Electronics > Projects, Designs, and Technical Stuff
Ultra Short, Ultra Fast LED Flash
JAndrew:
I appreciate all the input.
--- Quote ---https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1417075/#msg1417075
--- End quote ---
Definitely a worthwhile read through. The schematics he posts at the end to show his completed projected aren't in a format that I am used to reading.
EDIT:
I've studied his schematics and everything he's doing makes sense. I am assuming the length of the pulse is being controlled by the software on the MC.
I get it now on the MOSFET Driver, I haven't used one before, but it makes sense. All looks very doable. I wish he posted an update on the how the project turned out. I checked out his YouTube channel but that video looked to be the last he had posted on the topic.
Zero999:
--- Quote from: JAndrew on March 05, 2019, 11:06:50 pm ---I'd like to try and take photos like this on my own. I could go out and purchase the commercial unit for $1000+ or I figured I'd see if I could build it using RGB LED's. The reason I'm using RGB LED's is the phosphorus layer on a "White" LED Glows for a unspecified amount of time after the blue LED shines on it. This may extend the length of the flash beyond my target of 500ns.
--- End quote ---
You mean phosphor, not phosphorous, which is flammable.
One problem I've had with over-driving LEDs is colour shift, especially with green LEDs. I haven't noticed it with red and didn't test blue, but green LEDs shift towards the blue end of the spectrum, overdriven. This effect is clearly visible, when I tried drivinga green LED at ten times the rated current, but at a low duty cycle its colour drifted from a bright green to turquoise, almost cyan hue. It wasn't a problem with any of the phosphor LEDs I tried.
Some phosphor LEDs have a significant afterglow, others don't. The green phosphor LEDs I tested were very fast. No measurable afterglow, with instant on and off times. The yellow ones were the worst, with a slow on time and lots of afterglow. If I get the time, I'll update with part numbers and an oscillogram.
See the paper linked below about LED pulsed power. At the start of my tests I used a similar circuit, with the MOSFET driving the low side and the current sensor in series with the source, but I abandoned it in favour of using transformers for both driving the LED and current measurement, as the galvanically isolating the test equipment gave better, more reliable readings.
https://core.ac.uk/download/pdf/30991250.pdf
Marco:
--- Quote from: JAndrew on March 10, 2019, 01:24:42 am ---I've studied his schematics and everything he's doing makes sense. I am assuming the length of the pulse is being controlled by the software on the MC.
--- End quote ---
Microcontroller control only really makes sense if the micro triggers both the chain of events you want to photograph as well as the flash.
If you are say pulling the trigger on a gun with a string it makes much more sense to just keep everything analogue and trigger on sound or a light gate, no micro necessary.
Sceadwian:
Is this even possible with white LEDs? I thought the phosphor delay was WAY more than the nanosecond range?
Zero999:
--- Quote from: Sceadwian on March 10, 2019, 04:12:02 pm ---Is this even possible with white LEDs? I thought the phosphor delay was WAY more than the nanosecond range?
--- End quote ---
It depends on how fast and the LED. I posted an oscillogram of the CMA3090 being pulsed at 1µs here. As you can see there some delay in the on and off time, but not much. As I said in my previous post, some phosphor LEDs are worse, others are better.
What's odd about phosphor based LEDs is, although the initial decay is fairly fast (even the slow yellow one I tested had a time constant in the µs range) a very faint afterglow persists for a long time after the LED is turned off. This is no problem for a photoflash application, because it's several orders of magnitude too small to cause a problem: it's just about detectable, with dark adapted eyes, in a very dark room. I've seen it for myself. I ruled out persistence of vision and charged capacitors by closing my eyes, exposing the LED to a backlight (UVA) whilst my eyes were still closed, turning off the UV source and looking at the faint afterglow from the LED. I have only done this with white LEDs and don't have the equipment to make quantitive tests, otherwise I would.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version