Hi, newbie here please help me understand, i am trying to drive 2 LEDs with 1 or 2 Li-ion 18650.
LEDs have Vf=3.4-3.8V If=700mA (that's max i assume).
Driving them in series, i need 6.8-7.6V supply and about 650mA current.
So i fed 8.6V to an LM317 (in current control configuration with 2 Ohm resistance) which worked for 1 LED but not for 2.
To my surprise, the LM317 seems to be presenting a resistance of about 10.5 Ohm, making the 8.6V insufficient for driving both LEDs.
it also would consume close to 1W alone and at 650mA it heets up a lot, making the use of an additional heat sink necessary.
I then fed from the same source to the LEDs with just a 1.43 Ohm resistor in between (plus about 1.45 Ohm which seems to be my cables/connections resistance) and worked just fine, with about 1.07W consumption (apart from the LEDs) and with 7x10R resistors in parallel everything was pretty cool to the touch.
So no heatsink needed, no higher voltage and the power consumption the same.
What am i not getting here ? What's the use of the current controller then ?
Is this a "proper" way to power the LED's ? What would be ?
This is supposed to be a handheld flashlight, so size of components matters.
I apologize for the post being so long