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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.

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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?

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    \$\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
    Commented May 9, 2015 at 17:27
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    \$\begingroup\$ Use three 68R resistors not one. \$\endgroup\$
    – Andy aka
    Commented May 9, 2015 at 17:36

1 Answer 1

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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.

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