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I'm new to electrical engineering and I've studied the basic elements like resistors, capacitors, and inductors in DC and AC circuits, but I haven't studied LEDs or diodes in general.

Right now, I'm in a situation where I need to cut and connect a 1.3m, a 0.35m, and a 0.5m strip in parallel with the source. From what I've been reading, LEDs are largely resistive loads. With that said, I'm assuming that shorter LED strips have lower resistance values and vice versa.

If I use the current divider rule to determine the current going through the strips of varying lengths, the shorter LED strips would theoretically have greater current flowing through them because they have lower resistance values. In effect, that would mean more power is supplied to the shorter strips and they become brighter than the longer ones.

For now, I'm assuming LEDs have the same working principles as resistors:

enter image description here

I need them to have the same brightness. Please help and correct me if I'm wrong.

Also, how do I determine the current being supplied in the circuit?

Thank you!

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    \$\begingroup\$ 12V LED strips usually consist of parallel sections with a few LEDs in series and one resistor in series. (3 LEDs and one resistor per section is a common configuration). There is usually a 'cut here' line between each such section. Connecting multiple such strips in parallel is fine, that's what they are designed for. \$\endgroup\$
    – KristoferA
    Commented Jun 16, 2021 at 11:30
  • \$\begingroup\$ Isn't the 'cut here' line initially designed for another series connection? Because you might have a gap in your house that you might not want to light up so you can cut the strip to later connect it with a wire. \$\endgroup\$
    – Jamie
    Commented Jun 16, 2021 at 11:47
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    \$\begingroup\$ Although it may feel like a series connection, it is actually a parallel connection. Each section of 'cut here' groups are connected in parallel within the strip. \$\endgroup\$
    – KristoferA
    Commented Jun 16, 2021 at 11:56
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    \$\begingroup\$ Oh okay! That explains a lot! Thank you. Is that also why when a section dies out the whole strip still functions normally? \$\endgroup\$
    – Jamie
    Commented Jun 16, 2021 at 13:05
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    \$\begingroup\$ Yes, that is correct. \$\endgroup\$
    – KristoferA
    Commented Jun 16, 2021 at 13:15

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The brightness depends only on the current through the led. This current is determined by supply voltage minus total voltage drops across each led divided by the resistance in each series strip. Some led's have 2V, some 1.3V across them. It can be measured from the tracks. This means i) the total number of led's in a series cannot have a total drop >/= to supply, and ii) for the brightness in all series to be the same, the resistors in the strips should be shunted out and a single resistor used at the head of each series to control the current.

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If the pieces of "strip Leds" belong to the same cut "strip Leds", the Leds have, in principle, the same characteristics.

Here there are short. Be careful (if "very long" ...) to feed them with the "same resistance" series of cables. Preferably use identical lengths of "strip Leds".

The LEDs at the end of the strip can however be a little less illuminating (voltage drop along the strip). (google translate).

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With a standard cuttable LED strip, cutting it shorter will increase the resistance, not decrease it. An LED strip is actually lots of LED + resistor combinations, wired in parallel. The more you chop off, the fewer LEDs there are to conduct.

Connecting several strips in parallel is fine. You're simply adding more LED + resistor blocks in parallel again.

The brightness may vary slightly, because of the resistance of the wires, including the copper tracks in the strips.

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enter image description here

Figure 1. A 2835 LED strip. Image source: LEDnique.com.

enter image description here

Figure 2. Schematic. Image source: LEDnique.com (created by me).

The 12 V rails run the full length of the strip.

In each section three LEDs are wired in series with one or two current limiting resistors.

  • The strip can be cut at any scissors-marked point without breaking a series string.
  • Strips can be connected in series but if fed from one end note that the first strip will have to carry the current for all the LEDs and may overheat and fail. Additionally, voltage drop along the line will cause the voltage to droop. For this reason it is better to run wires from each strip straight back to the power supply.

I need them to have the same brightness. Please help and correct me if I'm wrong.

Provided the strips aren't too long (and yours are not) they will all be the same brightness.

Also, how do I determine the current being supplied in the circuit?

Either,

  • look up the current requirement for the full reel and proportion it down for the length you're actually using, or
  • measure the current in one length of strip, and divide by the number of scissors segments to work out the current draw by each string.

Either way you can then calculate the current draw for any number of segments.

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  • \$\begingroup\$ The schematic shows three LEDs and two resistors in each cutable segment. Yet, from the picture of the PCB only a single resistor is visible in each segment. Is the second resistor in each segment on the reverse side of the flexible PCB? \$\endgroup\$ Commented Dec 30, 2023 at 12:53
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    \$\begingroup\$ Good catch, @ChesterGillon. They're not the same strip. Some have only one resistor. Others use two to spread the power dissipation and run cooler. There's nothing on the back because they're usually coated with double-sided tape. \$\endgroup\$
    – Transistor
    Commented Dec 30, 2023 at 12:57

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