i´m still running into connecting problems with the "4x CREE XP-E2 M2/M3 on 23mm star PCB".


Can someone help me out how do i need to connect this pcb to run all four leds? Just connect it to the power source with a resistor (on -) on the + / - labels on the pcb wont work. if i connect the power source directly on the left and right side of the led i got it light up. but just one led at a time...

thanks a lot :)

enter image description here

Edit: There is a picture of the pcb. enter image description here

My testing power source is small 18650 3100mAh 3,7V battery. i know that to get 100% out of the leds i need about 12V The resistor is a 1,2 Ohm 3 Watt.

  • \$\begingroup\$ Please edit your question to explain your power source voltage and current rating and the resistor value. \$\endgroup\$
    – Transistor
    Feb 3, 2019 at 20:26
  • \$\begingroup\$ please show a picture of the PCB instead of the drawing \$\endgroup\$
    – jsotola
    Feb 3, 2019 at 20:29
  • \$\begingroup\$ why are you putting the resistor across the +- terminals? \$\endgroup\$
    – jsotola
    Feb 3, 2019 at 20:31
  • \$\begingroup\$ i dont putting a resistor acreoss the + - \$\endgroup\$
    – Amexy
    Feb 3, 2019 at 20:43
  • \$\begingroup\$ You need a heatsink driving rated current with >60cm^2 surface area then Read how to regulate current >12V using Ohms Law or CC regulator with lowest practical drop or PWM from 14V \$\endgroup\$ Feb 3, 2019 at 22:12

2 Answers 2


You can't light 4 x white LEDs in series with a 3.7 V source.

enter image description here

Figure 1. The Cree XP-E2 datasheet shows the relationship between voltage and current.

The graph shows that for one LED you would put > 1000 mA through the LED and eventually this will destroy it - probably fairly quickly.

With four LEDs in series on your 3.7 V supply you have < 1 V per LED so the current will be only a couple of mA and the LEDs may be just visible in pitch darkness.

enter image description here

Figure 2. All LEDs have a non-linear relationship between current and voltage. For small LEDs the forward voltage, Vf will be quoted at some nominal current - typically 10 or 20 mA. Source: LEDnique IV curves.

Note that Vf varies with the colour of the light emitted because the wavelength is a function of the dopant added to the semiconductor.


If you are going to power with an Li-ion 3.6V battery, you will also need a DC-DC constant current boost converter. (search for "boost constant current").

If you want a driver that has about the same size as you LED PCB search for "flashlight boost constant current"

enter image description here

With a constant current source you do not need a current limiting resistor.

Make sure that 12V is in the specified output voltage range.

An adjustable output current would be advisable. These PCB can handle maybe 300 mA without additional thermal management. NOTE: The page you linked to says min 350 mA, that is not true. The data sheet Vf graph goes down to 100 mA and the LEDs will work down to about 10 mA based on my experience.

When selecting an LED for a battery powered project the most important characteristic is "efficacy" (lumens/watts). The Cree XP-E2 is good, but not the best choice. Today (Feb. 2019), the best choice is the Cree XP-G3 @ 185 lm/W. The XP-E2 ranges from 58 to 133 lm/W.

If size is not an issue then a linear strip would be a better choice.
I like the Bridgelux EB Gen2 and Samsung F-Series Gen3. You would need a 24V CC driver.


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