constant voltage driver

DIYLite · Sep 16, 2004

Hi,
I just bought from a DIY store a simple constant voltage driver circuit (at 4V) to power 4 white LEDs. Stated on the manual that it can support 0.8V to 5V input (& it also that it has approximately 65% efficiency, /ubbthreads/images/graemlins/confused.gif). If I will like to use 3x AAA batteries (4.5V) to power my LED, i just wondering will i be silly to use this driver? or is it better to just using current-limiting resistors for each LED? /ubbthreads/images/graemlins/ooo.gif

thanks!

Steelwolf · Sep 16, 2004

I would suggest that if having a constant level of illumination is not important, then go with direct drive or current limiting resistors (keep resistor value small).

4.5V in with a small resistor will be much more efficient than that constant voltage driver circuit, but the brightness will drop throughout the life of the battery.

I would like to suggest that you leave out the resistors completely. I have an old Trek7 from CCrane. 7 LEDs on 3 C cells, no resistor. Works fine. Starts with nice brightness and just keeps glowing and glowing even when the cells are practically dead.

andrewwynn · Sep 16, 2004

the only way to tell would be to bench test... the difference is most important when the batteries are mostly dead.. the switching boost drivers really suck a battery dead dry.. they can over-come the losses by the regulator... typically you have the same integral of light... total light output over time.. but with DD it's brighter at first an dimmer in the end.. with the regulator it'll be dimmer than DD at first and brighter than DD in the end... plus it's just way cool to have constant illumination.

-awr

Hallis · Sep 16, 2004

what he said /ubbthreads/images/graemlins/smile.gif

DIYLite · Sep 16, 2004

Thanks Pal,
guess I will have to go buy 4 more white LED when I am available & set it up to do a comparison between the 2 methods... BTW another newbie question, how "small value" should the resistor be if I do DD 4 white LEDs? I have smoked a white LED accidentally connecting 3x AA direct /ubbthreads/images/graemlins/frown.gif

andrewwynn · Sep 16, 2004

you can calculate the DD resistor value by knowing the Vf of the LED and the current you want to put through it.

example.. with 4.5Vbatt.. and 2.5Vf... you need to drop 2V on the resistor... you take voltage and divide by current to get resistance so 2V / 20mA (example) = 100ohm... then you take the current x voltage (or current x current x resistance) to get power.. or 2V x 20mA = 40mW which is way less than 250mW so a normal 1/4 resistor will be fine... 40mW is a fair amt to loose to a resistor in a ckt like this.. so see how much current you can run through the LED.. the higher the better for efficiency.

-awr