You guys are confusing me.  If I read this right, you just want to regulate current
through the inductor, right?  If you regulate current, the voltage drop will be
relatively stable, granting temperature will have a slight effect.  Then you have
regulated power within reason.  So add a small resistor in series with the inductor
to sense current, and use one of two methods to regulate current.

One, use a Unitrode switch mode controller, little 8 pin job.  They make a long
list of varieties.  They directly accept the current feedback resistor drop to clip
at peak current, but you would have to experiment with amplifying that Rdrop into a
voltage to change it from voltage feedback mode to current feedback.

Two, use a single op-amp with one pin driven by the voltage on the resistor
(possibly gained up).  Then you add an RC delay in that feedback loop.  The OP-amp
output has a FET as a power driver.  When you add the RC, you make it unstable, and
instead of a linear current regulator, it falls into self oscillation.  It won't
have the steady frequency of an official switch mode controller, but it will have a
frequency that varies as it adjusts to contentment.

Chris Eddy
Pioneer Microsystems, Inc.

Harold Hallikainen wrote:

> On Fri, 4 Feb 2000 23:46:55 +1100 "Paul B. Webster VK2BZC"
> <paulb@MIDCOAST.COM.AU> writes:
> >
> >
> > > When the FET is opened, the coil current will ramp down, through
> > the
> > > inductor.  I think it's important that the FET be off long enough
> > to
> > > allow the current to ramp all the way down.
> >
> >   Sorry, that's the very thing you want it *not* to do.
> >
>
>         My concern here is that without some sort of feedback, this "continuous
> mode" switching will drift one way or another, possibly having the coil
> current increase a bit more each time the FET is on. I agree that a small
> current variation in continuous mode would make the solenoid quieter, but
> can you do this without feedback? How about feedback of a current sense
> (either a small resistor in the source lead or use of a SenseFET) so that
> when the current goes above some level the FET is turned off, below some
> level it's turned back on? No duty cycle calculations required... and the
> hysteresis would determine the switching frequency.
>
> Harold
>
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