Hi,
I am looking run some LEDs off an arduino but I need a dedicated power supply for the LEDs because they aren't bright enough.
How would I wire this up? I don't want to short the arduino out. Thanks!
Ground to Ground
power supply to LEDs
Very dodgy drawing indeed.
Nothing showing.
We really need a diagram of your current version and your proposal as to say you need "a dedicated power supply for the LEDs because they aren't bright enough" suggests you are doing something badly wrong.
Hi, I have updated the post.
At the moment I have LEDs linked to pin 13 and ground. But because there are two LEDs making a colour on the dot matrix square it won't light up properly as there are too many LEDs to light and the Arduino can't power it.
I am afraid that diagram shows us nothing.
simonjcox5:
But because there are two LEDs making a colour on the dot matrix square
? Post a link.
The diagram shows five LEDs in series, not a matrix.
Do you know the forward voltage (Vf) of the LEDs used.
Single colour LEDs, e.g. red, are ~2volt.
That means you need at least 10volt for a string of five LEDs and another 2volt for a current limiting resistor.
The Arduino with it's 5volt output is not going to work.
Post links to the parts if you want proper help.
Leo..
Sorry Leo, yes you are right. I am a bit tired.
I have updated the diagram now (Dropbox - File Deleted - Simplify your life).
The LEDs in the matrix are linked as follows: 
If I link all the red (so the whole matrix is red) or green (so the whole matrix is green) pins together they work fine, but the moment I connect the red and green pins together (to make orange) that's when I have the problem and the green LEDs are not bright enough.
The LEDs are linked in parallel, my apologies. The LEDs which are in the matrix are 5V (were in a display with a 5V power supply).
Sorry for any confusion and thanks for the help in advance.
(most) Red LEDs have a lower forward voltage (Vf) than green LEDs.
If you connect a red LED and a green LED in parallel, the red LED with the lower Vf is going to win.
Therefore red and green LEDs need their own current limiting resistors.
Don't connect a LED to an Arduino pin without current limiting resistor.
Leo..
How do I work out which limiting resistor I need??
Depends on how are you connecting the matrix to the Arduino.
Do you just want to light up the whole thing?
Dedicated chips are normally used to drive these matrixes (text/graphics).
If you want to drive it straight from an Arduino (and control each individual LED), you need 24pins. So a Mega.
Each COL pin of the matrix needs a resistor (16 resistors).
Value depends on the Vf of the LEDs.
Leo..
Yes I want to light up the whole thing and I have an Arduino mega.
Leo..
Wawa:
Bi-Color 8x8 Matrix | Adafruit LED Backpacks | Adafruit Learning System
Leo..
Thanks for the link Leo. I already have the LED matrix displays though and I cannot see any mention of resistors on there??
Also I'm thinking about using 12v LEDs as well so how would I power these.
There are no 12volt LEDs.
Things sold as "12volt LEDs" are usually three ~3.3volt LEDs (~10volt) in series (or several parallel strings of three LEDs), with the remaining 2volts dropped by an internal or external current limiting resistor.
You need the specs of the LED (Vf and max current) to be able to work out the value of the current limiting resistor (with Ohm's law).
If the (cob) LED is >3watt, it might be better to use an active constant current LED driver (less heat).
Leo..
Wawa:
If the (cob) LED is >3watt, it might be better to use an active constant current LED driver (less heat).
Hmmm. Now just what do you mean by "an active constant current LED driver"?
If you mean a constant current driver using pass transistors, then the heat is exactly the same, it is just dissipated in the transistor instead of the resistor, and you need a heatsink for that.
If you mean a switchmode constant-current driver, then this generates minimal heat.
Yes, I meant switching.
I think OP has to try to understand common single 20mA LEDs first, before diving into matrixes and power LEDs.
Leo..
Look at the MAX7219 driver chip; it is designed for driving LED arrays with common cathode.