A friend gave me a bicycle head lamp. It has 3 white leds connected in series and mounted to a heatsink. The leds look a lot like the 3 W ones SF carries, mounted on little flower shaped pcbs.
Is there a generic spec for forward voltage and current for white leds? For a typical red led, I’d assume 20 mA and 2.1 V or so.
I have a bench supply that I can vary the voltage as well as current on, can I use this to figure out the best way to drive the lamp? I’d like to use the Femtobuck and a 12V battery to make a portable light.
Thanks
Dave
Dave Mueller:
It has 3 white leds connected in series and mounted to a heatsink. The leds look a lot like the 3 W ones SF carries, mounted on little flower shaped pcbs.
Is there a generic spec for forward voltage and current for white leds?
I think your garden variety 20 mA white LED is about 3.2-3.4V. The higher power ones will be slightly higher. Those 3W ones that SFE sells ...
https://www.sparkfun.com/products/13105
Additionally, each LED requires a forward voltage of 3.2-3.8V at 750mA.
https://cdn.sparkfun.com/datasheets/Com … %20LED.pdf
I understand all that, but if someone hands you a high power white led with no specs or data, how do you figure out how much current it wants?
Dave Mueller:
I understand all that, but if someone hands you a high power white led with no specs or data, how do you figure out how much current it wants?
Good question. All I can think of is by measuring it's temperature. Increase the current allowed, when the case get's "too hot" it's time to back off. Assuming you can instrument the case, what's "too hot" ? I haven't looked but I'll bet someplace there's a generic Tjc for the type of LEDs you have. Figure a junction temp of 125C (to be safe) max and you have a max case temp to shoot for. Use 150C if you want, but then I think you're starting to cut into the lifetime of the device.
The only thing I do is give them voltage and current until I am satisfied they are bright enough… I get unknown LEDs from Ebay “lots”.