hi,
i want to connect 8 rgb leds in parallel. do i need the same Ohm resistor as i would use for 1 or do i need less Ohm resistor?
One resistor for each LED.
One resistor for each LED.
One per color, so 3 resistors for each RGB LED.
And of course, an Arduino output pin can't directly supply enough current to drive 8 LEDs.
@DVDdoug
Thanks! Only worked with the old fashioned LEDs...
Railroader:
@DVDdoug
Thanks! Only worked with the old fashioned LEDs...
The tungsten kind?
+1! 
You can’t parallel leds and use a single resistor ; each led will have a slightly different forward voltage and you’ll find some won’t light .
Use a resistor for each led .
DVDdoug:
One per color, so 3 resistors for each RGB LED.And of course, an Arduino output pin can't directly supply enough current to drive 8 LEDs.
well these are common cathode leds. does the 5v power 8 leds at once?
mgysgtdevon:
well these are common cathode LEDs. does the 5v power 8 LEDs at once?
Easily. Common cathode to ground, one resistor for each of the 24 anodes of course.
But take note:
DVDdoug:
And of course, an Arduino output pin can't directly supply enough current to drive 8 LEDs.
You would have to allocate by selecting resistor values, about 3 mA for each LED.
made a mistake. they are common anode rgb leds. does that change the situation or could i still use 8 in parallel?
mgysgtdevon:
made a mistake. they are common anode RGB leds. does that change the situation or could i still use 8 in parallel?
Easily. Common anode to ground supply, one resistor for each of the 24 cathodes of course.
If they are from the same production batch, you can often (surprisingly) parallel LEDs without a resistor on each one. Keep the current low to avoid thermal runaway which would unbalance the load. You would have to parallel each colour separately, as they are usually completely different LED dies internally, so the voltage drop on each colour is different.
It works for entertainment, but not commercial purposes because it's a potentially unstable design.
8 leds x 3 junctions each.... you could give 1 mA to each junction and see them in the dark.
If you put the leds in series and drive them with 18V-24V through a resistor, you can make them all bright.
GoForSmoke:
If you put the leds in series and drive them with 18V-24V through a resistor, you can make them all bright.
This is not an option with common anode/cathode multiple LED packages. Only possible if just one colour is illuminated.
You're right. Oh well.
Easily. Common anode to ground, one resistor for each of the 24 cathodes of course.
My way 
: Use a 500Ω pot wired as a variable resistor in series with a 150Ω resistor to set your first LED to desired brightness. Disconnect power and measure resistance across pot AND fixed resistor, then use the closest standard value resistor to replace the test rig. Power up the first LED and use the test rig to match the brightness of your second LED to that of the first and so on.
@aarg
Maybe the granite kind....