25 Leds, 9V battery, which resistors to use?

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Discussion Overview

The discussion revolves around wiring 25 RGB LEDs, each requiring 3V, to a power source (9V or 12V battery) using resistors. Participants explore the most efficient configuration for powering these LEDs, considering series versus parallel arrangements, current requirements, and resistor values.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the best way to wire the LEDs and whether to use series or parallel configurations.
  • Another participant questions the type of RGB LEDs being used, specifically whether they have a common anode or cathode, suggesting that this affects how they can be wired.
  • Some participants propose using a resistor in series with the LEDs to limit current, suggesting values like 220 ohms or 390 ohms based on supply voltage.
  • There is a suggestion to assume a safe current of 20 mA for calculations, leading to a formula for determining resistor values based on supply voltage.
  • One participant expresses confusion about the voltage requirements for different colored LEDs, noting that blue LEDs typically have a higher forward voltage.
  • Another participant mentions the possibility of parallel wiring for LEDs with only two wires and questions the internal structure of the LEDs.
  • Several participants share online design tools for calculating resistor values and configurations for LED setups.
  • There is a discussion about the potential for confusion regarding the designation of "RGB" LEDs when they may not function as expected with only two wires.

Areas of Agreement / Disagreement

Participants express varying opinions on the wiring configurations and resistor values, with no clear consensus on the best approach. Some participants agree on the need for resistors, while others debate the specifics of LED wiring and current requirements.

Contextual Notes

Participants note the lack of information about the maximum current ratings for the LEDs, which complicates the discussion. There are also uncertainties regarding the internal structure of the LEDs and how they achieve color changes with only two wires.

Who May Find This Useful

This discussion may be useful for hobbyists or students working on LED projects, particularly those interested in wiring configurations and resistor calculations for RGB LEDs.

Nelmeco
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I'm wiring together 25 rgb leds, each take 3V and I was wondering what would be the most efficient way to wire up as many as possible off one power source using resistors. Which resistors should I use? Series or Parallel? 9V or 12V battery?

This is for a small project I'm working on myself.
I know this is a noob question but I'm just starting phys 2 in college.

Thanks for your help
 
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You will need to find out a few things first.

RGB Leds.
So, 3 leds (red, green, blue) in each package? Do these have a common anode or a common cathode?
If so, then there is no chance of having LEDs in series.

They would all have to be in parallel.

Next, do you know how much current each LED is supposed to get? It may be different for the different colours to get similar brightness from them.

Last question, do you know how you intend to drive them? With a computer?

I know a lot of questions make you want to run away, but please work through them. They are essential to get a proper answer for you.
 
The red green and blue are not individual leds, they are together in one led, each led changes from red, to blue, to green after a certain amount of time.

When you say drive, i supposed you mean power? If so, then I will power them with a 9V or 12V better.

And they each require 3V.

And questions don't scare me, the faster i answer, the fast i get to start soldering :)
 
Last edited:
How many wires come out of each LED?

Do you know how much current the LEDs are supposed to use?

If you just want to try one, get a 220 ohm resistor and put it in series with one of the LEDs and put the combination across 6 volts. This should give a safe current of about 13 mA.
If you only have 9 volts try a 390 ohm resistor.

It will probably only work one way, although I have some LEDs that give a different colour if the polarity is reversed.
 
I have no idea how many mA they can take. I bought them off ebay, and there wasent much information given. Is there other way to find out the max?

There are two wires coming off each led, annode and the cathode. And about the resistors... ill have a buch of them, but ill check the ohm and get back to you
 
No, I don't think you can work out a maximum current (except by destroying a LED and then not using that much current again !)

Assume 20 mA is safe.

So you could work out the resistor value as follows:

R = (Supply voltage minus 3 volts) divided by 0.02

eg 12 volts - 3 volts = 9 volts.
9 volts divided by 0.02 = 450 ohms
so you would buy a 470 ohm resistor.

Try this with one resistor and one LED first to test the LEDs and see what they do.

It may be possible to put 2 or 3 LEDs in series with one resistor.

If you had two LEDs in series the resistor would be:
R = (Supply voltage minus 6 volts) divided by 0.02

If you had three LEDs in series the resistor would be:
R = (Supply voltage minus 9 volts) divided by 0.02
 
Sounds good. I had a bunch of two types of resistors and i believe one of them was a 470, ill check when I go over to a friends house tonight, we are working on it over there.

So i would only get 4 leds with one 12V?
 
Nelmeco said:
Sounds good. I had a bunch of two types of resistors and i believe one of them was a 470, ill check when I go over to a friends house tonight, we are working on it over there.

So i would only get 4 leds with one 12V?

No, you need some voltage across the resistor. At least 25% of the total voltage. The smaller this resistor is, the more sensitive to supply voltage the LEDs become.

Maybe 3 LEDs and a 150 ohm resistor with 12 volts.

Resistors are cheap. They cost a few cents each.

LEDs will be destroyed instantly if you put them across any source of power (greater than their turn-on voltage) without a resistor. You get a bright flash and then nothing.
 
There are several online design tools to help with this. Here are a couple:
http://led.linear1.org/led.wiz
http://ledcalc.com/
 
  • #10
There are several online design tools to help with this. Here are a couple:
http://led.linear1.org/led.wiz
http://ledcalc.com/

Thanks for those.

The second site seems to do a better job.

The first one gave 4 LEDs and a 1 ohm resistor in series when I asked for 4 LEDs @ 3 volts each and a 12 volt supply. This is not good design.

The second one saw the hazard and designed it with two strings of two LEDs and a better choice of resistors.
 
  • #11
I am a bit confused. How come all LEDs take the same voltage. I thought blue ones always have a higher gap voltage. Are you sure that the resistors are not build in.? Do you have an ampere meter? If you connect 3V and see something between 10mA and 25mA then the resistor is build in. In that case you could just wire them in 3 parallel chains on one 9V battery or four on a 12V (I assume you want to make some colored ambient light and not some changing pattern). But a normal 9V block would be empty quickly. They deliver maybe 2500mAh afaik, so they are empty within 5 hours. Or a third of that if you make white light.
 
  • #12
I am a bit confused. How come all LEDs take the same voltage.

He's right. Not all LED's have the same internal bias.

Check out this http://www.sparkfun.com/commerce/product_info.php?products_id=9264".

The wiring diagram should be useful.

By the way, a little internet sleuthing should turn up a data sheet. Do the arrays have any numbers written on them? Better yet, do you know who made 'em?
 
Last edited by a moderator:
  • #13
These are supposed to be RGB LEDs but with only two wires. I am waiting to see how they do this.

I have some that are parallel red and green LEDs that conduct in opposite directions.
So, you can get Red with one polarity and Green with the other or Yellow with AC.

But I haven't seen any that can do RGB with only two wires.
 
  • #14
vk6kro said:
These are supposed to be RGB LEDs but with only two wires. I am waiting to see how they do this.
You must have missed post #3...

Nelmeco said:
The red green and blue are not individual leds, they are together in one led, each led changes from red, to blue, to green after a certain amount of time.
It's a little misleading to call them "RGB" LEDs -- maybe "color changing" LEDs.
 

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