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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

Schematic

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  • \$\begingroup\$ Part numbers (and preferably datasheet links) to your LEDs and LED driver? \$\endgroup\$ – Null Apr 18 '17 at 21:37
  • \$\begingroup\$ Drive it with constant current and report back. Simplest way is to remove one LED and insert (and adjust) a resistor instead in your series string. I would suspect the claimed versus real Vf differ, hence the low current. \$\endgroup\$ – winny Apr 18 '17 at 21:37
  • \$\begingroup\$ When you say the LED driver is 6A is that a 6A input or 6A output? \$\endgroup\$ – user253751 Apr 19 '17 at 0:31
  • \$\begingroup\$ 2.4v x 700mA does not make 3W per LED - it's only 1.68W. You probably need a higher voltage if you need to drive 4 in series. \$\endgroup\$ – brhans Apr 19 '17 at 2:23
  • \$\begingroup\$ The driver is something like ebay.com/itm/… the 6A one The leds are something like ebay.com/itm/… Each "strip" is 1 blue and 3 red LEDs. The voltage seems to be ok, I meausured 11.6V which fits the specs exactly (2.6*3+3.6) but the current is strange but the LEDs are bright as they should and produce quite a lot heat. \$\endgroup\$ – Martin Novák Apr 19 '17 at 18:20
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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:

schematic

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.

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  • \$\begingroup\$ I tried to take 1A 12V LED driver I used to use and use it with just one strip (4 LEDs or 3LEDs + resistor) and the current was correct (0.6-0.7A) as well as Voltage (11.75V) so there is something wrong either with my 6A LED driver or with the way I have connected those LEDs \$\endgroup\$ – Martin Novák Apr 19 '17 at 18:38
  • \$\begingroup\$ @MartinNovák Try the 12V6A driver with 1 strand of 3LEDs+resistor & see what happens. ... Let me know in here & I can help you troubleshoot this problem further, if you like. \$\endgroup\$ – Robherc KV5ROB Apr 19 '17 at 18:41
  • \$\begingroup\$ I tried it with 4 LEDs first as I have it connected it in this way right now, 11.77V and 0.6A which is about right. So it seems to be problem with the driver having 6*4 LEDs connected to it or the way I have wired up those 6*4 LEDs. By the way thanks for help, I'm kinda electronics noobie. \$\endgroup\$ – Martin Novák Apr 19 '17 at 18:57
  • \$\begingroup\$ I have tried to connect 2(parallel)*4(serial) which resulted in 0.8A current (which is too low for one strip - 0.4A) and then 3*4 which resulted in 1A (which is 0.33A per strip, even lower). So I still thing my 6A led drive is broken or I'm using it in a wrong way. \$\endgroup\$ – Martin Novák Apr 19 '17 at 19:08
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    \$\begingroup\$ Yes, it sounds like it's a 1A source, instead of the 6A source it was sold as. \$\endgroup\$ – Robherc KV5ROB Apr 21 '17 at 17:57
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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 Vf. 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 Vf 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.

enter image description here

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  • \$\begingroup\$ I don't think that was a good idea, I have tried to use one of my LED string with only 3 LEDs and use it with my 12V 1A driver, it light them for a brief moment and then it went down so I guess that 4V/LED destroyed one or maybe all of them. \$\endgroup\$ – Martin Novák May 9 '17 at 20:09
  • \$\begingroup\$ @MartinNovák you can connect the power supply directly to the LEDs if (ONLY IF) the supply is a constant current supply like the Mean Well HLG. A constant voltage supply will burn them up if there is no current limiting resistor. The posted schematic shows a 1 amp source so... You do make a good case for making that comment in my answer. \$\endgroup\$ – Misunderstood May 9 '17 at 20:35
  • \$\begingroup\$ It's a constant voltage source (at least I think from how it behaves). In fact it is sold as a LED driver which supposed to be constant voltage source with some addnational components that keeps the (not sure if voltage or current) same during the runtime of the LED as it's characterstics changes as it heats up. In the schematic the A stands for ampermeter, the source should give something like 4.2A but it gives only 1A as stated in the description. \$\endgroup\$ – Martin Novák May 9 '17 at 20:49
  • \$\begingroup\$ In your post you said "I have a LED drive as power supply". Now I just saw your comment with a link to the supply. Yes it is constant voltage, the "strips" it refers to use a constant voltage supply. It is not a LED driver supply. They are likely all burnt out. I'll buy you replacements if you'd like. \$\endgroup\$ – Misunderstood May 9 '17 at 20:59
  • \$\begingroup\$ It's ok I have like 50 LEDs of each color as reserve because I have expected I will burn some of them during my experiments. It appears I have burned only one or two of them. Anyway do you have any idea what is the issue with my setup? \$\endgroup\$ – Martin Novák May 9 '17 at 21:31

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