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I am working on a project where I have to use string LEDs running on 5V (USB power.)

I had purchased one such string, which is 10m in length and has roughly 50 LEDs inside.

My specification requires me to use 20m of this string light , so I tried to join them together. That didn't work as the brightness of the second string attached was way lower than the first one connected.

I found that at the joint for the second string I was getting 2.4V (starting with 5V.)

This becomes a problem as I need them to be equal brightness.

I am not able to find why this would happen as all the LEds are in parallel and should get equal voltage along the string.

There is another seller selling the same LEDs in parallel and they work just fine. They have 3 wires running instead of 2 , and I can't figure out what the 3rd wire does exactly. (It's too expensive for me to break it apart and reverse engineer it.)

Please help me out.

LED string lights.

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  • \$\begingroup\$ What to check for when buying an electronic component or module. Find the data sheet, be sure of product quality. \$\endgroup\$
    – Andy aka
    Commented Jan 8, 2021 at 12:45
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    \$\begingroup\$ "I found that at the joint for the second string I was getting 2.4V (starting with 5V.)" Then joining them that way is putting them in series. Instead, join both to the supply, one with 10m of wire. \$\endgroup\$
    – rdtsc
    Commented Jan 8, 2021 at 12:51
  • \$\begingroup\$ You get voltage drop in copper depending on wire thickness, wire length and current drawn. In addition, LED forward voltages may vary quite a bit. \$\endgroup\$
    – Lundin
    Commented Jan 8, 2021 at 12:54

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Those string lights have tiny wires with considerable resistance. It is unclear how you are attaching them, but if you are wiring them sequentially in parallel, the voltage drop along the wire results in much less voltage at the far end of the 10m line.

There are two options that I can see:

Run a separate, lower gauge wire to each strip and posibly to both ends of each strip so that current doesn't have to pass through the more resistive strip wires.

Wire the strips in series and connect them end to end. This will require a 15v power supply.

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  • \$\begingroup\$ hi , each string has leds in parallel attached in it , im simply taking the other string and attaching to the end of the first one(in series as u have stated). Also inside the individual string provided i have noticed the first led is brighter than the last one , which means with length im getting lower voltage on each led . What i cannot understand is how the other seller selling the 20m long one has equal brightness in all. \$\endgroup\$
    – Dracus_Loki
    Commented Jan 9, 2021 at 11:39

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