Powering x-opto white leds

Discussion in 'Beginner Electronics' started by Alex Bird, Oct 27, 2004.

1. Alex BirdGuest

Hi,
I'm making some LED lights for illumination as part of some costumes,
in fact I have to put these things together tomorrow!

In my prototype I used some ultra-bright reds I had lying around, and
my series resistance calculations were fine. The device is simply a
D-type flip-flop, reed switch and npn transistor, enabling the light
to be switched on and off by waving it near a magnet.

For the real things I ordered these:

http://www.rapidelectronics.co.uk/r...CAT_CODE=30319&STK_PROD_CODE=M61376&XPAGENO=2

Made by X-opto. Who they?? I cannot find any online presence.

Lens Material V F max V R max Chromaticity [email protected] View angle
water clear GaN 4.2V 5V X=0.31, Y=0.32 3000
15°

As the intensity is quoted for 20mA, I factored in the forward drop of
0.7V per LED and calculated the resistance. Nothing. I Checked all
my cables, measured voltages.
Eventually I tried upping the current, and finally with 5V connected
directly across them they are putting out what looks like 3000mcd to
me!

Am I right in thinking that I can do this because VFmax is 4.2V, and
4.2 + 0.7 = about 5? Am I wrong to assume a forward drop of 0.7V?
Does anyone know anything about these LEDs?

Thanks for any help,
Alex

2. Robert MonsenGuest

You are wrong to assume a forward drop of 0.7V. Different types of LEDs
have a different forward drop. Red LEDs usually have a 1.4V drop,
whereas white LEDs have a 3.6V drop. The current goes up 10 times for
each 60mV the forward voltage goes up just like normal diodes.

The typical forward voltage for GaN white LEDs is about 3.6V. Taking
them up above that will cause them to draw more current. It sounds like
you have a multimeter, so measure the current through the thing as you
vary the voltage. It should be about 20mA at 3.6V. If you take it up
higher than that, you risk destroying the LEDs. They will get brighter,
but their life will decrease.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.