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Powering x-opto white leds
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/rk...1376&XPAGENO=2 Made by X-opto. Who they?? I cannot find any online presence. Lens Material V F max V R max Chromaticity Int.mcd@20mA 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
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Alex Bird wrote:
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/rk...1376&XPAGENO=2 Made by X-opto. Who they?? I cannot find any online presence. Lens Material V F max V R max Chromaticity Int.mcd@20mA 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 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. |
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