1
\$\begingroup\$

I have a string of led Christmas lights. They are wired in parallel with two wires coming from the power supply and terminating in a single led. It's missing a power source so I'm trying to get them working by connecting a DC wall wart directly to them. However, as the lights get further from the power source they get dimmer. Beginning by very bright, and ending in barely visible. It's only a string of 80 or so. My understanding is that in parallel, all leds should be the same brightness? I've tried different voltage PSUs starting at 5v and up to 9v. The leds are brighter with the higher voltage. I'm scared to go higher as this may blow the entire chain? Any help appreciated 🙏🏻

\$\endgroup\$
1
  • \$\begingroup\$ (1) You are using a power source different from what was originally being used. This means we have to trust that you had enough information and knew how to use it when considering a replacement unit. It's possible that a different power supply might yield different results. (2) The wiring has resistance. So the further from the power supply the less voltage each bulb has access to as the voltage drops down the line. (3) The problem could be a combination of (1) and (2). With more details it's possible someone here could be of more help. But with so little detail, less likely. \$\endgroup\$
    – periblepsis
    Commented Dec 13, 2023 at 21:15

1 Answer 1

Reset to default
2
\$\begingroup\$

Your understanding with parallel vs. series in an ideal situation is correct.

Where you're missing pieces is that the wire in between the lights are not zero resistance.

Particularly when there is a lot of current in the wire, there will be a voltage drop across the copper, and therefore, with a lower voltage drop across the LED, you'll have a lower current lighting up the LEDs at the end of the string.

Your options are, in order of what I personally would do:

  • Cut the LED strings - More strings, shorter, each with their own power supply
  • Splice in another power supply at regular intervals
  • Thicken the wires connecting LEDs, in order to reduce their resistance
\$\endgroup\$

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