enough for the highest voltage LED, then calculate series resistors for each LED separately.