Electronics 2000

I needed to bring this to the forum to see if somebody can shed some light.

Ill try to keep it short.
I recently had a project involving the installation of approximately 60m of LED strip.

Strip Spec: 2835 12Vdc 14.4W per m (60pcs per m)
PSU: 12Vdc output 16A 192W

Either I dont have a grasp of Ohms law or I am not aware of how the electronic circuit works.

The kit was supplied by the client.
There were 4 power supplies and the LED strip was to be split over the 4 PSUs in differing lengths. The uniformity of the light output was not to be an issue as it was for effects only.

My quick calculation 60m x 14.4w = 864w / 4 = 216w approximately per PSU so assumed we may need another power supply.

Now the runs of LED strip were split over 8 runs (2 per PSU) the longest being 8m & 9m.

My ohms law calculations and volt drop allowances indicated I would need much bigger cable than the 7 strand speaker cable supplied and should run more cable.
The client explained that this is what had been specified for the project and could not be done any other way.

The supplier explained that the PSUs had some tolerance and the 14.4w per m for the strip was worse case scenario! (whatever that meant).

I was not happy being told this as surely Ohms law cant be wrong.

After a few discussions the client asked to do a test rig in the workshop. We ran about 27 m split over 4 runs (so approx 7m per run) on one power supply. I would have expected around 390w to be drawn from this set up (well over 16A rating of the PSU). Unfortunately I didnt have a amp tester. But we certainly had a volt drop of about 4V on the longer runs.
We ran the LEDs for 5 hours. The protective device on the PSU held out ok. The PSU only became warm and the 7 strand speaker cable feeding all 4 runs only became slightly warm.

"so that's all ok" the clients says and goes ahead with the project.

What I do not understand is why Ohms law suggests I should be using more PSUs bigger cable and shorter runs. But in reality it was quite a different matter.

Can anybody explain from an electronic perspective why it should be like this? Is it the circuit layout and components of the strip? The volt drop and lower powered LEDs? Do the manufacturers over specify there products? Whatever the answer the LEDs could only have been drawing a fraction of what had been calculated.

Hi, phhhhhhheeeeeeeeeeewwwwwwww, any chance of a diagram. I have read that but its kinda in one ear and out the other. I am sure if I study it for a couple of hours I will sort of he my head round it. I do these sort of problems because I learn from my and other peoples mistakes ( I am not suggesting you have made any ).

The laws of physics must hold true or be rewritten, I am fairy confident Ohm got it right.

The drop across the LED is not linear or proportional to current, spec varies between 1type and another.

Diagram pls.

Tar&stuff.

Ahoy Pirate!
Sorry, I probably gave to much info.
I think some mistakes may have been made but it works and that's all the client was concerned with,
I suspect there may be a longevity issue a bit further down the line but they don't care as its only temporary.

For clarification its quite a simple diagram really;

PSU 240Vac input, 12Vdc output. (PSU rated at 16A 192W output)
2 cables connected to output.
Cable 1: 3m (7 strand 2 core copper cable) running 8m LED strip (12V, 14.4W per m)
Cable 2: 0.5m (7 strand 2 core copper cable) running 9m LED strip (12V, 14.4W perm)

If you want a diagram of the LED circuit, I am afraid its not available.
But can tell you it looked like one resistor every 3 LEDs from memory it could have been 470 stamped on the LED but not sure about that.

From the details above. 17m x 14.4W = 245W (That's all ready more than the PSU is supposed to be capable of).
245W / 12V (not including Vd) = 20A (that's 4 amps more than the output rating)

During testing we had about 27m running off one PSU. So there is no way the LED strip was drawing 14.4W per meter.

Using a volt drop calculator and assuming the cable diameter is 0.5mm (it may have been less) then the volt drop would have exceeded the 12Vdc supply!
I think we had about 8V on the end of the longest run.

Thinking out loud!
I am wondering if I am answering my own question here?!?
If it was 470 stamped on the resistor I wonder if that is the resistance?
And if 12V/470ohms = 0.02A. Does this mean every 3 LEDs is drawing 0.025A maximum?? (is that right?)
Then for a 9m LED strip with 60 LEDs per m would be 4.5A for 9m.....

Im not sure I have got this right but one way or another there are lots of factors to consider when carrying out a simple Ohms law calc!

Head swimming... I will try to do the numbers, just having my dinner.

Not including Vd... Explain?

14.4 watts per meter is perhaps the max power dissipation, this would require a higher voltage, so you are well within the limits.
470 is a standard resistor size, and ( not sure where it says that ) so you have a 470 ohm resistor in series's with 3 LEDs?

Any way dinner, back in a bit.

Hi, I have looked at some of the numbers, I think the supplier sales rep has done a short course, you seem to be a genuine pro, I am a sort of tinkerer aboutera myself, though I do get paid sometimes.

The 14.4/m is the max, it must be designed for a range of PSUs, like you say the wires would get very hot if you used a different PSU, but then it would probably have thicker wires and higher voltage output.

470 Ohm in series with 3LEDs ( from what I can tell) = (assuming 2V drop across LEDs) 6V/47O=0.0127=12.5mA.
Lots of 12.5ish circuits in parallel.

Tell you what... When he has finished with the strips... I'll ave em. PM me...
It's the PSUs and wires that are toiling, the LEDs are OK.

Tar&stuff.

I make that...

60 meters X 60 LEDs = 3600/3 = 1200x0.0127A = 15.24A X 12V = 182.88 Watts approx.

Pushing it,hope you've got a fuse.

Tar&stuff.

So, the equivalent resistance of the strips is...

12/15.24=0.7874 Ohm in series with cable resistance ( excluding Vd PCB copper ( got my head around that bit now))

Back later to finish it off.

If you have a 4V drop in the cable, ie 8V applied to the strips then You have only got 2V across the resistor and a current of 0.004A much easier on the PSU but I would be surprised if you can see the LEDs in the dark!

Any way ... Things to do back later to fine tune.

Well, I can't. be bothered doing all the maths. You have not taken the drop across the diodes into your figures. They usually drop about 2V regardless of current, any way the rest falls into place.
The other possibility is LEDs in parallel, but I doubt it because the PSUs would fry.

Tar&stuff.