# LED Strip Light - Power supply issues

I'm making this right now - https://www.youtube.com/watch?v=qlMtakaa7u8

Each 5m roll of LED takes 60W power (http://www.ebay.com/itm/182028302443?rmvSB=true). He's using two of these rolls, so a total of 10m LED Strip.

My question is, that he's using a 19v 3.14A adapter which gives out a wattage of 60W. Isn't this lesser than what is required for the overall 10m (60w + 60w = 120w?). What's the effect of using this lower wattage on the LED output?

Instead, I thought of connecting the two rolls in parallel but the 19V adapter and connecting in parallel will probably burn the LEDs due to the fact that the LED strips are rated at 12v. Can I connect the two sets of 5m LEDs in parallel to a 12V 5A (which will result in 60W, the maximum wattage the LEDs can handle) DC Power supply? I'm assuming that then both the 5m strips are running at full brightness then. Or am I wrong and since a total of 120W is required, should I go for a 12v 10A DC SMPS Power supply? I studies Physics a long time ago in school and I'm pretty clueless here. What I'm looking for basically is the best way to extract the maximum light out of the LEDs. Thanks a lot guys!

Most of these strips are laid out in a repeating pattern of three LEDs and their current-limiting resistors along a flexible circuit. There will be scissors marks every 10 cm or so.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Cut along the dotted line.

If it's this type of strip then the LEDs are effectively wired in series parallel along the length of the strip. You can connect the strips end to end but it should be clear from the schematic that you're actually connecting the individual strings of LEDs in parallel.

If you daisy-chain the strips then be aware that the first strip has to carry the current for all the downstream LEDs and the copper may overheat.

Alternatively wire each strip individually back to the PSU.

Figure 2. All you need to know.

• You need a 12 V supply.
• The current rating needs to be at least that required by the LEDs.

$10~m \times 1.5~A/m = 15~A$.

$P = VI = 12 \times 15 = 180 W$.

Be careful daisy-chaining the strips. The start of the first one will have to carry the full 15 A, falling to zero at the end of the second. It would be safer to feed 12 V in both ends. That way each end would carry 7.5 A, falling to zero in the centre.

• Thanks a lot for your time and effort! Really appreciate it! But I'm worried that the 180W power supply, while providing the maximum brightness, will heat the strips a lot, with nowhere for the heat to dissipate. Keeping this in mind, I'd rather keep the input Power lower than the calculated 180W. Sounds good? – Amit N Aug 9 '16 at 18:54
• You control the power by reducing the voltage rather than buying an under-spec. power supply. Yes, the power will be reduced but you could expect longer life from a 180 W PSU running at 10 V and, say, 140 W than at 100% power. See if you can find an adjustable voltage unit to suit your requirements. You are right to be concerned about temperature rise though. – Transistor Aug 9 '16 at 19:00

Yes, a 19V @ 3.14A adapter will light up a series string as you describe. But not well, at all. I have similar 12V @ 5A LED strips, myself, and I can operate them at a lot less than 12V. But the strips won't operate at their rated output using that adapter.

If you want the full brightness, and you say you do, and if you want to run two of them from the same power supply then you will need to support 120W of power. This means a supply that is rated as better than 24V @ 5A or else better than 12V @ 10A. However, it's probably best to use 12V supply capable of more than 10A and to star the two strings to the power supply directly. Don't chain them, if possible, as it's likely that there will be voltage drops in the wiring at some length away.

If you locate your power supply in one place and the LED strings somewhere else (let's say, as lighting underneath a stairway railing) then you must consider factors such as voltage drop across the wiring between the two systems, heating that may occur in that wiring, heating that occurs in the power supply located in some enclosed space, tripping hazards, etc. It gets less and less fun as you work through all the necessary details, in cases like that.

I see another answer here cites a "per isolated meter" figure. But I don't gather you are operating these in isolated meter lengths. Just a note.

• Thanks for your reply! Yes, I'm not operating in isolated meter lengths. I'm going to be cutting them down to 0.3 metre strips and line them up. The wiring from the mains will run for a minimum of 3 meters. Keeping in mind the answer given by @Transistor and the heat generated by such current flow, would you rather suggest I go for a 12V 10A power supply, or a 12V 15A supply to light the 10m supply? – Amit N Aug 9 '16 at 19:00

In the video, he has sections of 8 parallel strips, which is then wired in series after the 8th strip. He does this on both sides, each side which is individually wired to their own PWM module.

Because he has each section haphazardly wired in series, the current through one section is shared by the other. But since it's less than max voltage, the current also drops. He's estimating 9.5V through each section.

Here the voltage and current is less than you expect, not even counting the PWM power. No way it's taking a full 60W. And I doubt he's using 10 Meters on that panel as well.