Ask Electronics

The strips use 3V white LEDs, power to which is delivered via resistors or linear current regulators. Unless you see any inductors, there is no buck converter from 5 V to 3 V.

Why does this matter? Well, with resistors and linear regulators, 𝐼~in~ (current in) pretty much equals 𝐼~out~ (current out). So the efficiency 𝜂 = 𝑃~in~ / 𝑃~out~ = (𝐼 × 3 V) / (𝐼 × 5 V) = 60 %. Extra cable resistance will reduce the current, brightness and power, but still exactly 40% of power leaving the USB charger will be wasted before it gets to the LEDs.

However, I would advise against multiple, cheap USB connectors in the circuit: when moving the setup, their resistance changes somewhat and you would get blinking. The worst thing that could happen is a switch, such low voltage cannot spark over an oxide layer and eventually even small movement will blink the lights. I would get a good thick USB cable and solder it directly to one of the strips instead of whatever it came with, connecting the other with some thick wires.

So it does not matter, if you want better efficiency, use 12V (75%) or 24V strips (87%), or get just an LED array without resistors that needs a constant current driver (theoretically 100% but CC PSUs are slightly less efficient). Or make a constant current driver by fine-tuning the voltage of a PSU (by adjusting the feedback resistive divider) to 0.5-1 V above the LEDs’ voltage drop, then using an appropriate resistor to limit the current.

I’m slightly confused by your question. Are you trying to power two thingies off a twofer adapter, or are you trying to multiplex something?

It sounds like you’re just trying to do power so:

See how much each strip draws (either via data sheet of amps per meter times 0.69, or measure it) and if that amount exceeds the capacity of the adapters you want to use, it’ll either not work or shorten the life of the gizmos (or if it’s really cheap crap or you’ve gone way the heck over the capacity of any component it might catch fire a bit).

Note the current on the datasheet wont be linear per length, but for 27” chonks if should be close enough.

Edit: I see now your LEDs are 3V. Personally I’d recommend replacing them with 12 or 24V, or at least 5V to run properly on the 5V thing you’re plugging them into. Why did you go with 3V product?

There are two questions to answer:

• which current and voltage do the LED strips need (5V, 2A would be something you could support with USB 2).
• what output does the charger provide as a maximum? Is it enough to power both strips (5V and two times the A).

If these match it should work. Another topic could be the cable itself. Theoretically it could start to burn, if you try to channel to much current through it, but with simple USB I doubt it. If it is getting hot after some time, scrap your setup. This would be a fire hazard.