Paul, you are correct.
In my previous post I recommended to wait the current to stop, but I had
a "transformer" image in mind (coils, magnetic field...), in this case,
as much constant the current, better. Thanks for get us back to reality.

The following circuit using an op-amp or comparator would control
current (500mA) upon a 5V square wave. The resistor divider 10k/560
actually generates 265mV, but your PIC or oscillator will also not
generate exactly 5V. Increasing the 560 Ohms resistor value would
increase the average current over the LOAD. The square wave is necessary
to create a non-linear current regulation.

Again, costs still below $12.


                                 +80V
                                  |
                                 LOAD
Square Wave                       |
0-5V -_-_-_                       |
o------.         .--------o--- FET
       |         |        |       |
       |        / \       R 10k   |
  10k  R       /   \      |       |
       |      /_____\    Gnd      |
       |       +   -              |
       |       |   |              |
       o-------'   '------o-------o 250mV @ 500mA
       | 250mV            |       |
       | -_-_             |       |
  560  R                 ---      R 0.5 Ohm 1/2W
       |                 ---      |
       |                  |       |
      Gnd                Gnd     Gnd


"Paul B. Webster VK2BZC" wrote:
>   Try to remember the goal of the exercise *in this case*.  You want the
> same current to flow *as if* it were excited by 12 or 14 volts.  You
> want a steady current.  Some fluctuation of the current is allowable,
> but you must balance certain effects.