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.
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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
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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.
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