> I just had a luminous idea: what if you use a upconverting autotransformer
> and a relay to switch it off when the voltage increases enough ? You need
> only 3:1 or so and it can be a cheapo audio transformer (about $1.50 in
> qty.). A small relay or a VDR can separate the transformer when it is no
> longer needed (at higher voltage).
>
> After all, there is a minimum speed you need to take into account, as the
> alternator will drop towards zero (and ugly pulsed waveforms due to
> core saturation) as it slows down.


I assume that you mean that I use the alternator AC to feed the
auto-transformer?
If so its not really likely to be viable as the frequency increases with
voltage and the range is about as extreme as the voltage range :-)
Also the higher voltages need to be stepped down once they get much over
target voltage.

I have "discovered" the very old truly horrible but incredibly cheap
MC34063A. I think Noah used these in the ark to eke out every bit of energy
from the storage batteries for below decks lighting. Not current mode but
does have a current trip but it appears to ONLY be a high side current trip
which is no good when my high side exceeds the chip Vcc max but I can add a
1 transistor current detector to drive it and ..... .

It's major advantages are cheap, runs on a few volts minimum, cheap, uses
few external components (coz it doesn't do much :-) ), cheap and it's also
cheap.

That and the UCC3803 seem the two leading challengers,.

The MC34063A (and a range of higher current brethren) look rather useful for
REALLY cheap and (really) nasty designs where every dollar counts. despite
its age its available in DIP and SO packages.

Another interesting smps IC is the MC33364 which is cheapish and excellent
for low cost offline supplies with very low standby power levels.
It uses a mode I hadn't met before which they refer to as "critical
conduction" mode. It provides "just in time" switching of the inductor as
energy is "drained" - the penalty is variable frequency operation which can
be an advantage from an EMI point of view.
Not suited to my current project but worth looking at generally

Some interesting material and excellent app notes at

    www.onsemi.com

The suspicious close resemblance to Motorola is not coincidental :-)

.
    Russell

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