Roman Black wrote...

>That circuit is looking very cheap.=20

Cheap + simple + efficient + good control =3D elegant.  Richard's
circuit is definitely a winner.

>Is 2.2mH enough?

That, or even less, should do fine.  The deciding factors will be how
high a peak current the transistor can deliver (Ipeak increases as
inductance decreases) and how fast can it turn off.  I haven't
simulated this circuit with a large range of inductance values, but I
will before this weekend and give some results.

>What speed does it operate at?=20

The higher the output current, the higher the oscillation frequency.
At Iout =3D 20 mA, it looks like a couple of kilohertz.

>It would be nice to be
>able to use one of those tiny 2.5mH inductors, the
>resistor sized green ones that are available for
>20c each etc.

They should work OK, provided they don't have too high a series
resistance (I'll check this out, too) and they don't saturate at the
currents involved.

>I think a circuit similar to this could be adapted for
>low power use, ie 9v battery to 5v PIC applications,
>or 12v or 5v low power use.

In doing my simulations, I've tried input supply voltages down to 9V
with this circuit (using different resistor values, of course) and it
should work just fine.

>What would be great if someone could build and refine
>some, ready for any newbie to build, with a standard
>parts list etc. Like pre-made ones for:
>
>9v to 5v, 0 to 10mA
>9v to 5v, 0 to 25mA
>12v to 5v, 0 to 10mA
>12v to 5v, 0 to 30mA.

I can give us a starting point on that; I might get at it this evening
after I get home from work.

>I think within these small ranges the price, size and
>efficiency of each design could be optimised. I might
>have a bash at it this weekend.

As might I.  This weekend is our long end-of-summer holiday up here in
the States but alas, I'm grounded: my youngest son is in college and
with $$$out =3D $$$in, I'm not going anywhere.  Might as well fiddle
with hobby stuff. :(

One thing people should keep in mind about ALL of the switching
regulators we've talked about in this thread: they're going to have
output voltage tolerances which are pretty wide.  The Zener diodes
themselves will have a 5% tolerance, with other factors (transistor
Vbe's, for example) making the error band even wider.  These are NOT
precision devices.

The other limitation to keep in mind is that these things are, after
all, switching regulators: they WILL have ripple in the output
voltage.  Powering an A/D converter off the output of one of these
things might not give very good results.

Both these limitations can be overcome by setting the output a little
high and then following the switcher with a good, low-dropout linear
regulator chip as a post-regulator to clean things up.  I like
National's LP2951 which, in the 8-pin version, also gives me a
reliable undervoltage-reset output.

Dave

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