| Electronics > Projects, Designs, and Technical Stuff |
| Ultra Short, Ultra Fast LED Flash |
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| JAndrew:
At a previous job, I did some high speed stop action photography. (See Attached Pictures) The way this is accomplished is to use a dark room, with a long exposure on the camera, and to control the exposure with a short duration flash. The shorter the duration of flash the faster an object can move and still capture a clear image. Its important that the flash is bright, short and abrupt. We used Air Gap flash that generated a flash duration of 500ns. Hack-a-day explains how to build one but ours was a commercial one. (Uses high voltage caps, something I'm not real comfortable toying with) 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. The issues I'm having is trying to build a driver for the LED's. I plan on triggering it with a 5v high pulse driven from an ardiuno or some other simple electronic trigger. My thought was to use 25 of these 3 Watt LED for a total of 75 Watts: https://www.superbrightleds.com/moreinfo/high-powered/vollong-3w-rgb-high-power-led/899/2214/ Any thoughts on how I might go about building a driver for this? EDIT: The direction that I think I am heading in, is to purchase some power MOSFET's. Probably one for each leg of the LED's. The larger power MOSFET's would be driven by a single smaller MOSFET. As far as switching goes it seems like MOSFETs are fast and can handle higher amps and voltages without being expensive. Thanks in advance, Jay |
| dmendesf:
You should take a look at this: https://www.analog.com/en/analog-dialogue/articles/led-driver-for-high-power-machine-vision-flash.html Enviado de meu moto g(6) plus usando o Tapatalk |
| David Hess:
Many years ago I designed a low power LED flasher like that which easily generated pulses below 500 nanoseconds. It used +5 volt logic to drive the emitter if an NPN transistor with the base tied to the +5 volt supply and operating as a high voltage cascode so transistor drive was ideal and the resistance between the emitter and gate output controlled the current very precisely. Scaling this up would not be difficult with a power MOSFET on the low side and NPN RF power transistor as the cascode. The advantage of running at higher voltages is that unavoidable parasitic inductance has less of an effect. In practice the cascode is probably not required although it makes current regulation more precise. If you operate with such short pulse widths, then the power can be much higher although this makes the power switching more difficult. If the pulse width is only 10s of nanoseconds, then a charged transmission line and bipolar avalanche transistor might be worth doing; the voltage with that many LEDs in series is high enough. But my guess is that you do not need to go below 200 nanoseconds. |
| StillTrying:
There are a few threads on fast LED flashes on here. https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1414506/#msg1414506 https://www.eevblog.com/forum/beginners/microsecond-led-flash-ringing-issue/ https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg1751210/#msg1751210 If you go by the half light point the rise and fall times of medium-white LEDs is quite fast at about 100ns. |
| Zero999:
I've done this before. The requirement was for a 1µs. Phosphor LEDs are variable. Some are fast and others are slow. Attached is a schematic of the test set-up I used to drive LEDs with 1µs pulses of hundreds of amps, hence the low value current limiting resistor. It's possible to overdrive LEDs by a factor of 10, for short pulse lengths. Note transformers are used to drive the MOSFET and measure the current, to avoid large currents passing through the grounds of the test equipment. |
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