LED Power vs current and voltage

Question

I have 6 parallel connected led "strips" each having 4 leds in series connection (see image below).

Those leds should be 3W each (2.4V and 700mA max each) I have a LED drive as power supply (should be 12V and 6A max)

I tried measure the voltage and the current and I found out, that there is only 1/6A = 0.16A running through each LED with the voltage 12/4 = 3V each (rounded) thus having 0.16*3 = 0.48W.

I can't find what is wrong with my observations and/or calculations or if those leds are not really a 3W LED. I guess there is something wrong with my calculations or observations because the LEDs in this setup are bright enough and based on my original calculation it should work this way.

Thanks for help

Answer 1

LEDs have no internal current regulation & require a series resistor to keep them from overcurrent that can cause them to melt/explode/die horribly.

Also, the listed "voltage" of an LED is the "knee" voltage of the diode...more of a "no light comes out below this voltage" than an ideal operating point.

Try running a series of 3 LEDs, with a 7.5ohm series resistor to see if you get the LEDs to react closer to "datasheet specs."

Something like this:

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: After testing it with a 7.5ohm resistor, you can check the current flowing through each strand, then use more/less resistance to get closer to your "goal" 700mA.

Answer 2

The problem is the LED driver supply's voltage is too low. With insufficient voltage you will not get enough current flowing.

You can try parallel strings of 3 LEDs but without the resistors. (IF AND ONLY IF THE SUPPLY IS A CONSTANT CURRENT SUPPLY, A CONSTANT VOLTAGE SUPPLY WILL BURN THEM UP) With an LED Driver (i.e. constant current) the resistors are a complete waste of power and takes away voltage from the LEDs and may not allow the LEDs to fully turn on. There may be nothing wrong with your eBay supply. It's very likely the LEDs did not get enough voltage.

You will never get evenly matched current in parallel strings. The more LEDs in the string the worse it gets.

Either you need a Driver for each string or you need to balance the load in each string. The simple (and most effective) way to balance the strings is to use a TI LM3466 which cost about $0.50 each.

You are better off using a higher voltage driver, e.g. 48V (e.g. Mean Well HLG-60-48 @ $30, 7 yr warranty, 94% efficiency) to drive 2 strings of 12 LEDs with two LM3466 to balance them.

UPDATE

Now we have established the power supply is not an LED driver but a constant voltage supply. Now it's about the amperage. Your power supply is likely a 1 Amp supply. If it were 6 Amp all 24 LEDs would have burned.

The power supply must be greater than the LED's V_f. You measured 11.6v so 4 (red red red blue) in series with a 0.62Ω current limiting resistor should work fine.

2 blue and 2 red may not work well. 4 red should always work well. 3 red 1 blue may occasionally exceed 12v and the outcome is unpredictable, may work or may be too dim.

It is best to calculate the resistor value with the measured V_f for optimum efficiency.

700mA may be too much and the LEDs will get too hot without any thermal management e.g. heatsinks and fans. You should be able to touch the star PCB for a second or two without it burning your skin, or about 50-60°C.

These are two burnt Royal Blue Luxeon Rebel Color LEDs that were not screwed down to the heatsink with 1 Amp flowing through them. 700mA would make them about as burnt. Amazingly they did not cause the strip to fail. They did get hot enough to begin to melt the solder. The 2 oz. copper pads bubbled up.