# Powering up LED strip - How many amps?

I want my Arduino to drive a LED strip.
The LED strip in particular is the 3528 non-waterproof type.
To power this up, I need a 12V power supply.
My LED strip is going to be 5 meters.

Now my questions are:

1. Is it important on how many amps, my powers supply will feed? One guy did it with 12V, 5A power supply. Considering the length (5m), is a particular amperage value necessary?

2. Can I power up the Arduino Nano with the power from the power supply? Voltage wise, I am clear, since the nano can handle 12 volts... What I am scared of is the amperage... Considering that I need a particular value of amps to power up my LED strip, can the nano handle this many amps? Is there a limit to how many amps the power supply line that I will feed my Arduino has to be?

Usually strips have not individual LEDs, but are composed of a lot of "pieces" in parallel, and each piece is composed by three leds in series along with a resistor.

Usually the strips are composed by 60 leds/m (i.e. pieces of 5cm), but sometimes also 120 leds/m or 30 leds/m. Let's assume it is the 60 leds/m version (you can check the length of the piece you can cut - which should be 5 cm - and that it contains 3 leds).

This means that in 5m there are 100 pieces. Now, usually the 3528 leds absorb 20mA of current for each segment (the 5050 version should be rated at 60). This means that the whole led strip should absorb 2A @ 12V.

Now the power supply. 2A is the nominal current the whole strips should absorb. Add about 200mA for the nano, and increase a bit the current requirement not to stress the power supply: you get a 12V 3A power supply at least (note that, while 12V should remain the same, you can take any current greater than this value - if for instance you have a 12V 5A power supply it is perfectly fine).

Now, the nano. It surely can't handle the 2A current. This means that you can power the arduino nano from the 12V. BUT beware: you should connect the 12V power to the VIN input (so the onboard regulator lowers it to 5V). Then you can't power the LEDs directly from the nano, but you need a transistor to decouple them. You can take any nMOS with a drain current greater than 3A or an NPN with collector current >3A and build the appropriate curcuit to drive them, then connect the + terminal of the strip to the +12V wire, the transistor source (or emitter) to the ground wire and the gate (base) should be driven by the nano.

• Thank you so much! Out of ignorance, i didn't mention that these are UV LEDs for PCB production. So each led, has not three leds inside, that means even less power consumption! Something like this: ebay.com/itm/… – user1584421 Jan 17 '17 at 20:13
• @user1584421 The 5050 version of LED has three leds inside each package, that is why the 5050 is rated at 60mA and the 3528 at 20mA. If you look at the photos you will see that they are different. Usually the color does not matter a lot, since the limiting resistors are trimmed to get 20mA out of each segment. As you can also read from the description, the input should be "12V 2-3 A for 5meters 3528 300LED light strip", which is what I told you (2A is the minimum, but use a bigger one). – frarugi87 Jan 18 '17 at 8:01
• One more thing: when I built my bromograph they discouraged me from using LEDs, because tubes are more homogeneous. With 5m of LEDs you can glue them very close, so probably you will get a nice exposure also with leds, but try to glue them very close, make a "zig zag" pattern and place the PCB a bit far, so the light can spread evenly.. – frarugi87 Jan 18 '17 at 8:04
• Thanks a lot! On a slightly unrelated note, given this setup, and with a 2.5A power supply, how much the exposure time you think should be for a PCB? – user1584421 Jan 18 '17 at 14:01
• @user1584421 Probably you are asking the wrong person, because I was never able to obtain a truly good PCB this way ;) anyway I know that exposire time varies with a LOT of factors, including primarily the photosensitive film you are using, its "freshness", the developer concentration and type, even the climatic conditions (even if these should have a little impact). What I suggest is to make a "trial" piece, with some small traces and gaps, cover almost all the board and start it. Then after 10 seconds uncover the board some more, in order to have different exposures in the board. continue... – frarugi87 Jan 18 '17 at 15:16

1) Is it important on how many amps, my powers supply will feed?

Yes. You need the PSU to be able to supply as much current as you will need for everything you are powering form it.

You don't say how many LEDs will be on this strip (only the length of the strip) and I am not sure how much current/power a single one consumes, but for illustration let's imagine there are 100 LEDs on your strip and each one takes 0.8W.

100 LEDs x 0.8W per LED = 80W.

P = VI, where P=Power, V=voltage, I=current.

So I = P/V = 80 / 12 = 6.66A -- call it 7A.

The amount taken by the Nano will be small in comparison - if you said 0.5A you'd be safe.

So with these figures you'd need a 7.5A PSU.

Considering that i need a particular value of amps to power up my LED strip, can the nano handle this many amps?

You can power the Arduino from the same power supply, yes. The Arduino will draw whatever current it needs. (Think of it this way: mains sockets are 13A - a desk lamp won't draw 13A.) What matters is that the supply can supply at least enough current, and that the voltage is correct.

As mentioned in the comments, you can't drive 3528s directly from the Nano - but you don't ask about that so presume you either know how to do it, or will ask another question.

Because there are three LEDs in series, you cannot drive these LEDs from a 5V supply. The LED strips say "+12V" on them to mark the anode and that's the maximum voltage we suggest. We've found that if you're ok with them being a little dimmer, even 9VDC works very well.

Each segment of 3 LEDs draws approximately 20 milliAmperes from a 12V supply, per string of LEDs. So for each segment, there is a maximum 20mA draw from the red LEDs, 20mA draw from the green and 20mA from the blue. If you have the LED strip on full white (all LEDs lit) that would be 60mA per segment.

To find the total maximum current draw per meter, we would multiply 60mA x 10 (ten segments per meter for the 30/LED per meter strip) = 0.6 Amps per meter OR 60mA x 20 (twenty segments per meter for the 60/LED per meter strip) = 1.2 Amps per meter. Again, that's assuming you would have all the LEDs on at once and that you are powering it from 12V. If you're going to be PWM-fading between colors, maybe 1/2 of that is what you'll be drawing. Still, you do need to have a fairly decent power supply to run this strip, all those LEDs add up!