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I need to confirm / refute my assumption about current in LED strips. I know they are usually wired in paralel blocks of several serial connected diodes. This means that current consumed by the LED strip flows thru the entire strip power wiring, right? So if I connect for example 2 strips in series, I will double the current flowing thru both strips. Is this right?

If it is, how much current can typical LED strip handle? I could not find this information for every single LED strip I tried, I only assume this is given by the width and thickness of the copper wire of the flexible PCB used.

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Let's say that this is a LED strip:

schematic

simulate this circuit – Schematic created using CircuitLab

The voltage at the ends of D1...Dnv line is the same as the voltage across D1a..Dna and D1x...Dnx and is equal to the supply voltage because they are connected in parallel.

The current drawn by the whole strip is the sum of currents drawn by each series strip like D1...Dn. The current drawn by a single LED is equal to the current drawn by the other LEDs in series with it. Only the voltage is lower because they are connected in series.

So, to supply similar devices with the normal voltage/current:

  • parallel: voltage remains the same across every element of the circuit and it is the voltage of the supply. The supply must be able to provide a current which is at least the sum of all currents drawn by each parallel device
  • series: current remains the same for every element of the circuit. If the devices draw different currents then they shouldn't be connected in series. The voltage of the power supply must be equal to the sum of voltages required by each device.

If you connect LED stripes in series, then you'll need the same current as for a single LED strip but a voltage n times greater, where n is the number of stripes you are using.

Only connect in series identical LED strips with the same ratings!

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Be careful about what you mean by "in series". If you mean that you're connecting the two inputs of one strip to the two outputs of another, you're really connecting them in parallel electrically, and yes, your concern about the maximum current that the first strip in the sequence (the one closest to the power supply) can handle is valid.

On the other hand, if you connect the positive input of one strip to the negative input of the other, you have them in series electrically. In this case, you would need twice the voltage, but at the same current as a single strip.

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  • \$\begingroup\$ This is the most correct answer here!! \$\endgroup\$ – Michael Karas May 31 '14 at 17:13
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So if I connect for example 2 strips in series, I will double the current flowing thru both strips. Is this right?

No that is not correct. For any given voltage that is powering a load of (say) 10 ohms, if you put another 10 ohms in series, the current will halve. Basic ohms law. For each 10 ohms you put in parallel the current will increase so if the first 10 ohms took 1 amp, adding a 2nd 10 ohms in parallel means there is another 1 amp taken from the power supply.

LEDs can be viewed this way but they have a non-linear resistance that could mean if 1 strip of LEDs took 1 amp, putting two in series might take nearly 0.5 amps or it could be much lower at just a handful of milli amps.

enter image description here

Here's another picture that shows how a series resistor can affect things: -

enter image description here

No two LEDs can be regarded as the same. Some LEDs have a limit of a few milliamps while others can take an amp. Read the data sheet for the device - this is very important and if you can't locate a data sheet for the device then the product is probably cheap and crappy.

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Your premise is built on a false assumption. While Voltage across parallel circuits is the same, and Current across Series circuits is the same, you are forgetting one thing. Each of these 12V led strip segments are composed of 3 leds AND ONE RESISTOR set for ~20mA @ 12.

If you were to place two segments in series, you would not get the same current if you do not double the voltage. In fact, 6 3.2V diodes in series would require more than at least 16V to even light up.

schematic

simulate this circuit – Schematic created using CircuitLab

In essence, you have Doubled the Voltage & Doubled the Resistance, thus keeping the current the same.
I = V/R is the same as I = 2V/2R.
But total power is the same. 12V * 0.04A = 48mW. 24V * 0.02A = 48mW.

As for the current rating of the FPC, keep in mind that RGB led strips have 60mA per segment (3 Colors x 20mA), and a typical 5 meter roll has 300 leds, so 100 segments. 6 Amps are full power. 12V * 6A = 72 Watts. But keep in mind, that FPC has a high resistance per meter. You quickly start seeing color fading at the far end of the strip. Ideally, any long runs are have power injected at 2.5 meter increments.

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