Down hole isn't an issue, there alway is some gas coming out of the=20
hole, so oxygen isn't available. When the top of the 6" steel casing=20
(pipe) is open, it make a nice hand warmer for the drilling crew,=20
burning with a 2' high yellow flame. Then the cable tool comes up, it=20
takes the flame up where the gas is quickly burned out and the flame=20
goes out. :)

I'm just a hobbyist, with limited electronic design background. I do OK=20
with Pic's and programming in C or basic sort of thing. I try to keep=20
things as simple as possible with off the shelf stuff, and following=20
data sheet examples of circuits and applying Ohm's laws. :)
On 10/19/13 12:31 PM, Peter wrote:
> 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 consta=
nt
> 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 als=
o
> 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 cab=
le
> must be treated with respect for any spikes and impedance issues. The
> professional solution above ground is to use high voltage on the cable an=
d
> 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 partiall=
y
> 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 t=
o
> avoid sparks when hitting the walls anyway.
>
>

--=20
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