I am looking to purchase some LED light strips, and I have seen these:

7.5 Watt RGB LED Tape

They are 7.5w, and have 150 LED lights over a span of 5m and the website states that they are 12V. But I am not sure what transformer/driver to use for these lights. The QA section, below the product description mention:

" To work out what transformer you require this is quite simple. You take the amount of LED tape you have in metres and x this by the wattage of the LED tape you have, again per metre. For example 7m of 4.8w LED tape: 7 x 4.8w = 33.6w Therefore you would require a 60w transformer. "

So for 5 meter of 7.5watts the calculation is 5 x 7.5 = 37.5W The website has power transformers, but at 100W and 150W, and when I rang them they were out of stock and the prices for the transformers were about 3 times what I can see on ebay.

Firstly, can I use a transformer that is producing 40W which is just above the 37.5W that I need, instead of a 150W transformer that they sell and claim is compatible for this LED strip?

Here is one transformer, that looks equivalent to the one they sell, at a relatively affordable price:

This gives me 12v x 12.5A = 150W, but I can see a 15V x 10A, which also gives 150W. Which of these is suitable for me (as they both produce the same wattage, but have different Volts/Amps), and what do the different Amps and Volts mean?


Yes, you can use a 40w transformer to power your 37.5w load. However, given the quality of these small power supplies, running at 90%+ all the time will cause heat and premature failure probably.

You will need to drive the LEDs at 12v. Running them at 15-18v (or anything over 12!) is a bad idea for the life of the LED. You will need to put a resistor on an 18v supply to bring it down. I'd recommend simply getting the 150w 12v supply. Wattage/amperage is different than volts, where you can use a higher powered power supply and the connected lights will only draw what they need (example: running a 12v 50w light bulb from a 12v power supply rated at 150w is fine. Running a 6v 50w bulb on the same power supply is not fine).

  • Is a 150W transformer not inefficient. I know the LEDs will draw however much they need, but would a 150W transformer not get a lot hotter than say a 60W transformer or a 40W transformer? If I get a 60W to be on the safe side (as my requirements are only for 37.5W load) - that would be a little bit more capacity than needed and less wasteful than 150W - am I right?
    – Husman
    May 20 '14 at 13:41
  • A power supply (PSU) will only draw as much power from the wall as it needs to power the load. PSU's have a sweet spot for efficiency which could be in the 40-60% of rated load. So, if you need 75w, a 150w would be a good choice. In your example, a 60w running 37w or a 150w running 37w will have a negligible difference of actual consumption. Go for the 60.
    – mikegreen
    May 28 '14 at 14:17

I believe you have misread the listing for these lights. They are 7.5 watts for the whole string. These are incredibly bright and efficient lights that I have used for under-cabinet lighting and for accent lights. For the under cabinet lights, the sections are 1 - 2 meters long and only use a couple of hundred milliamps of power. I use the wall transformer power supplies designed for home CCTV cameras - 12v at 500 milliamps. They work great, they are small and can be hidden easily.

  • Are you absolutely sure you have the same lights? the listing does specifically say under the specification tab: "7.2 Watts per metre" and later on says: "Reel length can be 5m or 10m"
    – Husman
    May 22 '14 at 14:45
  • I'm sorry but I did misread the description, but I still think their specs are not clear. This is the item that I have.tmart.com/… As you can see it's only 20 watts for the whole strip. So calculating for the 7.5 W per meter, it's 7.5 / 12v = .650 amps
    – user76732
    May 22 '14 at 15:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.