I am making an LED strip installation. I will be using 15 meters of APA102 LEDs. They require constant voltage, I believe. https://cdn-shop.adafruit.com/datasheets/APA102.pdf

I calculated that I need 23.4A at 5v, or around 120w. Adding 20% for safety, I figure I need a 150W 5v supply.

I will be buying a power supply from Digikey: https://www.digikey.ca/product-detail/en/tdk-lambda-americas-inc/LS150-5/285-1817-ND/1918828.

I will be running large wires from the power supply to each of the strips (ie in parallel), while running the LED strips' data lines in series. The Arduino will control the strips, and that will be powered by the supply as well (that should be fine, right?).

I will encase the power supply and Arduino in an aluminium encasement. This will hang from the ceiling outdoors, and be subject to low temperatures in winter (Canada). I am figuring on a temperature range of -10C to 30C. I will be testing it indoor at first, of course.

My confusion is with safety of the power supply. I am relatively new to electronics, and the high current and power makes me a bit nervous, so I want to verify things first.

I think that I need to hook up the live AC wire to L on the power supply, the neutral AC to N, and the ground AC to GND. I then need to hook up the Arduino and LEDs Vcc to V+, and Arduino/LED GND to V- (correct?).

The installation instructions from the power supply http://www.us.tdk-lambda.com/ftp/manuals/ls_manual.pdf states "This power supply has a possibility for hazardous voltage to appear at output terminal depending on the type of failure. The outputs of these products must be earthed in the end equipment to maintain SELV. If the outputs are not earthed, they must be considered hazardous and must not be made user accessible."

Would you be able to explain what this means I need to do? I have read quite a bit about this, and have an idea of what it means, but I want to be absolutely certain.

I think it means that I need to, in this situation, hook up V- to the AC GND. Do I have this correct?

Also, do I need to hook up the chassis of both the power supply and the encasement to GND?

Thank you for you help.

EDIT: Thanks for your answers. I found this series of articles that made things much more clear for me. https://www.acromag.com/white-paper-electrical-ground-rules

Also, I am still wondering how to verify that the PE and Neutral on the AC in my house are actually independent, up to the mains board. This is the best I have found: https://soundforums.net/features/192052-basic-receptacle-testing


2 Answers 2


I think the way you want to do the connections certainly makes sense. It is as I would to them as well.

And as you said, for safety reasons and to fulfill the requirement of the manual, you should connect the V- wire to the protective earth (FG) terminal. The point is, if something goes wrong and there is current flowing out of the V- terminal, it will find a low impedance route to the earth through this connection and the earth wire coming from your home's electrical installation and not through the person that potentially comes in contact with the circuit.

That is also the reason, why you should hook up the chassis of both the power supply and the encasement to the earth. However, I make the educated guess that the chassis of the power supply already makes contact with the FG terminal (you could easily check this out with a multimeter) and so no special care should be taken there. The connection of the chassis of your encasement to the earth could be done either with an extra wire or with the mounting screws/holes -if any- that you will use to mount the power supply, or with both ways.

All these of course under the condition that the PE terminal of your house's installation is indeed wired to earth.

One last point: If you indeed indeed to hang the whole thing from the ceiling, pay attention that this method of mounting is NOT recommended, as you can see in Chapter 5 of the manual you have linked. The reason is, as they say, that in this case the heat from the electronics will be trapped and not be able to leave the enclosure. That would mean probably that the output power will be reduced and/or the power supply could shut-down due to high temperatures or worse be damaged. And you are anyway quite at the limit if you see the derating curves, so I wouldn't do it like you intend to.

Lastly, I assume you have already considered this, but it is also imperative that your enclosure has air vent holes.

  • \$\begingroup\$ Thanks, but I am still confused. Connecting V- to FG (which I assume is the same as PE) means connecting it to Neutral or Ground of the AC? \$\endgroup\$
    – Fed
    Jul 6, 2017 at 14:53
  • \$\begingroup\$ FG (or PE otherwise) is indeed the Ground of the AC as the name says. But we should be very careful here. I don't know how your building is wired, but the most proper and safest way would be that N and PE are NOT connected. The PE should normally go to an earth electrode. If N and PE are shorted together at your building, then connecting the enclosure to PE could under circumstances be more dangerous than if not. \$\endgroup\$
    – nickagian
    Jul 6, 2017 at 15:34
  • \$\begingroup\$ Should I use a multimeter to measure the resistance between N and PE? If it is very high then all is good, correct? \$\endgroup\$
    – Fed
    Jul 7, 2017 at 0:16
  • \$\begingroup\$ @Fed It's not about resistance (N and PE can end up connected in the end in some installations), it's more about PE being independent. Should N wire get damaged and be exposed to high voltage, the PE should still remain safe to touch. \$\endgroup\$ Jul 7, 2017 at 7:11
  • \$\begingroup\$ Ok, but is there a way to test to make sure N and PE are independent? \$\endgroup\$
    – Fed
    Jul 7, 2017 at 7:14

The connection diagram provided in the manual is what most users would want:

enter image description here

(Note there is no connection between Protective Earth and outputs)

However, if you need your installation to have a SELV rating, you will need to additionally connect the PE terminal to either V+ or V-. In this case it is also advised to check that the PE terminal in your mains socket is actually wired correctly, because if it isn't, this connection will make things much worse safety-wise than no connection at all.

Considering you plan to hang it on the ceiling level (where it cannot be easily touched), I doubt you actually need SELV. Personally, if I wanted to have SELV, I would just pick a different power supply which guarantees that high voltage never reaches its output, without the need to connect the outputs to PE.

  • \$\begingroup\$ Do you have an example of such a power supply? \$\endgroup\$
    – Fed
    Jul 6, 2017 at 14:59
  • \$\begingroup\$ @Fed Yes \$\endgroup\$ Jul 6, 2017 at 15:21
  • \$\begingroup\$ I did not notice. On the app the blue is faded and on a 3 letter word did not show up well. Sorry. I will delete the other comments. \$\endgroup\$
    – Fed
    Jul 10, 2017 at 10:25

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