Electronic – Why doesn’t the 5050 led strip draw as much power as I expect

ledpwm

I've been playing with a 5m long RGB LED strip, with 300x surface mount 5050 RGB LEDs, but can't understand why the strip isn't as bright or drawing as much power as I was expecting.

I've had a look at Arduino and 5A 12Volt driver, which appears to be talking about the same sort of product, but the answers there don't help me understand.

From the spec:

LED Light Source  5050 SMD LED
LED Beam Angle    120 Degree
LED Power         14.4W/Meter, 0.24W each LED
LED Quantity      60pcs LEDs/Meter
Working Voltage   DC 12V
Common Type       Anode

The reel however says something a little different:

Model:            5050-1M-60LED
Color:            W/RGB
Voltage:          DC-12V
Power:            72W/5M/5A

The IR controller is connected up via a 4 way ribbon cable, one wire for each colour and one for the 12v line. Each 5cm section contains three 6 pin RGB surface mount LEDs and three 1206 SMT (resistors?) labelled 151 (for Green & Blue I think) and one labelled 331.

The IR controller manual & case detail the following spec:

Output:           Three CMOS drain-open output
Connection mode:  common anode
Output current:   < 6A (on case)
                  < 2A each color (in manual)

So, I was expecting when I powered it up and set it to full brightness Red, Green or blue, for 2A to be drawn, and when I switch it to white, for 6A to be drawn.

That's not what I'm seeing though. At 11.95v I'm seeing each colour on it's own draw between 1 and 1.3A, while full brightness white only draws 2.2A, or substantially less than the three combined!

On a hunch, I cranked up the supply voltage to 14.4V (given the 72W/5A on the reel) and now I get much closer to what I expect, but the full brightness white is still drawing well below the 72W I was expecting. The full results were:

Red   (full brightness)    1.325A  11.95V  15W    2.000A  14.4V  29W
Green (full brightness)    1.021A  11.95V  12W    2.000A  14.4V  29W
Blue  (full brightness)    0.996A  11.95V  12W    1.978A  14.4V  28W
White (full brightness)    2.218A  11.95V  27W    3.961A  14.4V  57W

Is there something I'm misunderstanding about how these circuits should behave?

Is it likely to just be the IR controller limiting the current available to the LEDs, thus resulting in them being less bright and drawing less current?

Could I just hook the 12v supply directly to the LED strip without the LED controller to measure current and get a feel for brightness, or am I likely to burn out the LED strip without the 'proper' controller?

I haven't yet cracked open the IR control box, to look at what components it contains, but would be happy to do so if requested…

… Opening up the IR controller, the PCB is marked EC-LED-19A, so it is probably this product but that still doesn't give me a datasheet. The significant components seem to be an unlabelled 14pin IC (presumably a PIC), an FT24C02A serial eeprom, a 78L05 regulator and 3 more SMT caps and 68ohm SMT resistors. Each of the RGB channels have a 10k resistor, a 2k resistor and a 3 pin SMT package labelled WFAON, which I can't find a datasheet for.

It is possible that LED strips: 46% resistive losses? may be able to help answer this question more comprehensively.

Best Answer

I once had the same problem and could not figure it out until I measured the voltage on the other end of the strip: It was dropping whole 3V accross the entire length! You could even see the difference in brightness when comparing the first and last LED.

Consider the resistance of the 5 meter trace of copper. These strips are usually produced on cheap flexible PCB with the default (35um) copper layers and have very high resistance.

It is clear that these LED strips will, at least in one piece, never meet the spec printed on them.