[TR] Accessory power in +ground TR3

[Randall]

> Maybe I haven't grabbed the concept here but if the plan is 
> to run a GPS, phone charger or laptop - or two out of three 
> at any one time, what's wrong with plugging in to a cigar 
> lighter socket? I do it all the time with just three 
> different leads relevant to what needs to be charged.

The problem here is that Carl has kept his TR3 positive ground as original.  I daresay you have later cars that are negative ground.

One issue is that the shell of the cigar lighter is assumed to be negative by most electrical devices.  Some can be damaged by
reverse polarity, others just won't work.

Of course, it is possible to find cigar lighters with the shell isolated from the mount (or build an isolated mount yourself) and so
wire it so the shell is negative.  But then that introduces the chance of a sneak ground path to the car body causing a short.

As an extreme example, a friend of mine once wired a Citizens Band radio to his positive ground MGA that way (although he wired it
directly instead of using a cigar lighter).  He clamped the antenna to the rear bumper and we set off together on a 3000 mile trip.
The radios worked fine at first, we had a lot of fun talking back and forth while driving.  (This was long before the days of smart
phones and laptops.)

Somewhere in North Carolina, the rusty bolts in his back bumper finally wiggled around and shed enough rust to make good electrical
contact to the frame.  The first thing he knew, the cockpit filled with smoke!  Of course the ground lead for the radio had no fuse
(although there was one in the hot lead) and he had not thought to add one to the "ground" lead.  And the antenna clamp was grounded
through the antenna lead to the radio.

He got it safely stopped and put the fire out, but the wiring harness was pretty well toasted.  We robbed a headlight lead to run a
hot wire to the coil, so he could drive to a car parts store where we bought some fresh wire and jumpered around the burned sections
of harness.  We completed the trip that way, but I don't know if he ever did get the harness repaired.  He eventually moved to
Germany and the poor MGA got relegated to a barn in the states, where the rats had their way with the leather upholstery and
probably whatever unburned wiring was left.  Might be there yet (but I doubt it).

-- Randall