Timing requirements for LED's to start illuminate

Its very rare for LED manufacturers to specify the time characteristics of their devices. I spent a lot of time pulsing violet, blue and green LEDs to produce pulses with a risetime of less than

2nanoseconds and pulse widths of 4ns. Some LEDs work, others don't, nothing in the datasheet tells you what will happen. You want to pulse at a much slower rate than I did. I think you can assume the process is less than 10ns and so essentially instantaneous.

thanks for the reply.

yes

i am assuming the rise time you are mentioning here is the rise time of the illumination it self. Not the current through LED.

thanks for the reply.

yes

i am assuming the rise time you are mentioning here is the rise time of the illumination it self. Not the current through LED.

I think I went down this road one time. I believe I ended up looking up the specs on photodiodes to make a photo response meter to test the LED optical rise and fall times. D from BC

Is there a reason they need to pulse at 600hz vs. simply powering on constantly for the duration that you need supplemental lighting?

Chris

WOW. Powering 400 XR-E's at 1amp may produce around 30,000 lumen. Assuming a polar of ~ 1 steradian this will be around a 360,000 Cd source. This will be illegal to use.

Why must you flash it?

I dont fancy making the power supply (~1.5kW!) if it is to run off some AA NiMh batteries.

Why so many leds?

What is it you are trying to achieve?

If you give some more details then we may be able to help.

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Presumably some form of stroboscopic photography. Of course, a plain old krypton or xenon strobe light has worked fine for those purposes in the past, should have no trouble operating at 600Hz, and would likely be cheaper than 400 LEDs, but there might be other considerations that lead the OP to want to use LEDs.

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Cats, coffee, chocolate...vices to live by

typical white leds output only two colours: blue and yellow, so they aren't well suited for colour photography,

if it's just for a stroboscope it may be OK but the yellow fluorescent dye may lag the blue LED producing fringing on moving parts.

600Hz shoudn't be a problem 38000Hz is commonly done for remote controls.

Bye. Jasen

I think the biggest problem will be getting the voltage applied to the LEDs in a short time. You will be making a pulse of about 1V and 40A with perhaps a 100A spike at the rising edge. This needs to be spread over the array of LEDs.

I would suggest using the RGB type of white LEDs and not the phosphor based ones. The red/yellow part of the spectrum will come up later than the blue part with the phosphor type.

For speed, you want to drive the LEDs to the operating voltage more quickly than a constant current would. If your operation is continuous, you can ramp the voltage you pulse to up until the LED current is the desired amount. I would switch the cathode connections with N channel MOSFETs. Breaking the LEDs into groups will allow reasonable sized MOSFETs to be used. If you want a very fast turn off, a second MOSFET could short the LEDs just after the main one turns off.

thanks for the reply.

the original problem is this.

im constructing a gray scale image capturing system that captures images at a rate around 580 frames per second. the used CMOS sensor is KAC-9630. currently i have a continuous light source, powered by 12 V car battery, and it has 400 LED's. The surface im targeting is a moving surface. with the current scheme, pictures taken are NOT blurred for very low speeds of this surface, but becomes blurred at higher speeds. The solution i see is to limit the illumination to a very short time period, so that i can get an instantaneous picture of the surface. For this i need to have this flasher and it should be able to produce a very high illuminations for a very short time period (of course not illegal harmful level !!).

please let me know if you have better iseas.

thank you CMOS

You mean like taking a photo of a disk rotating at 3600 rpm and reading the text printed on it ? Done it with 2 different brands of digital camera - Fuji and Canon.

GG

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Yabbut - he wants to do it 600 times a second. =:-O

Cheers! Rich