Hi everyone, just a quick note to say i've been testing
a stripped down version of Richards 2-transistor SMPS
buck 5v regulator, I removed a couple of parts and have
been testing it in its simplest possible configuration.

The good news is that the tiny 1.5mH RF choke i mentioned
before is working pretty well, efficiencies over the
current range from 10mA out to 70mA out(!!) seem pretty
constant, so I think the saturation issue is not as bad
as Dave D. and myself thought it would be! Excellent
for a resistor-sized inductor you can buy for a few cents.

I really haven't tried to get the efficiency right up
yet, i'm only using a 1N4148 for the flyback diode,
I will try a low Vf diode next. Also the main buck
transistor is only a BC327, so I will try a low Vce
sat transistor there too. The little cheap inductor
has a 9.6 ohm DC resistance, so I'm going to try
a lower H and lower R inductor, and play with it to
find the upper speed limits of the circuit. Since
these BC3?7 transistors are good for 100MHz I think
we could get good efficiencies right up in the 100's
of kHz range.

Even with the parts I have in there now, current gain
is about 2:1, at 12v in 10.3mA : 5v out 20mA. Not bad.

All in all, for a few cents worth of parts it's looking
like it could be refined to be very impressive!
-Roman

PS. Russell, I tried a variation using NO zener like
you were suggesting, using a R:R voltage divider on
the output rail and the Vbe of a transistor as the
regulator. It regulates ok, and saves one part, but
efficiency is not as good as the transistor must be ON
to turn Q1 off, where Richards original circuit is
very efficient as nothing turns on until the output
voltage drops, which is better for low current
situations.

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