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I have an LED power supply - constant current with output 1.4A/96W/23-69VDC. I want to buy a number of LEDs and connect these in series.

What do I need to calculate to know how many of the following LEDs I can add to this power supply?

Can I just do it all by Ohm's law?

https://www.led-tech.de/en/CREE-XM-L2-T4-on-Star-PCB

My calculation results in approximately 48 LEDss (though I would only want to push the power supply to say 80% which gives more like 38.)

Just wanted to see if I made this right here since it was a while I did this.

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    \$\begingroup\$ Unimportant gave the answer, but... why use so many XMLs? Some COBs would be cheaper, same or higher lumen/watt, higher CRI, less soldering... What are you trying to achieve? \$\endgroup\$
    – bobflux
    Commented Oct 31, 2020 at 14:50
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    \$\begingroup\$ Maybe some overgrown 7 segment LED indicator for a football field. \$\endgroup\$
    – h22
    Commented Oct 31, 2020 at 15:49
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    \$\begingroup\$ The Ohms law is not valid for the constant current source. It should push 1400 mA through all your LEDs that is a little bit too much for (700 mA typical) them but bearable (3000 mA max) \$\endgroup\$
    – h22
    Commented Oct 31, 2020 at 17:32

3 Answers 3

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According to the datasheet, the LED's Vf is 3.05V @ 1500mA, which is close enough to the 1400mA constant current provided by your power supply.

The power supply's maximum output voltage is 69V.

69V / 3.05V per LED = approximately 22 LEDs in series. Say 20 to have some margin.

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  • \$\begingroup\$ Thanks ! but how did you get 3.05V? \$\endgroup\$
    – electro_music
    Commented Nov 6, 2020 at 8:29
  • \$\begingroup\$ It is in the manufacturers datasheet, linked in my answer. Under "characteristics", "Forward voltage @ 1500mA". Your power supply is 1400mA, but close enough. \$\endgroup\$
    – Unimportant
    Commented Nov 6, 2020 at 8:38
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What do I need to calculate to know how many of the following LEDs I can add to this power supply?

enter image description here

If the supply can handle 1.4 amps then you can use two parallel strings of 24 LEDs because that produces an end-to-end voltage of 68.16 volts and you can mop up the 0.8 or 0.9 volts up with a series resistor. So two series strings in parallel gives you 1.4 amps.

That's the maximum (48) of course. It's slightly more light efficient to run the LEDs at 700 mA current so, you get out more light out overall but dispersed over more LEDs compared to running (say) 22 or 23 in series at 1400 mA: -

enter image description here

But you don't really need to stop here. If you want to know how many LEDs you can add then why not use three parallel strings with each string taking 466 mA. LED forward volt-drop might be about 2.79 volts at 466 mA meaning you can still use 24 LEDs in series to make a string (total volt drop = 66.96 volts with a little left over for a padding resistor). So now you can have 72 LEDs producing a slightly bigger net light output than before.

Where do we stop? If each string took 200 mA the LED forward-voltage would be about 2.69 volts and 25 LEDs could be in each string AND there would be 7 strings so, in total there could be 175 LEDs. If you want more LEDs then use more parallel strings. If you want fewer LEDs then use a single series string operating near 1400 mA.

It's up to you and your application.

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    \$\begingroup\$ The two strips in parallel may have slightly different brightness. But 24 LEDs sequentially give lots of statistical averaging for the total forward voltage so may not be much. \$\endgroup\$
    – h22
    Commented Oct 31, 2020 at 17:38
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Ohm's law has nothing to do with it.

Your power supply puts out 1.4A. The datasheet says your LEDs are designed to operate on 700 mA to a maximum of 3000 mA. The power supply current falls right in the middle of that range. Make sure you have enough heat sink capacity for each LED and you should be fine.

Your power supply can supply 1.4A between 23 and 69 volts.

Your LEDs will have a forward voltage of about 3 volts at 1.4A. That's a minimum of 8 LEDs in series, or a maximum of 22.

Any number of LEDs between those two extremes will be OK.

80% of 96 watts is about 55 volts at 1.4A. That's about 18 LEDs.

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