I just started playing with LEDs and Im trying to make a sign with them. I have the LEDs in series and I have a resistor on each led. I can barely get one letter to light with about 36v( two 18v battery's in series.) I'm clueless about what to do and don't really know how it all works.

  • 1
    \$\begingroup\$ I'm assuming ~10 to 12 leds in a string? What resistor value are you using? Can you power a string of 5~6 on a single 18v battery? Is the battery charged or old? \$\endgroup\$
    – Passerby
    Nov 23, 2013 at 1:07
  • \$\begingroup\$ LED is a diode, and it has a forward voltage drop. Read up on this, and you'll understand why you have problems with the present version of your circuit. \$\endgroup\$ Nov 23, 2013 at 1:19

2 Answers 2


If you want to power all of the LEDs in series, you only need a single current-limiting resistor for the entire string. However, your power supply voltage must be at least the sum of all of the LEDs' forward voltages. If that's not convenient for your application, you should divide them up into separate strings and connect the strings in parallel. Importantly, each string should have it's own current-limiting resistor.


If you have 100 LEDs in series you're looking at about 160V DC to drive them. Or 240V for green or nearly 400V for blue or white.

Separate them into smaller strings. If they are different colours, adjust the number in each string to give a convenient drive voltage - e.g. 10 red LEDs and 2V across the current limit resistor will work reasonably well off 18V.


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