Q about pwm-controlled luxIII

hi all - new guy from norway here
(hope i post this the right place..)

since winters in norway aren't exactly sunny, i'm planning on making my own bikelight based on luxeon 3W stars.

i want to control the voltage delivered to the LEDs by using an atmel uC with PWM that in turn controls a FET by duty-cycling.

SO - here comes my question:
is it safe to feed the led directly from the battery via a "duty-cycled" FET with a (much) higher voltage than the LED is supposed to have? e.g. having a 24V battery with a 50% duty-cycle making it an effective 12V delivered to the LEDs(3 in series making it ~12V).

hope i made myself clear:thinking:


:thanks:

longleg said:

SO - here comes my question:
is it safe to feed the led directly from the battery via a "duty-cycled" FET with a (much) higher voltage than the LED is supposed to have? e.g. having a 24V battery with a 50% duty-cycle making it an effective 12V delivered to the LEDs(3 in series making it ~12V).

Click to expand...

OK - after playing with the search-function a couple of hours i've come up with the following answer to my question:

if V_batt is much higher than V_led, you can reduce the duty-cycle by means of a pwm-controlled FET to get the V_load you want. BUT this will not reduce the current in each of the time intervals. to reduce the current one will have to introduce a series-resistor, which in turn will decrease the efficiency of the circuit. aka a not-so-good idea..


am i on the right track here?
(meaning: one should try to match V_batt and V_led as closely as possible, and use pwm mainly to dim the light.)

:thanks: again!

I think you are on the right track. You should make the VF of the led and use the uc just for dimming. I use the PWM in the FLuPIC with 4.2V from li-ion and that is good. The response time of a led is very very fast so giving it a shot of 12V might kill it before it shuts off. Cheers and good luck!

I have just got running a 'high' speed current limiter suitable for pwm (at least TaskLed's D2Dim) drive. This is a linear buck regulator so the battery voltage should be not much above the led voltage at the current you want to drive. I am driving 6x3W leds at 2.4A total, 800mA to each pair. Powered from 6xAA NiMH. The circuit also has overdischarge voltage protection for the batteries.
The drop out voltage of the regulator is <100mV at 2.4Amps. That is it will maintain 2.4Amps as long at the input voltage it 100mV above the Led voltage.
matthew