Re: Simple Linear Supply (for teaching my son)

On Mon, 27 Aug 2007 15:41:10 -0400, "Paul E. Schoen"
<pstech@xxxxxxxxx> wrote:

"Jonathan Kirwan" <jkirwan@xxxxxxxxxxxxxx> wrote in message
news:ps66d3h9ohrucq07nh60g87167uhodlm78@xxxxxxxxxx

2. High-side series pass elements are more easily implemented with PNP
devices, although they are usually more expensive and not as rugged. The
base drive is then just a current path to the common return.

Yes. But this was very easy to explain as a start and, also, the
arrangements for controlling the voltage from the ground reference
work as simply designed negative feedbacks against the current source
at the top. If I were to use a PNP (aside from the 'more expensive'
and 'less rugged' issue), I'd probably need to add more parts and the
details needed to fully apprehend the arrangement would increase, as
well.

3. You can get very good regulation by using a zener reference

Indeed. But I eliminated that for several reasons. One of them was
nothing less than it was fewer parts to start. Simply less to deal
with, at the outset. I don't want to teach by including at the
beginning everything required for a "good design." Just one that is
workable, from a beginners' point of view. Understand the basic
structure, first. Then worry about adding detail later on.

I also like to learn by seeing what doesn't work so well, too. If you
have something simple to start and then you discover a few "problems"
with the result, you have some motivation for finding something a
little better to fix that without breaking other things in the
process. Then as you get more sophistication, it all makes good sense
in the end. Too much at once reduces motivation to understand the
quantitative details because they are so daunting.


When I was a teen, my father had me build a simple power supply with a
couple of 2N3055s with the base controlled by a pot across a zener, in an
emitter follower configuration. I still have and use the supply today,
although it could benefit from more modern design. It is about as simple as
can be.

I've done that one to death with him, already. It was my first shot
at this and he "gets it" very well.

I don't quite follow the logic of omitting a zener to reduce parts count,
yet using a forward B-E junction as a reference. The whole circuit would
need to be redone to do it right.

Because it works sufficiently to show the operation of a summing
junction. Which is the next step in what I'm trying to show. I could
be wrong about that approach, though. So as always, I love to have
informed criticism about anything I'm trying to do. Thanks for your
kind time and that of others here, as well. Much appreciated.

I would just use a pot with a series
resistor across the raw supply at first, to demonstrate adjustability and
show the poor regulation.

Assuming you are talking about "with a bjt," here... done that. Well,
actually I didn't need to. It was easy enough to show by equation
form and he is gaining proficiency at differential equations so he had
more than enough math skill to have no problem seeing the details just
from this relatively simple algebra (ignoring details like temperature
variations.)

Then add a capacitor across the pot to show
capacitance multiplication by using the gain of the BJT, and then add a
zener for voltage stabilization. The emitter follower still suffers from
poor load regulation without direct feedback (you have indirect feedback
via the relatively constant B-E drop). So you add a diff-amp with output
voltage sense, and then you have a stable and useful supply. Add current
limiting, then make it fold-back, and you have a good all purpose supply.

I'll take this advice. Worthwhile doing.

After that you can go further and design for higher power and greater
efficiency by adding an SCR phase-fired front end to optimize the voltage
drop across the series pass transistors, and then you will need to add
inductors to minimize switching spikes. Also, as the design progresses in
complexity, things like transient response and feedback can be learned.

By that time, I'll be learning as I go, as well. These are things I
yet need to apprehend well and haven't had time in my own hobby life
to explore well. But that's part of what makes this a good thing for
both of us to share. At some point, I'm pushing myself to keep up
some pace and we both learn together.

Good luck. A power supply is a solid basis upon which to build.

I think so, too. There is plenty detail to master in something as
"simple" as a linear power supply. You can dig deep and continue to
find nth-order effects to try and cope with until you lose interest, I
suppose.

Thanks for the thoughts and additions. Appreciated.

Jon
.

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