LED Frequency Ranges

Discussion in 'Electronic Design' started by LightBoy, Mar 17, 2005.

  1. LightBoy

    LightBoy Guest

    Hello,

    I am working with some "super bright LED's" - so called - and I have
    been looking for "specs" on the upper limit of how fast (Frequency) at
    which LED's can be modulated. Measureably switched on and off.

    I can find nothing doing a Google search on the subject. Can anyone
    give me a place where I can go (on the Internet that is - LOL) where
    such information might be obtainable? Or information that you may be
    aware of on this subject? The place I bought these LED's has lots of
    specs - but none relating to this capability.

    Thanks so much for any help!

    Don H
    LightBoy, Mar 17, 2005
    #1
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  2. "LightBoy" <> wrote in message
    news:...
    > Hello,

    Hi.
    > I am working with some "super bright LED's" - so called - and I have
    > been looking for "specs" on the upper limit of how fast (Frequency) at
    > which LED's can be modulated. Measureably switched on and off.
    >
    > I can find nothing doing a Google search on the subject. Can anyone
    > give me a place where I can go (on the Internet that is - LOL) where
    > such information might be obtainable?


    I once tried to find the same kind of data, and found a SLED
    (superluminescent diode) with the feature "pulsed operation"
    http://www.nolatech.ru/nolatecheng.htm#b6
    I wrote to the manufacturer to see if they would part with
    some data as to what that meant, and got a nice reply with
    some frequency response data. Sorry I cannot lay hands
    on it. (It was for a one-time job.) But you should be able
    to retrace that path.

    > Or information that you may be
    > aware of on this subject? The place I bought these LED's has lots of
    > specs - but none relating to this capability.


    All I remember was that it was much faster than necessary
    to be considered flat in the 100 KHz to 10 MHz band.

    You might see if you can find anything on the diffusion
    capacitance of GaAs diodes. That relates directly to
    carrier lifetime which in turn controls emission frequency
    response since emission is directly related to carrier
    density/population.

    > Thanks so much for any help!

    Good luck. You'll need it as I recall.

    --
    --Larry Brasfield
    email:
    Above views may belong only to me.
    Larry Brasfield, Mar 17, 2005
    #2
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  3. LightBoy

    Paul Mathews Guest

    Virtually all LEDs have rise/fall times in the range of 10nsec to
    1usec. For 100% modulation, the upper limit for LEDs is in the
    neighborhood of 20MHz for types designed to be modulated (e.g.,
    communications devices at 850nm wavelength). The upper limit is
    usually just a MHz or less for devices optimized for illumination
    energy output.
    It doesn't take much equipment to make your own measurements. Just be
    sure to use a really small photodiode, a small load impedance, and to
    keep LED and photodiode far enough apart that you're not looking at
    capacitive or magnetic coupling between them. You can check for either
    by blocking the light with an opaque non-metallic object: any light
    coupling should go away.
    Paul Mathews
    Paul Mathews, Mar 17, 2005
    #3
  4. LightBoy

    LightBoy Guest

    Thanks to both of you - Paul and Larry for the information. At least I
    now have some clue and something more to go on - which is more than I
    had when I originally wrote my question. I just wanted to be sure they
    would modulate to at least 500KHz and from what you are telling me -
    that should not be a problem.

    Best to both of you!

    Don H
    LightBoy, Mar 17, 2005
    #4
  5. LightBoy

    James Meyer Guest

    On 16 Mar 2005 22:29:49 -0800, "LightBoy" <> wroth:

    >Hello,
    >
    >I am working with some "super bright LED's" - so called - and I have
    >been looking for "specs" on the upper limit of how fast (Frequency) at
    >which LED's can be modulated. Measureably switched on and off.
    >
    >Don H


    So far, all the responses have concerned "bare" LEDs. I'll be willing
    to bet that "white" LEDs, the ones that use a UV LED to excite a white phosphor,
    will have dramatically slower rise and fall times because of the phosphor.

    Jim
    James Meyer, Mar 18, 2005
    #5
  6. LightBoy

    Tom Becker Guest

    > ... modulate to at least 500KHz ...

    I use conventional T1-3/4 LEDs driven by a stiff square 50% 40mA source at
    6MHz. You will have no difficulty at 500KHz.

    Tom
    Tom Becker, Mar 20, 2005
    #6
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