Timing requirements for LED's to start illuminate

[[email protected]]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

for this i checked several datasheets, but non of them mentions
anything like that.
im wondering whether LED's are instantaneous, so that "T on" and "T
off" are 0. Is this correct?

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

thanks

 

[John McMillan]

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.

 

[[email protected]]

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.

You want to pulse at a much slower rate than I did.
yes

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.

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

 

[[email protected]]

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.

You want to pulse at a much slower rate than I did.
yes

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.

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

 

[D from BC]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

for this i checked several datasheets, but non of them mentions
anything like that.
im wondering whether LED's are instantaneous, so that "T on" and "T
off" are 0. Is this correct?

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

thanks

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

 

[Christopher Ott]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

for this i checked several datasheets, but non of them mentions
anything like that.
im wondering whether LED's are instantaneous, so that "T on" and "T
off" are 0. Is this correct?

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

thanks

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

Chris

 

[RHRRC]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

for this i checked several datasheets, but non of them mentions
anything like that.
im wondering whether LED's are instantaneous, so that "T on" and "T
off" are 0. Is this correct?

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

thanks

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.

 

[Ecnerwal]

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

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.

 

[Jasen]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

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.

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

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

Bye.
Jasen

 

[MooseFET]

im trying to build a camera flasher that uses 400 white LED's.
For timing purposes, i need to know timing requrements for LED to
illuminate once
it is forward biased (ON time). Similarily need to know OFF times as
well.

for this i checked several datasheets, but non of them mentions
anything like that.
im wondering whether LED's are instantaneous, so that "T on" and "T
off" are 0. Is this correct?

the reason to use LED's for this flasher is because it needs to
continuously work at a rate of 600 flashes per second.

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.

 

[[email protected]]

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.

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

 

[Glenn Gundlach]

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

 

[neon]

I don't know your application but it is not uncommon for super bright to have 10k mcd so why so many LEDS most of the specs that I read are sayin the range of 10ns to 200ns rise time 600 hz cannot be considered into the time since these diodes are fast. 400 leds in series i presume makes this sckeme unpleasent at 4 volts times 400 LEDS makes 1.6kv this becomes bulky and very bad concept. if you manage to biud such a monster and use it 10kmcd it will be the same as 4 millions candles. I know tha you can buy green 16k mcd each. different color will give you different spectrum in nm. from 550 nm green to 650 nm red. then of course there is angular displacent to consider. If you don't know what i am talking about you should not aptempt to biuld such a device. one wrong flash and a wrong look at. you my friend are blind

 

[Rich Grise]

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.

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

Cheers!
Rich

 

[neon]

a led operate in the nm range 560 green,580 yellow and red is 630nm. NOT ALL ARE CREATED EQUAL some can have a turn on of 2 to 4,5 volts and the current availableto turn them on can also changes the brightness. Lot of variables the question is can your eyes detect such changes the answer is NO. the best is to have all colors in a 100 LED string and a current source of 50ma voltage means nothing since every led will determine itself when to turn on. then the question remains how bright each one is at 50ma that is also variable with leds SO YOU MUST CHOOSE leds THAT HAVE THE SAME LUMEL OUTPUT AT 50MA. Put them in 400 in prallel is just plain silly some will burn out before the other blink. that can be corrected by small resistors in series with each. that becomes a monster of a deal. even 100 in series is horrendeous needs 450 volt @50ma from ac to dc that s a good choise the you need 10 string in parallel of 40 LED each. and this ca be dangerous after all you are looking at a laser flash direct is a killer.