Current limiting diode

[Steve]

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

 

[John Popelish]

Steve wrote:

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

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

--
John Popelish

 

[Steve]

"John Popelish" <jpopelish@rica.net> wrote in message
news:41AC9F23.193BA725@rica.net...

Steve wrote:

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

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

--
John Popelish

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.

 

[Rich Grise]

On Tue, 30 Nov 2004 18:17:26 +0000, Steve wrote:

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

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."

(still learning),

May you learn well.

with only an inkling of what's going on.

This is the first stage of wisdom. Or maybe that was "confusion."

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.

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.