Just was thinking...
Is there a limit, other than heat generation, to how much current you could shove through an LED if you could cut the pulse width (single pulse) enough to stop it from heating up?
Could you get enough through in a thousandth of a second to get a decent camera flash out of it?
There is a limit. You will find it in the data sheet of any LED, as the absolute maximum current rating.
And you can't get anything like a camera flash out of a regular LED.
IIRR someone published a scheme - in Review Scientific Instruments - for getting near-UV out of a blue LED by significantly exceeding the absolute maximum current rating for about a microsecond, but the LED died after 10^7 flashes, which is a total on time of ten seconds.
---------------- Bill Sloman, Nijmegen
Get LEDs made for camera flashes. Lumileds, Nichia and Cree come to my mind.
In my experience, 5 mm LEDs can withstand about an amp for a microsecond. I think they should take 200-300 mA for a millisecond - NO GUARANTEE. Do consider that LEDs are normally not linear devices and usually have efficiency decreasing as current increases past about 80% of the maximum they are rated to handle continuously. Some, especially most white ones, have efficiency peaking at current around or less than half their maximum continuous current.
- Don Klipstein ( snipped-for-privacy@misty.com)
I have heard of this scheme, and I got the LED to last much longer - by using current around 3/4 of an amp. I have yet to blow an LED after running them like that for minutes. I suspect half an amp for a microsecond with duty cycle of .5% will be fairly safe.
Also, this trick for getting UV out of a blue LED by pulsing with high current only works well with GaN ones with peak wavelength (at 20 mA) 450 nm and spectral halfwidth that is wide (60-70 nm or whatever). These included Nichia's NLPB series and similar Panasonic ones that may have had the older Nichia dice. I got better results with the Panasonics - possibly the epoxy was more UV-transparent. All of these LEDs were obsoleted by 1997-1998 since InGaN ones were established.
I describe this and a circuit in:
- Don Klipstein ( snipped-for-privacy@misty.com)
Pulse 5,000 amps, and i will guarantee *brilliant* illumination from almost any diode...
Yabbut, won't the color balance be all off? ;-)
Thanks, Rich
Who cares if the color temperature is higher than that of the sun?? Once is enough...
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