Resistor Question for parallel LEDs

Hi I'm sure that there is probably a very simple answer to this question, only I'm a musician - not an expert in electronics.

I'm creating a design that will control 6 common anode RGB LEDs in parallel. By that I mean controlling three channels of PWM; channel one pulses the 6 Red cathodes; channel two the 6 blue etc

The specs of the LEDs are:

Blue and Green - forward voltage I wish to run them at 3.4 Red - 2.1 - current 20mA - from a 12v supply

I wish to control and run these LEDs at their maximum brightness for the best visual impact.

My question about resistors is that I will be putting series resistors on the cathodes of each colour, however when I use LED array calculators such as http://ledcalc.com/#calc and http://led.linear1.org/led.wiz. Their respective 'wizards' to give me the resistor values for each colour, I type in 6 LEDs of a colour, their specs etc. - but they only give me the calculations and diagram for using ONE resistor at the top of the power rail, which of course wont work very efficiently with RGB LEDs

There seems to be no way to get a calculation for using 6 resistors for each colour. - For example for Blue and Green it states that 71.76 Ohms is required, that 82 is the closest appropriate and that the resistor should be rated at 1 watt.

Should I be using 6 of those spec resistors for Blue, 6 for Green or should/could it be less as there are now 6 of them rather than one. E.g. 1 watt seems very high. Would 1/4 watt resistors be chosen instead? Or does it make no difference

My apologies if this is a simple question, only I have not been able to find good info on things like this

First you didn't mention how you are going to drive the cathodes of the LEDs. If trying to drive 6 LEDs is parrallel from one Arduino PWM output pin (one PWM output pin for each color), then 6 X .020ma = 120ma, which is more then the 40ma maximum rating of an Arduino output pin, so some kind of switching device, usually a transistor is required between the output pin and the cathode leads to be switched.

After that we can revisit the required resistor size. You have the option to drop the LED driving voltage (you stated 12vdc) to a somewhat lower value which should lower the power dissapation in the current limiting resistors. Also you should be using a resistor for every LED cathode lead, not one resistor for all cathodes of the same current. The LEDs will not share current evenly when driven that way, so each would require it's own resistor. I think using your calc link a given LEDs red cathode would use a 560 ohm 1/4 watt resistor. Just use the calc program but use one LED rather then 6 to get the resistor size for each color cathode.

That make sense?

Lefty

Lefty

You ought to be able to use the wizard for just one LED as that is effectively what you have when you wire them in parallel. Then use one resistor of whatever the wizard calculates per each of your 6 LEDs.

Yeah, what lefty said.

Yeah ok cheers guys & thanks for the quick reply.

And yes Lefty - I'm switching with logicl level mosfets from an external 12v power supply which I'm adequately decoupling etc I should have said that earlier

The results for singles are

470 and 1/4 is the B & G

560 and 1/4 is the Red

Thanks for clearing that up.

BUT, to further complicate - would running them as per the 'Guru' result from http://ledcalc.com/#calc for 6 LEDs at a time which gives a circuit like:

  • ________ R R l l
  • * l l
  • * l l
  • * _l_l Ground

Where * = the cathode of the LED And R= 100 ohm 1/8 watt resistors

Only this approach would reduce current by a great deal to 36 mA per colour

Would this a reliable set-up method for linking the cathodes of the common anode RGB LEDs, employing a similar approach for each colour or would there be current/voltage sharing issues from only using one resistor

I'm afraid I don't follow your question. If you could possibly create a real drawing maybe I can follow your question better.

Lefty