RussellMc wrote:

> Brent Brown wrote:
>=20
> Neil wrote:
> >
> > > I've been avoiding tying anything to the positive side of the load as
> > > that would be outside of the circuit.
> >
> > The flyback diode can only work (in the mode where it maintains inducto=
r
> > current
> > and minimses energy loss during PWM off periods) if it is across the
> > inductive load.
> > Appreciate that it may be inconvenient to place it there, but that's wh=
ere
> > it must go.
> >
> >
> Yes, but ... :-).
>=20
> I know you (Brent) know this:

I feel like I should know, but I'm still not quite getting it yet, thanks f=
or your patience=20
:-)

> The diode must be ELECTRICALLY across the load but can be from load to
> ground if the power supply impedance is such as to allow the recirculatin=
g
> (off) current to flow through it with minimal impediment.

I can picture this working with the zener diode fine, but not with a conven=
tional=20
diode - as Neil said he replaced the zener with a regular at some stage. Su=
rely the=20
conventional diode would be the wrong direction to become forward biased to=
 allow=20
the inductor current, which does not change direction, recirculate through =
the power=20
supply? Perhaps we're picturing different things.
=20
> Also, the current resistor must be in series with the diode and it is
> easier to have it ground referenced to allow a single ended comparator to
> sense current. If the diode is "across the load" in such a way that it's
> not ground referenced (except for sense R) then the sense resistor also
> must be not ground referenced and the comparator must do a differential
> comparison of the voltage across the floating sense resistor wrt the
> reference.

No doubt the rest will make sense once I get the first part right.



--=20
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