i just wanna start by saying i appreciate any help at all and am grateful for this forum! Also, the only electrical knowledge i have are that of self experimentation, no education except reading online.

So i have always been curious to how xmas lights were able to go so far, while providing the right amount of voltage and current to each light bulb.

I am trying to build something similar, using lithium ion 3.7-4.2v batteries. I am building a dettachable solar system, so each globe has about 5-12 standard LEDs (around 3v each) inside them, a lithium ion battery, and a TP4056 charger.

Problem is, sending 5V through the wires, only the first globe would receive the full 5v,,, the next few would not receive enough voltage to the tp4056 charger.

I thought about sending 12v across and using a step down module at each globe, but that just seems so bulky. Which is why i ask the question, how does the xmas lights do it... Or other appliances, you don't see a bulky piece before each light, so it's not a step down module.

I thought about using resistors, but that's different for each globe as the voltage goes lower and lower throughout the main wire leads.

How do other appliances seem to handle the voltage regulation precisely without using a bulky voltage regulator? Could someone point me in the right direction?


(more information about my appliance) i'm making each globe dettachable so they can be used and charged individually and portable. When brought all together and re-attaching back to the main wire leads, they will all be used long term stationary with continuous charge.

  • \$\begingroup\$ You seem to have only considered parallel wiring so far, but Christmas lights are often wired in series. \$\endgroup\$
    – Dampmaskin
    Aug 3, 2018 at 23:57
  • \$\begingroup\$ There are 12Vdc systems series parallel strings and series high Vac string types \$\endgroup\$ Aug 4, 2018 at 0:33
  • 1
    \$\begingroup\$ Your assertion does not make sense. If have connected the globes in parallel, there should be a very minimal drop in voltage when you get to the last globe. If this is not true, you are either using extremely thin or otherwise high resistance wire or you have very poor connections producing high resistance. I'm not going to ask you for a schematic since you are a novice but please draw us a diagram or give us a good photo of what you have. \$\endgroup\$
    – DoxyLover
    Aug 4, 2018 at 0:59
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    \$\begingroup\$ Add schematic to your question. \$\endgroup\$ Aug 4, 2018 at 5:05

3 Answers 3


The standard solution to evening-out the variable voltage you get from long thin wires driving a number of items in parallel is to feed from both ends.

Compare the following two parallel arrangements. I've shown the resistance of the wires as discrete 1 ohm resistors.


simulate this circuit – Schematic created using CircuitLab

In the first configuration, the voltage to each 100 ohm load drops steadily as we get further away from the power feed.

I'll let you work out what the voltage supplied to each 100 ohm load is in the second diagram.

  • \$\begingroup\$ Yes very nice. Like it. \$\endgroup\$
    – K H
    Aug 4, 2018 at 5:59

Why not just have +5V (or whatever voltage you want) run on 1 wire, and use another wire as the ground? Just attach the bulbs as needed between the 2 wires. Some Christmas tree lights are done like this.

  • \$\begingroup\$ hi, thanks for your reply. I don't quite understand your solution though. When i run 5v through the wire, only the first globe would get the 5v. Voltage running past the first globe would have lower voltage, so charging the next globes would not have sufficient voltage to fully charge. \$\endgroup\$ Aug 4, 2018 at 0:05
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    \$\begingroup\$ @KarTuneTran: Based on what? \$\endgroup\$ Aug 4, 2018 at 0:14
  • \$\begingroup\$ @KarTuneTran Voltage on two wires with multiple taps does drop due to current drawn BUT this drop can be small compared to the bulb voltage. \$\endgroup\$
    – Russell McMahon
    Aug 4, 2018 at 14:55

Especially if whatever you're building will be run continuously or is semi-permanent, it is very important that you measure the current running to it and size your wires accordingly. You don't want to become part of an electricity related house fires statistic. The solution for a problem of volt drop over wiring, is to either reduce current or upgrade the wiring. Depending on your load and wire length, it is easily possible that using a slightly higher voltage and converting it down inside each globe will cost less than upgrading the wire. 12v to 5v switching mode regulators are efficient, small (bit bigger than a 7805 chip), and you can probably get them for a few shmeckels each. That said, here's a list of things you can do: Note that the currents here may vary depending on battery level as well as if you're setting it up to run at the same time as charging. You should plan for the largest possible current. It would help to know if you plan to charge the planets and then put them on display running or if you want them to be charged by the display itself and you simply want them to be able to run independent of it.

-Use a higher voltage, which will give you more power at the same current. You can do this by stepping down the voltage in each globe as above, or redesign your LED drive circuit to directly use the higher voltage.

-Measure your current and use the proper sized wire. While you're at it, make sure the wire has proper ratings for whatever you want to do with it. If you're putting it inside a wall, you need different wire than if you're putting it in a heat vent, or inside mobile equipment or a display. Also note that it sounds like Pluto might need significantly smaller wire than the sun, due to the number of LEDs involved.

-Decrease the length of the wires. If the wires are significantly oversized for the current they're expected to carry, sometimes it's nice to leave a bit of extra length in the system for future modification or repair, but if the wire is tightly sized to the current, or in cases where wire size is limited by space, etc, remember that every extra inch of wire is extra resistance.

-Alter your wiring pattern. You can give a heavy load it's own wires if necessary. You can have a larger wire backbone and smaller taps off of it for each individual fixture. There is a lot of reason not to have a wire be smaller than it needs to be, and much less reason not to have it larger.

-Check to make sure that your 5v source is capable of providing enough current for all attached chargers, as this could be contributing to voltage dropout. Evaluate if there is any disadvantage to upgrading the size and decreasing the length of the charging wires, equalizing their length in the process. The current to charge one lithium ion battery from a 5v source when the battery is near depleted is quite substantial and it may be better to have a 5v source for each one anyway.


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