> This reminds me of a technique I used in a battery monitor for an electri=
c
> vehicle. We wanted to prevent overcharging of some batteries in the serie=
s
> string, so if the voltage went above some threshhold, we'd increase the
> duty cycle on a "bypass resistor." The bypass resistor went from the
> battery positive to the drain of an N channel FET. The FET was pulse widt=
h
> modulated to give the desired battery voltage (increased duty cycle
> increased current, decreasing the battery voltage).

> In the "programmable load" situation, a 1 ohm resistor could be made to
> look like anything between 1 ohm and infinity by varying the duty cycle o=
f
> when the resistor is in circuit. The instantaneous current, of course,
> will be higher than the average current, but if the instantaneous current
> is not excessive, this may be a way of providing a variable load to the
> battery.

This works well, provided that your supply can tolerate the high
current/no current approximation to a constant value resistor.
I used this for an exercise cycle load with dissipation varying
between 0 - 500 Watts depending on load and speed.

If you add a series inductor and a catch diode this becomes a buck
regulator based variable resistor.
Sounds remarkably like my Peltier supply solution :-).
Inductor must be suitably dimensioned to avoid saturation. Air core
may be a good idea for  large currents.
Low (and not so low) frequency radio transmitters can happen this way :-).


                    Russell


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