> I'm not really a hardware guy, but I wonder if somebody could comment on
the
> variation of Olin's design shown in the attached schematic (sorry for the
> bandwidth!).
>
> This version replaces 2 op amps (the voltage follower for the PWM signal
and
> the differential amp for the current sense) with a single op-amp. It does
so
> by summing the PWM output and the lower end of the sense resistor to the
> positive input; and ground and the higher end of the sense resistor to the
> positive input.
>
> This should mean that the op-amp will try its hardest to ensure that:
>
> Vpwm + Vsenselo = Vgnd + Vsensehi
>
> or:
>
> Vpwm = Vsensehi - Vsenselo
>
> Thus the opamp should drive enough current thru the transistors to balance
> the PWM voltage.
>
> Did I analyze this right?
>
> Does it have a chance of working?

Yes, I believe it will work well enough for the purpose.  I evolved the
circuit the various sections of the circuit as I was designing them and
hadn't stepped back to realized that the diff amp and current driver could
be combined into one.

The only drawback is that the charging current now has a small dependency on
the battery voltage.  My original circuit buffered the PWM voltage thru an
opamp.  It is now connected to the battery voltage via a 2Mohm path.  This
is no big deal in case of a battery charger where 10% (or worse) current
accuracy is just fine.  The effect could even be eliminated in the firmware
since the battery voltage is known.

Nice work, Bob.


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