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I am building circuit to control some LED strips. I will be using APA102 strips. https://cdn-shop.adafruit.com/datasheets/APA102.pdf

I am using an Arduino to control the strips. The strips will have approximately 100 rgb LEDs, and so the maximum current is 60mA x 100 = 6A. They will normally be at near maximum brightness, including when turned on.

I have been reading a fair bit about inrush current. I am strictly amateur, so please excuse my ignorance and poor terminology. My impression so far is that from a cold start, a 6A circuit can have a significant inrush current, especially if I include a 1000uF smoothing capacitor across the power supply, as advised by the LED suppliers, to protect the LEDs:

https://learn.adafruit.com/adafruit-neopixel-uberguide/power : "Before connecting a NeoPixel strip to ANY source of power, we very strongly recommend adding a large capacitor (1000 µF, 6.3V or higher) across the + and – terminals. This prevents the initial onrush of current from damaging the pixels."

https://github.com/ManiacalLabs/AllPixel/wiki/Installing-Optional-Components#21mm-dc-barrel-jack : "1000uF Capacitor - This capacitor sits over the strip V+ and Ground lines and helps smooth out the power for the LEDs. Some strips are more sensitive to power rail conditions and recommend such a capacitor be installed for reliable use. Regardless of the strip type, there isn't any harm in installing this capacitor."

https://github.com/ManiacalLabs/AllPixel/wiki/Assembling-the-PowerTap also suggests a 1000uF cap.

From my reading, the large cap will result in an even larger inrush current, exacerbating the problem. Is this correct? Is the cap really necessary?

How can I measure the inrush current?

Can this inrush damage the PSU?

How can I deal with the inrush current. I am thinking of using a NTC thermistor, specifically the NTC 5D-15 https://www.icbanq.com/data/ICBShop/board/00000000-NTC-POWER.pdf

Are there better ways to deal with the inrush current?

Also, I would like to use a mechanical switch to turn the circuit on and off. Should this go before or after the cap and thermister?

I know this is a lot of questions, but the more I read about this, the more confused I get. Thank you for your help in advance.

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Your problem can be summarized by asking how much inrush is required to energize a reactive circuit. Reactance can be caused by either inductance or capacitance or a combination of both. Basically, any kind of Z will need to charge up and store energy.

What you are describing is pure DC feeding capacitors and trying to charge them before the LEDs light. That is basically what a ballast does for flourescent lamp. But that is because it needs to charge enough to achieve the breakdown voltage of the gas inside. Then it can basically feed simple power to maintain the light. If you recall the old "starters" they were the switches that would allow it to do this.

The best way to mitigate such a problem with LED strips is to simply use a large enough power supply that can handle the total current and do not add extra caps.

LEDs alone do not exhibit the kind of inrush current that any kind of reactance does, whether inductive or capacitance. LEDs present as pure resistors. They do not need a starting voltage.

Just feed them the power they need and the only limiting factor will be the resistance of the wiring of a string. I.e., if you do not provide enough power then you can only light a certain length. The strips do have conductors integral to them but they are not of a very heavy gauge so there is voltage drop just due to the length.

Instead of using capacitors to extend that length it would be better to feed along some larger wire along the string to directly power the next section.

I have had strips that worked fine until they used blue, then the far ends would not be as bright as they were when they were green or red. One power supply separately feeding multiple sections is the key to bright LED strips.

Bottom line is to think in terms of resistances. Both the wire and the current requirements of the LEDs.

And keep in mind that different LED colors require different amounts of current. That can mean that if your strings change color, you need to adjust your power supply setup as if they are all white (R+G+B) or all blue. Those take the most current. If it will work for white, it will work for all the other colors just as well.

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    \$\begingroup\$ Thanks for the clarification. I built the circuit, and it works fine without the cap. Why do you suppose they suggested to add the cap, then? Also, does the same reasoning apply for a larger installation, requiring 35A at 5V? \$\endgroup\$ – Fed Sep 1 '17 at 10:44

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