Unknown LED Parallel wiring.

I would like to thank you guys for hooking me up with the led driver for my hexacopter spot light.

Now I am moving on to LED navigation lights using the ones below.

These are the LED's http://www.extremeglow.com/store/p/27-Flat-Lights-2-Pack.aspx

I will power them from a 18650; Six led's in parallel. My basic searching is showing six 75 ohm resistors for the top battery 4.2v but for bottom 3.6v shows 42 ohm. So i am confused what to do, if I put the 75 ohm resistors and the battery drops as its used I would rapidly be loosing current; however if I use 42 ohm maybe they burn up.


judging from the use of the lithium 3v watch battery I am guessing they are not drawing more then 2.6 to 2.8 volts

I made basic guess of 2.8 fv 20mah current.

If I make the wrong choice and start over driving them will I only risk my leds or will I be risking the led and the battery. If they died in mid flight I can still get back from the spotlight or in emergency the RTH. However I wont be able to save the copter if the battery catches fire or worse explodes.

If you overdrive the LEDs, one failure mode is that they can short. That would be no problem as long as you use 1/2W resistors (1/4W if they are 75 ohms). Even if they all shorted (worst possible case), they won't overload the battery and cause a problem with it. But it would drain the battery quite a bit faster.

For a few dollars, you can get a really inexpensive voltmeter, and actually measure the voltage across the LED when it's being run. You can also measure the voltage across the resistor, then calculate the current. Keep in mind that as you put more current in the LED, it will have a higher voltage drop. You can always start with a higher resistor value, then switch to a lower value if you find the current is less than you want. That would be quite safe.

One of the inherent problems with resistor drive is that the light output will always fall as the battery voltage falls. There's no way around that except to use a more sophisticated driver. Seems to me somebody, maybe Pololu, makes a low-current driver that might be suitable. Or just accept that the light level will change.

I didn't know they made low current drivers I would much prefer that. Thanks I will start looking for one.