Quoting "M. Adam Davis" <stienman@gmail.com>:

> Something you might try is measuring using current from the shack
> side.  Set up a constant current source (LM317 or similar) and then
> measure the voltage at the output of the regulator.  The voltage will
> vary based on the position of the pot in order to make the current
> constant.
>
> It should help significantly, based on the description of the problems
> you're having.
>
> -Adam




     Vdd
    +----------+
              .-.
              | | 499R
              | |
              '-'
               |
               +------------+--------------------------------+----+
                            |                                |    |
             \ |            |                               .-.   |
               -            |                     500R Pot  | |<--+
    BFTranzorb ^ \          |                               | |
    eg. 10V    |            |                               '-'
               |            |                                |
     +---------+----------- | -------------------------------+
               |            |
              ===           |
              GND           |
                            |
                            |
                            |
                            |
                            |                      To ANx PIC
                           .-.     +------+---------------+
                        2K | |     |      | +
                           | |     |     ---
                           '-'     |     ---  47uF/16V
                            |      |      |
                            +------+      |
                                         ===
                                         GND


The voltage at the ADC input is

V = Rx/(499 + Rx) so it varies from 0 to about 2.5V for pot value from
min to max (ignoring wire resistance etc.)

It's a trivial exercise to solve for Rx as a function of ADC reading.

Best regards,
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
s...@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


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