Led Strobe - Page 1
Forum section: electronics / beginners

author: Neuromodulator
author_rank: Regular Contributor
timestamp: March 25, 2016, 07:46:29 pm
content: Hello,I want to build a led stroboscope and a have a few questions on how to accomplish it.So the specs are these:- 10us or shorter pulse with the aim of releasing as much as possible light energy during that period.- max pulsing rate: 200 pulses per second. (That is, the duty cycle would never surpass 0.2%)Now my question are:- Would a phosphor led work for such short timespan? Can't find any chart with the light emission rise and fall curve.- This of course can't be answered without testing, but how much more current do you estimate a bright led like an XML2 could handle in such short pulse? (x3, x10?, x30?, more?)- Any special considerations I should have or advice on circuitry design to deliver high current in very short pulses?Thanks

author: danadak
author_rank: Super Contributor
timestamp: March 25, 2016, 09:52:52 pm
content: "but how much more current do you estimate a bright led like an XML2 could handle in such short pulse? (x3, x10?, x30?, more?)"That answer best found from manufacturer as they do the characterization of thepart.I saw this online, seems like a good idea -However.... if your application requires white LEDs, then I'm afraid that the response time of the phosphor coating is much slower than that of the LED itself. Whether it is fast enough for your application will depend on the maximum operating frequency you need. Unfortunately I have no data on phosphor performance.If you need a really fast white-light strobe frequency, maybe you should consider using mixed color LEDs (RGB) instead of phosphor-converted white LEDs.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 25, 2016, 09:57:58 pm
content: From another google search on switching time -To address the question, first a distinction needs to be made between phosphor LEDs (#1) (e.g. white LEDs, possibly some green LEDs) and direct emission LEDs (e.g. most visible color LEDs, IR and UV LEDs).Direct emission LEDs typically have a turn-on time in single-digit nanoseconds, longer for bigger LEDs. Turn-off times for these are in the tens of nanoseconds, a bit slower than turn-on. IR LEDs typically show the fastest transition times, for reasons given ahead.Special purpose LEDs are available, whose junction and bond-wire geometries are designed specifically to permit 800 picosecond to 2 nanosecond pulses. For even shorter pulses, special purpose laser diodes, in many ways operationally similar to LEDs, work all the way down to 50 picosecond pulses.As pointed out by @ConnorWolf in comments, there also exist a family of LED products with specialized optical beam shaping, that boast pulse widths of 500 to 1000 picoseconds.Phosphor type LEDs have turn-on and turn-off times in the tens to hundreds of nanoseconds, appreciably slower than direct emission LEDs.

author: Neuromodulator
author_rank: Regular Contributor
timestamp: March 26, 2016, 02:17:57 pm
content: Thanks for the reply, well my plan is to work with 10us pulses (maybe longer if i can't get the led to emit enough light), so I suppose phosphor coated leds will suffice. Cree has a document were they discuss overdriving of leds, but it's not even close the parameters range I plan to use, they go to as low as 5% duty cycle at 1000hz and overdrive it to a max of x3 the specified current limit. I guess I'll have to do some destructive testing to figure the numbers out

author: SeanB
author_rank: Super Contributor
timestamp: March 26, 2016, 02:27:10 pm
content: Look at the datasheet and see the absolute maximum current they quote. That is the bondwire destruction level, going above that will blow the chips nearly instantly, and running at close to that will result in a high failure rate of those with less than perfect bonding. You probably want to aim for a current peak of around 70% maximum as a drive. If you need more light output use either more devices ( and ballast resistors or duplicate the driver circuit per section) or find some with higher current ratings.