> Does the use of the diode mean that if the device works it's OK, and if it
>doesn't work there is no harm done -- just reverse the plug?
That is exactly what a diode will do for you. Two things to consider -
1) the diode must be sized to accommodate the maximum current draw you
expect to have and 2) even with a diode in the line, the circuit should
be fused at a level that will protect the wiring from releasing all the
smoke should the device that is being plugged in have an exposed ground
that touches ground on the car since the device being plugged in will
have a negative ground and the car (unless converted to negative ground)
will be positive ground.
The original plug that fits the sockets on the dash of the TD are not
real long life items, since the prongs are just split brass prongs and
they become loose in a very short time. A better approach is to get a
auxiliary power outlet (looks like a cigarette lighter plug, but
completely isolated from ground). The one that comes to mind is a Sea
Bowld P/N 19-166-0042, available from boat supply stores such as
Boater's World.
Before installing such a set up, read through the discussion on it in
the T series TD-TF BBS under the thread title "The saga of the steering
wheel" (a two part thread), where the various safety concerns are
discussed and a link is provided to Barney Gaylord's web site where the
same subject is discussed. It is necessary to understand the safety
ramifications of plugging any kind of device that is of different
polarity than that of the car, even though it is easy enough to set up a
plug that will accommodate it. It is necessary to insure that no metal
portion of the device being used is connected to any portion of the
internal circuit in the device. If there is contact to even the
smallest screw on the device, the potential for an incinerated wiring
harness is almost inevitable. The best approach, if you want to run
some modern device off of the car's electrical system is to convert the
car to negative ground or install a polarity converter as Barney Gaylord
suggests in his article.
Cheers,
Dave
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