Lindy Mayfield wrote:

> Lindy Mayfield wrote:
>>> As I said, forget about the line going off the page, forget
>>> about C2 for now, and concentrate on just what R2 and D3 do.
>>
>> Did you mean R2 and D2 or R2 and D3?
> 
> I meant what I wrote.

> P.S. Results come from effort. And responding to your questions which
> you deemed simple weren't obvious or simple for me.  I've spent hours
> digging through my electronic books trying to understand this simple
> sub-circuit.

Maybe a few hints (I don't mean to disturb anything, just help keep the
ball rolling :)

For the purpose of this exercise (R2, D3), consider the 8.2V rail a given
(at first). Disconsider any other components. It then looks like this:

----+----8.2V
    |
   R2
    |
    +---- What's going on here? And why?
    |
   D3
    |
   Gnd

Another hint: the symbol Olin used for D3 usually means it's a zener diode.
Look up how these work; no fine details are needed for understanding this
subcircuit, only the basic principle. 

Search at your favorite manufacturer's site or at Digikey for a 5.6V zener
diode datasheet (e.g. BZX84 is a common type). Wikipedia has an article on
zener diodes. Probably other electronics sites do, too, and in most
introductory electronics book you should find them explained, at least the
basics.

One difference between reading a traditional program and an electronic
schematic is that in a traditional program, there is a clear causality, a
clear sequence from input through processing to output. In an electronic
schematic that's not so simple: an input is always also a load that
influences the source, often there is no clear input and output (like with
a resistor), etc. It's more of a complex system where everything influences
everything else. But what probably most people do when reading such a
schematic is that they start out with some simplifying assumptions, and
later on verify whether these assumptions are consistent with the
schematic. So in this case, we just assume that whatever is connected to
the connection between R2 and D3 is not relevant for them to work as this
basic, common circuit is supposed to work. Later on we need to see whether
this is true, when exactly it is not true if it isn't always, and what the
consequences are of this. Same for the 8.2V rail.

Gerhard

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