Current limiting resistor for RGB LEDs in parallel

I have 9 common cathode RGB LEDs. I want to connect them in parallel and control R, G an B channels of all the LEDs. To do so I am planning to use 3 transistors. (So, LEDs won't be individually addressed)

Each channel of LED requires 20mA so for 9 of them I need 180mA.

At first I was planning to attach individual current limiting resistor for each LEDs' each channel. But that makes too much resistor (3*9 = 27).

I tried to find a better solution but couldn't think of any way out.

Any better design suggestions on that ?

You can try putting them in parallel but there could be variations is brightness. I'd experiment with wiring them in parallel before you carve your design in stone. In parallel, they will all have the same voltage drop across them, but we can't be 100% sure they will all have identical current.

Calculate the resistor required for 180mA. Calculate the power dissipation (Current x Voltage). You may need a 1/2W or 1W resistor.

DVDdoug:
You can try putting them in parallel but there could be variations is brightness. I’d experiment with wiring them in parallel before you carve your design in stone. In parallel, they will all have the same voltage drop across them, but we can’t be 100% sure they will all have identical current.

Calculate the resistor required for 180mA. Calculate the power dissipation (Current x Voltage). You may need a 1/2W or 1W resistor.

Thanks for the answer.

Yes due to variation in Vf, brightness may vary.

As for power dissipation, for red channel;

(9-2)/0.02 = 350 Ohm.

P = I x V = 0.02*9 = 0.18 W

0.18 < 1/4 then I’m good to go with standard resistors. One question though; Should I consider the voltage drop due to LED when calculating the Power, meaning is the above mentioned calculation correct or should it be 0.02*7 ?

This is aside, my original question still holds. Apparently there is no way to eliminate those 27 resistors, right ?

No other soloution than having individual current limiting resistors.

Grumpy_Mike:
No other soloution than having individual current limiting resistors.

I feel the same way but what do people do when they build huge RGB LED cubes. Something 8x8x8 would result in 1536 resistors, do they really solder that much resistors ?

And, how about using common anode RGB LEDs ? Would that make any difference ?

zoom:
I feel the same way but what do people do when they build huge RGB LED cubes. Something 8x8x8 would result in 1536 resistors, do they really solder that much resistors ?

LED cubes are always multiplexed, so the resistors are shared amongst alternately enabled sets of LEDs. But they still have enough resistors.

zoom:
And, how about using common anode RGB LEDs ? Would that make any difference ?

Yes, you have to put the resistors in the cathode lines.

Paul__B:
LED cubes are always multiplexed, so the resistors are shared amongst alternately enabled sets of LEDs. But they still have enough resistors.

Oh, I see.

Paul__B:
Yes, you have to put the resistors in the cathode lines.

Yet I need 27 resistors, right ?

zoom:
Yet I need 27 resistors, right?

Certainly looks that way. :smiley:

Paul__B:
Certainly looks that way. :smiley:

Oh that's too much resistor on a board, I really want to find a way to reduce it :slight_smile:

How about multiplexing them as in the example of LED cube ?

How about resistor networks, can really reduce the amount of packaging involved.
For example, here are "isolated" 180 ohm resistor networks, SIP & DIP, thruhole & surface mount.

You can use the one common connector resistor package because that is what you need for the common ground.

The only way to reduce the resistor count is to use LEDs that are not common anode or cathode. Then you can put them in seriese The only one I know is a 5050 and that is surface mount.

You can use surface mount resistors between tracks of strip board, that takes up hardly any room.

The problem with multiplexing is that the circuits are more complex and take up more room than the resistors you say is too much. Also the LEDs are dimmer because they are not on all the time.