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Current limiting diode

Discussion in 'Electronic Basics' started by Steve, Nov 30, 2004.

  1. Steve

    Steve Guest

    I have been studying the design of a signal "booster" that is used in the
    DCC model railroading hobby. This booster's job is take the digital output
    of a control box and boost the current before sending it down the rails.
    This design is using an optocoupler for isolation purposes between the input
    to the booster and the boost portion of the circuit. The input from the
    control box is passed through a "current limiting diode" just before the
    optocoupler. First, what is a current limiting diode, and second, what is
    its purpose in this application? Is it necessary? Thanks for your input.

    Steve
     
  2. A current limiting diode is a junction field effect transistor
    (depletion mode, fully on and turned off with reverse bias on the
    gate) that has its gate connected to its source (or maybe a resistor
    between them) and the load connected drain to source. It acts as a
    current regulator, wasting all the extra voltage once its current
    limit is reached. In this case, it limits how much current can pass
    through the LED in the opto coupler.

    See:
    http://www.allaboutcircuits.com/vol_3/chpt_3/16.html
    http://www.allaboutcircuits.com/vol_6/chpt_5/15.html
     
  3. Steve

    Steve Guest

    Perfect. Thank you for the insight and for pointing me to the webpage. What
    a great resource, especially for hobbiest's like me, self-taught (still
    learning), with only an inkling of what's going on. I intend to read all the
    chapters on this page to help fill in some of the gaps of what I've learned
    during my short time with smoking silicon.
     
  4. Rich Grise

    Rich Grise Guest

    One demerit from the Apostrophe police for improper pluralization.
    Three demerits from the Grammar/Spelling police for hobby, hobbier,
    hobbiest.

    It's "hobbyist."
    Two or more of them are "hobbyists."
    May you learn well.
    This is the first stage of wisdom. Or maybe that was "confusion." :)
    Extremely good idea!
    [The Philosophizer wants to say: "No, no! Don't smoke Silicon! Smoke
    Geramiums! It's a much mellower buzz!"]

    Welcome to the Zoo!
    Rich Grise, Self-Appointed Chief, Apostrophe Police;
    De Facto Chief, Grammar/Spelling Police.
     
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