LED current limiting resistor

[Mooseman7]

So I bought these RGB LED's on ebay and I cannot figure out what size of current limiting resistors to use. It is common anode. Here is all the info they provided:

Specifications:
Forward Voltage: R: 1.8-2.2 G: 3.0-3.4 B: 3.0-3.4
IV: R: 5000-6000 G: 6000-7000 B: 2500-3000
XY/WLD: R: 620-625 G: 515-520 B: 460-465
Light Color: RGB
Size: 5mm length : about 4.5CM
Emitted Colour : Red / Green / Blue
Pins sequence: RED/Common Anode(positive terminal)/Green/Blue
Lens Color : water Clear/diffused
Luminous Intensity: 4000/8000/5000mcd
Forward Voltage (V) : 2.1~3.4
View Angle: About 25 degree.

Forward Dominant LuMen(LM) Reverse Power
Voltage(V) wavelength(mm)K current(uA) Angle (deg)
If=20mA If=20mA If=20mA Vr=5V 　
Min Typ Min Typ Min Typ Max 20-25　
Red 2.1 2.4 620 630 3000 4000 10 20-25
Green 3 3.4 520 530 6000 8000 10 20-25
Blue 3 3.4 460 465 4000 5000 10 20-25

 

[brispuss]

Resistors required depends on supply voltage.

First calculate equivalent resistance of the LED's.

R = V/I = V[sub]LED[/sub]/I[sub]LED[/sub], where V[sub]LED[/sub] = nominal LED forward voltage, and I[sub]LED[/sub] = nominal LED forward current = 20 mA.

For the red LED, R[sub]RED[/sub] = 2.0 V/20 mA = 100 Ω

For the green LED, R[sub]GREEN[/sub] = 3.2 V/20 mA = 160 Ω

For the blue LED, R[sub]BLUE[/sub] = 3.2 V/20 mA = 160 Ω


Say, supply voltage = V[sub]SUPPLY[/sub]

And say current limiting resistors = RL[sub]RED[/sub] for the red LED; RL[sub]GREEN[/sub] for the green LED; RL[sub]BLUE[/sub] for the blue LED

We want to limit current flow through each LED to 20mA for the supply voltage V[sub]SUPPLY[/sub].

Current limiting resistor value required = RL = V[sub]SUPPLY[/sub]/20 mA - R[sub]LED[/sub], where R[sub]LED[/sub] is the equivalent LED resistance.

For the red LED, RL[sub]RED[/sub] = V[sub]SUPPLY[/sub]/20 mA - 100 Ω

For the green LED, RL[sub]GREEN[/sub] = V[sub]SUPPLY[/sub]/20 mA - 160 Ω

For the blue LED, RL[sub]BLUE[/sub] = V[sub]SUPPLY[/sub]/20 mA - 160 Ω


For example, if V[sub]SUPPLY[/sub] = 10 V, then the current limiting resistors required are -

For the red LED, RL[sub]RED[/sub] = V[sub]SUPPLY[/sub]/20 mA - 100 Ω = 10 V/20 mA - 100 Ω = 400 Ω, nearest available value = 402 Ω @2 % tolerance

For the green LED, RL[sub]GREEN[/sub] = V[sub]SUPPLY[/sub]/20 mA - 160 Ω = 10 V/20 mA - 160 Ω = 340 Ω, nearest available value = 348 Ω @2 % tolerance

For the blue LED, RL[sub]BLUE[/sub] = V[sub]SUPPLY[/sub]/20 mA - 160 Ω = 10 V/20 mA - 160 Ω = 340 Ω, nearest available value = 348 Ω @2 % tolerance


Current limiting resistor power dissipation = forward current² x RL = 20 mA² x RL

For the red LED current limiting resistor RL[sub]RED[/sub], power dissipation = 20 mA² x 402 Ω = 0.16 W, so a 402 Ω resistor rated at a minimum of 1/4 watt is sufficient.

For the green LED current limiting resistor RL[sub]GREEN[/sub], power dissipation = 20 mA² x 348 Ω = 0.14 W, so a 348 Ω resistor rated at a minimum of 1/4 watt is sufficient.

For the blue LED current limiting resistor RL[sub]BLUE[/sub], power dissipation = 20 mA² x 348 Ω = 0.14 W, so a 348 Ω resistor rated at a minimum of 1/4 watt is sufficient.

The resistors are placed between the cathode of the corresponding LED and the negative voltage supply.