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MG MGB Technical - Cigarette lighter, is the centre positive?

I would like to be able to charge a mobile phone in my 1964 MGB. I have an aftermarket cigarette lighter socket, as in any modern car, but I need to confirm the polarity.

Am I right in assuming that the centre should be positive and the outside negative?

My MGB is positive earth, but the socket screws on to the side of the speaker console, and so I can wire it up either way. I just want to confirm the right way so that I don't do damage to the mobile phone!

John Prewer

John, Yes the center electrode is indeed the positive connection. If you are to mount this in a positivly grounded car, you will have to insulate the mounting bracket from the speaker surround unless the base is only touching the plastic housing. Ray

Positive and negative have nothing to do with the function of a simple resistance element, such as a lighter. Normally the center is "hot" and the case is "ground". Therefore the usual for a positive ground car is that the center of the socket is negative. If you isolate the case from all possible ground contact with the car, you can hook it up "backwards", so the center is positive. But you have to be sure there is NO possibility of anything bridging from the lighter socket to the car ground.
FR Millmore

John - Fletcher is spot on (as usual), and even with isolating the socket you run a very real risk of allowing the chassis of your mobile phone to touch the chassis of your car, with corresponding sparks and excitement. That would probably melt your charging cable in short order.

Probably not a good move on a "benefits vs. risk" basis - - Alec
Alec Darnall

Thanks guys.

I had a small loom fire years ago in an MGA I bought (NOT my wiring job!) and have been very wary since...

John Prewer

There is likely no connection between the works of the phone and the outside of the case. If you decide to do this, you could put a low amperage fuse in both ground and hot lines to the socket. Of course, it would then not work for a lighter, but neither would it burn the car down. If it were me, I'd switch polarity on the car. Other than a radio and possibly electronic replacement fuel pump, it's not as expensive or difficult as a fire or a new phone.
FR Millmore

Why not just wire in a new plug? Any shop that sells cell phones or radar detectors and other accessory equipment will have them. Then you can wire it with a fuse and tuck it away when not using it.
Bruce Cunha

Or just buy a set of cig connectors (1 male, 1 female) and wire them reverse of each other. For example center to outer, outer to center. Then just plug your new adaptor into the cig socket and your phone charger into the adaptor.
Steve Simmons

I didn't see it posted if your car is positive or negative ground. If it's already negative ground, no problem. Check the lighter socket with a voltmeter first or you might burn out your phone. I don't think you'd burn the wires in your car again, but it's good to be careful.

Why not convert the car to negative earth ? It's a very simple operation. I did it to a Triumph Spitty many years ago. From memory ( which isn't so great these days )I had to "flash" the dynamo & the rest was straightforward. I'm sure there will be something in the archives on how to do it. Barrie E
Barrie Egerton

You will also need to reverse the 12v and ground connections inside the tach if it is electronic. Heater fan may need to be reversed if it is has two different coloured wires going in to the motor, easy to check which way round gives the best 'blow'. Wipers may need a tweak to their parking position. All other standard electrics will cope with the polarity change.
Paul Hunt 2

OK - I've waited for days . . .

Two hydrogen atoms meet on the street. The first one says, "I'm afraid I've lost my electron." The second one asked, "Are you sure?"

"Yes, I'm positive."
John Z

This thread was discussed between 26/09/2005 and 30/09/2005

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