Hello all,
i have a 5W power LED
i want to use the very common LM317 at current limiting mode, with the known schematic:
(let's suppose that i always feed with a 12v stable voltage at 10A)
as i understand it - i have to set R according to the LED's current (i know the formula)
and the input voltage of the LM317 can be 12v, 15v, or anything up to 37v.
now,
let's say i have 2 types of 5w LEDs:
1400ma @ 3.5v
700ma @ 7v
all i have to change in the schematic is the resistor, one calculated for 700ma, or 1400ma.
my question is: how the LM317 knows the LED's voltage ?
if i take a 5W led (1400ma @ 3.5v) and a 10W led (1400ma @ 7v)
the resistor setting is the SAME (both leds are 1400ma),
but suddenly the LED need twice as much voltage !
same case if i add LEDs in series, current stays the same but the voltage doubles....
does the LM317 knows that it has to double the voltage ?
finally, i mean, is the VOLTAGE parameter of a power LED can be ignored (as long as i supply a higher voltage) ?
and also, how can i be sure that the LM317 will deliver the highest possible voltage to the LED (within the safe margin) ?
OK, it will deliver 1400ma that the LED needs, but how can i be sure that it will give the full led's Vmax (3.5v) and not 3.4v or a bit lower (hence causing the LED not to be the brightest possible) ?
Thanks a lot !
i have a 5W power LED
i want to use the very common LM317 at current limiting mode, with the known schematic:
(let's suppose that i always feed with a 12v stable voltage at 10A)
as i understand it - i have to set R according to the LED's current (i know the formula)
and the input voltage of the LM317 can be 12v, 15v, or anything up to 37v.
now,
let's say i have 2 types of 5w LEDs:
1400ma @ 3.5v
700ma @ 7v
all i have to change in the schematic is the resistor, one calculated for 700ma, or 1400ma.
my question is: how the LM317 knows the LED's voltage ?
if i take a 5W led (1400ma @ 3.5v) and a 10W led (1400ma @ 7v)
the resistor setting is the SAME (both leds are 1400ma),
but suddenly the LED need twice as much voltage !
same case if i add LEDs in series, current stays the same but the voltage doubles....
does the LM317 knows that it has to double the voltage ?
finally, i mean, is the VOLTAGE parameter of a power LED can be ignored (as long as i supply a higher voltage) ?
and also, how can i be sure that the LM317 will deliver the highest possible voltage to the LED (within the safe margin) ?
OK, it will deliver 1400ma that the LED needs, but how can i be sure that it will give the full led's Vmax (3.5v) and not 3.4v or a bit lower (hence causing the LED not to be the brightest possible) ?
Thanks a lot !
