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PWM for driving HPLEDs

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_helio_

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Hi,
in the newsgroup sci.techniques.microscopy we are discussing how it is possible to drive HighPowerLEDs (approximatively 12-20 V, 1-5 A) with 100 nS-10µS pulses.
Any answer welcome. :)
 

Boncuk

New Member
I don't see any sense why to PWM LEDs at such a high frequency. (10 to 100KHz)

Of course LEDs flicker when PWMed, but the human eye is too slow too recognize flickering at frequencies of <100Hz.

500Hz to 1KHz would also suffice.

Boncuk
 

Mr RB

Well-Known Member
It might be beacuse their microscopy requirement might be using a camera of some sort which would cause strobing at lower fequencies.

For helio; I would consider using a lower PWM with a lot of capacitor filtering to give constant light at the LED, or even a linear current source. That would give zero strobing.

Unless you require the LEDs to actually flash? I know some LEDs can give higher brightness at certain freq/duty but isn't that a factor caused by the human eye, which would not affect a camera?
 

BrownOut

Banned
It might be beacuse their microscopy requirement might be using a camera of some sort which would cause strobing at lower fequencies.

For helio; I would consider using a lower PWM with a lot of capacitor filtering to give constant light at the LED, or even a linear current source. That would give zero strobing.

Unless you require the LEDs to actually flash? I know some LEDs can give higher brightness at certain freq/duty but isn't that a factor caused by the human eye, which would not affect a camera?

Some LED's require constant foward current for constant brightness and color. Filtering the PWM would defeat that requriement.
 
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BrownOut

Banned
Sometimes, I need to go back to the top of a thread to refresh what we're talking about. We had at least 3 LED threads going yesterday. For some reason, I was thinking the OP wanted to use PWM for dimming.

Or maybe it was just my ADD kicking in!
 

_helio_

New Member
It might be beacuse their microscopy requirement might be using a camera of some sort which would cause strobing at lower fequencies.

For helio; I would consider using a lower PWM with a lot of capacitor filtering to give constant light at the LED, or even a linear current source. That would give zero strobing.

Unless you require the LEDs to actually flash? I know some LEDs can give higher brightness at certain freq/duty but isn't that a factor caused by the human eye, which would not affect a camera?
yes, my correspondent needs "a single full-intensity flash
with a variable duration of, perhaps, 100 microsecond to 10ms"

the fact that brightness depends on freq/duty is very interesting; IMO a camera objective is conceived to react as the human eye
 

Mr RB

Well-Known Member
Thanks for clarifying that. I think you might be able to achieve this simply using the same method a xenon flash uses; by charging a capacitor to a set voltage then switching it directly to a LED. Just a mechanical switch will do. Since the LED has a relatively constant forward voltage (ie is a low impedance load) the photographic intensity of the flash should be fairly proportional to the energy stored in the cap.

So by using larger sized cap and/or charging it to a higher voltage you will get a much bigger "flash" which is a similar system used to adjust pro xenon flash units for photography. It also leads to easy experimentation in that you can fine control the charge voltage to get fractional control of the flash intensity.

so it's basically the same as a xenon flash, instead of charging a cap and dumping into a xenon tube you charge a cap and dump it into a LED.
 
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