Re: Simple Linear Supply (for teaching my son)
- From: Jonathan Kirwan <jkirwan@xxxxxxxxxxxxxx>
- Date: Mon, 27 Aug 2007 09:18:38 GMT
On 27 Aug 2007 08:04:00 GMT, Fred <guest@xxxxxxx> wrote:
On Mon, 27 Aug 2007 01:03:13 -0400, John Popelish <jpopelish@xxxxxxxx>
wrote:
Jonathan Kirwan wrote:
I picked up a few 16VACrms, 1Amp transformers from a junk table at a
local supermarket store (I think they are used for doorbells and the
like) for $2/each, new. Cheap. Couldn't resist. Anyway, I just do
this stuff for a hobby but I'm thinking of teaching my son some of the
design issues for a completely discrete, bjt-only, linear power supply
that supports variable voltages from something just under 1VDC to
perhaps maybe 12V, at no more than say .6-.7Amps.
Here is the schematic I came up with to teach from:
http://www.infinitefactors.org/misc/images/ps1.gif
I intend to start by using a 3.3k ohm resistor for the "BASE DRIVE"
circuit, instead of the current source shown. Just by way of showing
that it can work either way and some of the trade-offs.
I also will start without including the current limit circuit, but
adding that in later on. Then once we get the current limiter put on
the high side, I'll also do the same thing for the circuit but instead
linking the current limiter into the ground return instead (and change
where the voltage control circuit 'grounds') so that I can illustrate
its operation in either configuration.
Might even make this into small, separate component boards that can be
linked together in pieces. That way, these steps will be easier to
add and pull apart.
I'd like any thoughts, though. Questions asked here would help me
make the presentation better, later on. Or perhaps I'm not thinking
about something I need to deal with.
Sounds like a plan, but substitute 1 amp rectifier diodes like 1N4001
(or 1N4002, 1N4003, 1N4004 or a 1 amp packaged bridge rectifier) in
place of the 200 mA 1N4148 shown on the schematic. Those tiny diodes
might blow just charging the storage capacitor the first time.
I was just using those as placemarkers for the schematic and forgot to
change them out before posting it up. Thanks for the reminder, though.
I'm thinking of two different "blocks" for the drive supply -- one is
just a 3.3k resistor and the other is the current source shown. Each of
these can be swapped in. Also, two or three blocks for the voltage
limit -- one a fixed voltage one using a pair of resistors and a bjt,
then one adding the ripple compensation cap only, and then the one as
shown on the schematic. Those can be likewise swapped. Finally the
current limit block, just to remind, can be attached either on the high
side or the ground side (with slight other wiring differences.)
What do you think of the schematic layout? Are the purposes and blocks
clear?
Thanks,
Jon
Jon,
there are lots of good 'bad ideas' to be improved on there :-)
so this indeed is a good circuit... to start with.
My son basically understands resistors, Ohms' law, less-than-EM1
hybrid-pi model of BJTs, a little about inductors (the differential
equation) and a little about capacitors (also, the differential
equation.) He knows about Thevenin but hasn't really had to apply it
much, and knows about the two basic forms of Kirchoff's
somewhat-obvious-today laws, though again he hasn't had to apply them
all that much. He also has a passing exposure to ideal opamps.
One of the more important things I'm trying is to avoid hitting him
with too many details and to allow him to see just how much reasonable
performance can be achieved with only a little bit of quantitative
theory and knowledge. Although I want him to see this in the broader
behavioral blocks (excluding having to look into exact details of
each), I also want him to be able to quickly approximate values within
a block almost on sight and without the need for much use of a
calculator. I also want him to see what can be done with just a few
simple components, which is part of why I'm completely avoiding
anything that looks like an IC. (No MOSFETS, for now, too.)
Plus, I like being able to swap in supposedly improved bits and pieces
and see how they help (or hurt) things.
He and I have already covered, in some detail, the issue of a bridge
rectifier and following capacitor -- including what happens to the
peak current when you just hang on a huge capacitor, because of the
much shorter moment available then for recharging it (load current
remaining the same.) Once we get through some variations on this
theme, and I feel he has mastered the details well enough, that will
be the time to start asking some new questions to complicate the
result a bit.
* Temperature sensitivity. How to improve his?
A large source of heat is with the 2N3055 and keeping it away from
other elements will be helpful. The other BJTs won't heat much on
their own -- I think they are probably in the few tens of milliwatts,
or so. I expect some dissipation in the bridge.
All BJTs have a lower Vbe with higher temperature, I think. Perhaps
dropping 0.23V in 100C change, just going from some vague experience I
recall. The main problem will be with the voltage comparison, as it
entirely depends on this Vbe as a reference value. As this Vbe goes
lower with higher temp, the output voltage will be driven lower as a
result. The output is about Vbe(Q3)*((R3+R4)/R5+1) so as Vbe(Q3)
drops with rising temp, it directly affects the output voltage,
dropping it as suggested in the formula. With R3=10k, this would be
some 2.8V change. At R3=0, the change would be only .2V.
The current source is a BJT, too, so the current there also goes down
with lower Vbe -- higher temperatures would tend to reduce the source
magnitude. But I set the base drive current source far above the
needed drive current, more than double what I expect to use. It would
be trivial to increase that even further to provide more margin.
Most of the improvement here would come from improving the voltage
control section. (Assuming one can live with the current limit
section's drift over temp.)
* Then, how to improve this while still satisfying the 1V min requirement?
Actually, I'd like to hear from you about this. One quick possibility
comes to mind, by adding a couple of diodes and capacitors to the
existing bridge to add a negative rail for some headroom.
* What will happen to Q3 if you suddenly dead short your supply while
it's set at 12V output?
* What will happen to Q4 if you suddenly dead short your supply while
it's set at 12V output?
C2 (now that I look, I see I didn't renumber the capacitors... oh,
well) will deliver a bit of a short current pulse. But are you
referring here to the equivalent C from emitter to base and base to
collector and the stored charge there at the time? Or?
* Is Q2 sized adequately?
Estimate beta=30 at .5A-.7A for Q1. Call it 600mA/30 or 20mA. But
probably less, really. Q2 will be at a beta of about 200, or so. So
we are talking about 20mA/200 or about 100uA. Again, probably less. I
think the 2N3904 can handle the 20mA collector current just fine.
* What is the transient resposne of this supply?
That would be well beyond where my son is at and probably something
for which I wouldn't mind an education and would likely miss some
important considerations as I went through what I know. Remember,
I've had no training whatsoever. It's just something I enjoy thinking
about, at times.
* Q1 will benefit from a E-B 1K or so resistor.
Yes!
Jon
.
- References:
- Simple Linear Supply (for teaching my son)
- From: Jonathan Kirwan
- Re: Simple Linear Supply (for teaching my son)
- From: John Popelish
- Re: Simple Linear Supply (for teaching my son)
- From: Jonathan Kirwan
- Re: Simple Linear Supply (for teaching my son)
- From: Fred
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