0
\$\begingroup\$

I have a 10 metre set of 12VDC "5050" LED lights (RGB). The manufacturer/distributor (from eBay) quote these LEDs as being 10.8W per metre (108 W for 10 metres). But they also quote (on their eBay advert) 1.2A for 5 metres (2.4A for 10 metres).

I currently have a 6A power supply driving these lights without any problems.

I'm confused because I thought Power = Current x Voltage - so the current should be 9A (not 2.4A?).

I'm guessing that the LEDs draw a lot less than advertised anyway as I have them on 'fade effect' so they just fade between solid colours (i.e. at most two of the RGB LEDs on at one time).

I want to connect another 5 metre set of lights to my installation. I've read that, due to current consumption, it's best to connect these in parallel.

However, I'm not sure how to calculate the power/current consumption in order to work out whether I will need to upgrade my PSU?

Also, I'm worried that due to the different resistances of the two strips (one 5 mtrs and one 10 mtrs), one will be significantly brighter than the other or it will cause other problems?

Could you shed any light :) on this please?

\$\endgroup\$
  • 1
    \$\begingroup\$ Your calculations are right, their figures don't match up. So frankly, any answers based on them would be pure guesswork. But if they need 12V and all you have is a 12V supply then parallel is your only option, though your supply may well not be powerful enough. \$\endgroup\$ – Finbarr Aug 22 '17 at 13:23
  • 1
    \$\begingroup\$ Get a meter and measure it? I'd have more confidence in that than in contradictory information from eBay listings. But IF it takes 10.8W per metre then you're right about 9A for 10 metres on top of the 4.5A for the 5 metres you already have. \$\endgroup\$ – Finbarr Aug 22 '17 at 13:34
  • 1
    \$\begingroup\$ Two loads in parallel require the sum of the currents for each load, yes. But I can't tell you how big a supply you need based on BS information, nor can anyone else. \$\endgroup\$ – Finbarr Aug 22 '17 at 13:53
  • 1
    \$\begingroup\$ You also have to take into account the voltage drop of the strip itself. While 1meter may draw X Amps, 5 meters will not draw 5*X Amps, but a bit less. This effect is more dramatic on denser strips (more LEDs per meter). \$\endgroup\$ – Wesley Lee Aug 22 '17 at 14:01
  • 1
    \$\begingroup\$ If they're designed to be the same brightness when powered at the same voltage, then yes they will be the same brightness when you put them in parallel. The only problem for the supply will be if it gets overloaded - but then we're back into guesswork territory again. \$\endgroup\$ – Finbarr Aug 22 '17 at 16:15
1
\$\begingroup\$

You're right to be confused because the figures given don't tie up. 10.8W per metre at 12V does indeed give you 4.5A for the 5 metre length and 9A for the 10 metre length. So which figure should you take? It's not uncommon for listings on eBay to be wildly inaccurate, possibly just cut and pasted from another listing or badly translated so the answer is... neither.

Measure the current consumption and rate your supply accordingly. If you don't have a basic multimeter, and you're planning to play around with things like this, get one - they're widely available at budget prices. The alternative is that you end up overloading your supply; if it's a cheap one with no overload protection and you're lucky it will just quietly fail, if you're not it could overheat and start a fire.

As both sizes are designed to run from 12V you simply connect them in parallel to a 12V supply and add the currents together, then pick a supply that can comfortably deliver that load; I'd suggest allowing at least 25% overhead for safe and reliable operation.

Provided the different sizes are designed to be the same brightness when powered at the same voltage, then yes they will be the same brightness when you put them in parallel.

Incidentally, LEDs are not linear devices so the concept of resistance doesn't apply to them. Some even have regulators built in and won't behave like resistors at all.

\$\endgroup\$

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.