How to choose/calculate the right resistors for the 8x8x8 Led cube?

https://www.instructables.com/id/Led-Cube-8x8x8/step14/Choose-your-resistors/

Some people like me seem not to understand how they can determine which resitor is to use for the led cube posted above!
Could someone PLEASE explain how to choose the right ones, maybe with some example?


please answer in the comments section too, so others who want to build that cube see it too!

thanks in advance!

best regards

The LED doesn't limit the current, the driver chip does. The maximum current you can use per LED is roughly 6mA, so use high efficiency devices. Find out your:
Your supply volts
Your LED forward drop
and calculate R= (V-Vdrop)/6mA

Steve
freaked1 (author)  steveastrouk6 years ago
hmm ok thank you very much for your answer now i understand how it really works, but there is another thing that bothers me now:

does the instructable (the page i posted above) really say that i can only run the leds at 6mA?
wouldnt that mean, that the LEDs only light at 1/4 as bright as they can?

i mean, the LEDs say, that they need 20mA, so isn´t there much of a difference when i operate them at 6mA?

so should i use: 5V - 3 V = 1,8 / 20mA = 100 ohm
or 5V - 3V = 1,8 / 6mA = 350 ohm

Yes, only 6mA, that's the price you pay for the transistors and drivers they used in the 'ible. Of course you can change the bits and get more diode current.