I have 40 LEDs pre-wired with resistors at 12V connected in a series and they won’t all light up. When I test a series of 5 lights together it works fine but any amount over 5 less in a series won’t work. I’m not sure why this is happening. My power supply is a 120V AC to 12V DC power adapter 5 ampere 60W. Should I be wiring in a parallel circuit instead?

I’m clueless on how to make all 40 work. I’m not electrician this is for a painting I’ve been hired to make, I’m an artist with little wiring experience.

  • 1
    \$\begingroup\$ N LEDs in series requires N times as much voltage. You also shouldn't use the ones that include resistors for this, since you only really want one resistor per series chain. \$\endgroup\$
    – Hearth
    Nov 11, 2021 at 5:58
  • \$\begingroup\$ If you have 40 LED in series you need much more than12V. It depends on the LED, I guess something like 80 ... 100VDC. \$\endgroup\$
    – Mike
    Nov 11, 2021 at 13:26

1 Answer 1


If the pre-wired LEDs are intended to work with 12V, then you should put them all in parallel rather than in series.


simulate this circuit – Schematic created using CircuitLab

It takes a certain minimum voltage to make an LED conduct (light up.) When you put them in series, that voltage adds up. When the sum of the forward voltages is higher than the power supply voltage, the LEDs can't light up.


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