> It is possible, but it will be difficult to get much power-handling
> ability this way. A TO-220 type package will be limited to around 20
> Watts absolute maximum continuous dissipation regardless of how big
> the heatsink is.
>
> One way I've done this before is to have two power resistors: one
> which is the minimum resistance (call this Rmin) desired and a second
> which is the difference between the max resistance and the min
> resistance (call thisRb). I put these two resistors in series and put
> a transistor (FET or BJT) across resistor B. By using this transistor
> to shunt current around resistor Rb, to a varying degree, I could vary
> the effective resistance from the min to the max and the maximum power
> the transistor needs to handle is around one quarter of what it would
> need to handle if it were the only dissipating element.
>
> Another way is to use a switching regulator. Let's say your battery
> voltage ranges from 10 to 20V. If you made a buck converter which took
> 10 to 20V in and gave, say, 2 to 8V out, then you could put a fixed
> resistor on the output of the converter and vary the voltage setpoint
> to vary the power drawn from the battery.
>
> Sean
>

This reminds me of a technique I used in a battery monitor for an electric
vehicle. We wanted to prevent overcharging of some batteries in the series
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 width
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 of
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.

Harold


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