i know this is possible but not sure how to work out the resistor value needed... I should point out that I am re-using a PCB that has had all the components removed.

I need 15 LEDs in 3 parallel circuits, each circuit having 5 LEDs in series. The 3 parallel circuits share 1 current limiting resistor between them.

    |                 |
+ --|-->-->-->-->-->--|--/\/\/---
    |                 |

Assuming 12v 2v forward voltage and 30mA, I know the resistor required would be 68 ohm. Am I correct in assuming the resistor I need would still be 68 ohm?

  • 1
    \$\begingroup\$ don't use common resistor between parallel led paths UNLESS there is a smaller resistor per leg to mitigate the NTC of PN junctions \$\endgroup\$
    – user16222
    May 9, 2015 at 17:27
  • 3
    \$\begingroup\$ Use three 68R resistors not one. \$\endgroup\$
    – Andy aka
    May 9, 2015 at 17:36

1 Answer 1


If you use ONE shared resistor it needs to be about (12-(5x2))/.030/3 =~~ 22R
BUT this is a bad idea as the 3 strings will not current share well. Better, as several say, is to use a series R per series string - souse 3 x 68 Ohms - one per string.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.