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How to measure on/off LED timing ?

Discussion in 'Electronic Design' started by Boki, Apr 17, 2007.

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  1. Boki

    Boki Guest

    Hi ,

    I need to provide the LED measurement result for compare
    specification.

    What I should do is to use a oscilloscope and check the current ?

    Thanks!

    Best regards,
    Boki.
     
  2. Compare what? Brightness?
    How will that help?
     
  3. D from BC

    D from BC Guest

    wha wtf???
    I'm going to try to translate the questions..
    1) How do I test a LED to compare to the data sheet?
    2) Do I use an oscilloscope to check the current?

    Sorry..need more info .

    I cringe at how nonenglish the details will be. :(
    D from BC
     
  4. Guest

    Did we fail to read the subject before we pounced on this poor fella?

    If you want to measure the on/off time of an LED you might try using a
    moderately high-speed photodiode; at least 2X faster than the LED
    datasheet specification (some Nyquist thingy or something ?). Make
    sure that the spectral response of your photodiode is appropriate for
    the LED you are testing (i.e. the photon energy of your LED is greater
    than the band gap of your photodetector). Silicon photodiodes will
    work out to about 1.1 microns, InGaAs will work between 900 nm and 1.7
    microns. Attach the output of your photodiode to the input of an
    oscilloscope (again, the analog bandwidth of your o-scope must be at
    least 2X the frequency you are trying to measure) and measure the
    voltage drop across 50 ohm input impedance. You might need to use a
    lens to focus the LED emission onto the photodiode in order to get
    enough signal to make this measurement across 50 ohms. If you have a
    high speed scope probe with small capacitance (~10 pf for a passive
    probe these days) you can also make this measurement across a 1MegOhm
    input impedance for a larger signal.

    I know nothing about the subject matter at hand, but I hope this
    helps :)
     
  5. It's in Bokish.


    Best regards,
    Spehro Pefhany
     
  6. Guest

    There is the old equation

    bandwidth=0.35/(rise time)
     
  7. Guest

    Yep, I agree for the subject matter at hand. Not related to the
    current topic, but you might find the following (dated) article of
    interest.

    http://www.edn.com/article/CA263113.html
     
  8. D from BC

    D from BC Guest

    lol :)
    D from BC
     
  9. D from BC

    D from BC Guest

    Maybe he just needed to check Vd at 20mADC...
    D from BC
     
  10. jasen

    jasen Guest

    usually it's a good idea to put the entire question in the message
    That'd work, hook it to a scope and once you select the correct scales and
    trigger settings etc the answer should be fairly obvious.

    Bye.
    Jasen
     
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