To deal with unknown cable resistance one can up the input voltage using a
small dc/dc converter (boost, like a simple switcher), operated in constant
current output mode, and then use a parallel (shunt) power zener like a
heatsinked 1A NPN and a zener and a few resistors at the far end. You also
need to take care that the inductance of the cable is not negligible.
Abruptly switching the contraption on or off can cause unexpected voltage
spikes at either end. For the same reason, pulse signalling on such a cable
must be treated with respect for any spikes and impedance issues. The
professional solution above ground is to use high voltage on the cable and
dc dc converters at both ends. This is used in ENG and so on camera cable
setups and is very okay but you probably do not want 150Vdc in a partially
air vented gas well, unless you want to compete in the space launch
business. All external metal parts in the downhole part should be brass to
avoid sparks when hitting the walls anyway.


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