# Multiple LED strips and one power supply

I have been given a purple, waterproof, high intensity (18 LEDs/foot) LED strip. I don't have any spec/datasheet for the LED's because they were acquired directly from a Chinese manufacture.

I can clearly see they are rated for 12VDC but cant find any other data on them. I am trying to lay 7 strips that are 5 feet each in length using one power supply. As of now I am using a 12V, 3A switch power supply that I recycled from an old laptop and the full strip of LED's turn on but they get warm to the touch.

My question is, whats the best way to hook 7 5 foot strips together? Series or Parallel? and how would I calculate the overall power consumption in order to pick a correct power supply? Can I add a 5 watt resistor to each strip and lower the current if necessary?

I have attached an image of the strip.

• I wonder what Morpheus would say: vimeo.com/7619378 Commented Jan 13, 2016 at 22:14
• picture of strip? Commented Jan 13, 2016 at 22:22
• I just added one. I can see from the LED its self its probably an RGB that was set to only product purple color. I also assume from other research that each LED array (red green blue within the module) pulls aprox 60mA. I've just done enough research that I've confused myself and want to make sure I get the correct circuit Commented Jan 13, 2016 at 22:29
• 60 leds per meter. Based on 150 Ω resistors, that's 20mA per series string, times 3 because 5050 leds, 60mA per 3 led segment. 60 leds / 3 leds = 20 * 60mA = 1.2 Amps per meter. 5 feet is 1.5 meters, so 1.8 Amps per 5 feet. 35 feet means 12.6 Amps. I suggest each 5 feet section has it's own power cables run to it, Parallel style. They will be warm but never hot to the touch. You can reduce the voltage to about 10 Volts for less power usage, but less brightness too. Or think about using a LED controller for PWM dimming. Commented Jan 13, 2016 at 23:03
• @Andyaka xkcd.com/566 Commented Jan 14, 2016 at 14:28

Most of these strips are laid out in a repeating pattern of three LEDs and their current-limiting resistors along a flexible circuit. There will be scissors marks every 10 cm or so.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Cut along the dotted line.

If it's this type of strip then the LEDs are effectively wired in series parallel along the length of the strip. You can connect the strips end to end but it should be clear from the schematic that you're actually connecting the individual strings of LEDs in parallel.

If you daisy-chain the strips then be aware that the first strip has to carry the current for all the downstream LEDs and the copper may overheat. You've no data sheet so you need to get a feel for what's normal temperature with one strip and add another, etc., until you feel it's getting too hot.

Alternatively wire each strip individually back to the PSU.

If you have a multimeter switch it to 10A DC and plug the red lead into the 10 A socket. Measure the current drawn by one string and work out how many strings your PSU can handle.

Update

The photo arrived after this post. Basic principle still stands but I can see a resistor per LED and the LEDs have six pins. You'll have to figure it out.

Calculating current requirement

simulate this circuit

Figure 2. Measuring LED strip current.

Hook up a multimeter as shown in Figure 2. and measure the current drawn by one strip. If using a digital multimeter then be sure to plug the red lead into the 10 A socket, and switch to 10 A range before powering up. The total amps will be the sum of all the individual amps readings.

Converting to watts

Let's say you measure 0.9 A. Power for that strip will be given by $P = V \cdot I = 12 \cdot 0.9 = 10.8 W$. You can then figure out how many of these strips you can power from a PSU of known wattage.

Your flashing LEDs indicate the power supply has shutdown - probably its thermal protection kicking in.

Plug your red lead back into the V socket before you finish. If you forget and hook it up to a battery, etc., a high current will flow, blow the internal fuse and possibly damage the internal shunt resistor.

• If I had the opportunity to use either method. Is there one that is more ideal? I need them to be cool to the touch and can change power supplies if necessary. and if I took each strip back to the PSU would I want to add a limiting resistor between the PSU and the negative lead to the LED strip? Commented Jan 13, 2016 at 22:50
• If you have a choice wire each strip back to the PSU. No external resistors required. The resistors are built in. See my sketch. You can cut just one section out and connect it to 12 V if you want. See the sketch. See the sketch! Commented Jan 13, 2016 at 23:15
• Okay, So ive hooked them all up to the PSU but when I have over 4 strip the LEDS flash every half a second... Any clue why this could be happening or a potential fix? I tried switching to PSU with 12V 5A -- 60Watt of power and that seemed to fix the problem. I guess I cant trust random LED strips from China to be require standard power. Commented Jan 15, 2016 at 15:14
• They're standard 12 V LEDs. You need to calculate your real power requirement. See the update. Commented Jan 15, 2016 at 18:31