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MG Midget and Sprite Technical - Using cigar light to power USB adaptor

Hi,

Been meaning come up with a way to have power to the dashboard so I can charge my phone and or Bluetooth speaker on longer journeys.

I have a 1965 MKII positive earth.

Thought the best way was to install the cigar lighter and buy a USB adapter and should be good to go.

Ordered the cigar lighter and fitted it using the brown wire as recommended on the wiring diagram that can be used for either cigar lighter or radio as it is a fused wired.

Turned up the other day so fitted on the weekend by making a bracket and bolting it to the chassis just above the bonnet release lever. Wired it up and the cigar lighter worked straight away. šŸ˜€

Decided to order a fancy USB adapter that has a voltage reading as well.

That turned up today and plugged it in but it didnā€™t work. šŸ˜ 

Thinking it could have been a dud took my other usb adapter from my daily runner which in knew worked, plugged it in and it didnā€™t work either.

Plug cigar lighter back in and worked fine. Plugged in portable tyre inflator and that worked.

Whilst wondering what could be the issue, plugged old USB adapter back into my daily runnerā€¦..now stopped working. šŸ˜” Plugged new USB into daily runnerā€¦.still not working! šŸ˜”

So now thinking Iā€™ve fried both in the cigar lighter šŸ¤¬ but I canā€™t think why as itā€™s just 12v.

Any suggestions or advice from anyone that has already fitted a usb charging port?

Thanks

James
James Paul

James, I suspect your issue is related to car your being positive earth. Most devices, USB chargers, Satnav's etc will be expecting negative earth. The cigar lighter will work with either polarity as it's just a simple coil of wire, the tyre inflator will also work as it's a simple brushed motor, albeit that motor is probably running in the opposite direction.

It's possible you've fried the USB adaptor but normally there's a fuse in them. Some you can change by unscrewing the front collar others have them soldered to the PCB.

I have a 12V socket fitted in my positive earth car, it was fitted by the previous owner. It has a 2 wire supply cable and is isolated from the ground.
R.A Davis

Hi,

Certainly crossed my mind but the tyre inflator out doubt in my mind.

Could swap the wires over and try again.

Might look into the dedicated USB port as and alternative

Thanks
James Paul

Anything bought nowdays for fitting into a cars electrical system will expect the feed to be positive and the earth negative. With your car being Positive earth anything you fit will receive the wrong polarity and blow straight away. The only way to do what you want is to have the cigarette lighter socket isolated from earth/chassis by mounting it on a plastic sheet to secure it phisically, feed a fire from the chassis/earth to the centre electrode on the socket and feed the frame of the socket from your battery live feed through a fuse. The centre electrode will then be positive and the frame negative. You can then safely plug in your USB adaptor to power a satnav or charge your phones. Note that you will need to take care with the metal parts of any USB lead to make sure it never gets to touch the body of your car. If you do happen to short to the body/chassis you will let the smoke out of your wires.
Rob
MG Moneypit

James, I agree with R.A. Davis. The USB needs positive on the centre pin. I'm guessing a cheaper Chinese device may not be protected. The tyre inflator doesn't care. A motor with a wound field will rotate the same way, only a permanent magnet field will be reversed.
Bill B

Ideally, you want something like this

https://www.ebay.co.uk/itm/144990129018

You can then connect the red wire to +ve (chassis) and the black wire to +ve (brown wire).


Dave O'Neill 2

Knowing what flavour USB connector would helpful, A B C.
David Billington

Iā€™ve just noticed Iā€™ve put ā€œ+veā€ twice.

Brown wire is obviously negative.
Dave O'Neill 2

Hi,

Thanks for all your help.

Sadly Iā€™ve shorted the wired and had lots of white smoke coming from the loom within 2 seconds of connection.

Fused didnā€™t blow which puzzles me.

Brown wire completely melted at both ends of the loom so know the loom is now unsafe and will need to be replaced.

Thought the cigar lighter body was isolated from the bracket.

So bloody cross with myself.

Time or order a new loom.

šŸ˜­





James Paul

Ouch.

The brown circuit isn't fused, so it always pays to add an in-line fuse when connecting any accessories to brown.
Dave O'Neill 2

Hi Dave,

The reason I thought it was fused is because according to the wiring diagram for my GAN3 1965 the brown wire runs from the fuse box to the dashboard or a radio or option cigar lighter.

https://theleisure.wordpress.com/wp-content/uploads/2009/03/wiring-2.jpg

Also the wired is melted at both ends and by the fuses as you can see in the photo.

But are you saying that just because the brown wire goes to the fuse box that doesnā€™t mean it is fused?

Thanks


James Paul

Hi James

Although it's attached to the fuse box, that's the unfused side. The purple side is fused, feeding horns and headlamp flash (if fitted).
Dave O'Neill 2

Thanks Dave.

Makes perfect sense! šŸ˜µā€šŸ’«

Everyday is a school day with these cars.

Loom ordered, think Iā€™ll forget about modifying the car.

James Paul

Now that you are faced with rewiring the car, why don't you change it over to negative earth? It was one of the earliest changes I made to my Frogeye, and one of the best. It's certainly far less trouble than trying to get modern accessories to work safely on a positive earth car.
Les Rose

Hi Les,

Decided I donā€™t really need modern additions to the car.

Itā€™s a fair bit of work changing it and Iā€™m not sure itā€™s worth it.

James Paul

James, I think I am right in saying that the solid wiring colours are usually supplying either a switch or a fuse. The wires with a stripe then are the ones from the switch (or fuse) to the item being powered. But I stand to be corrected on this.

Your cigarette lighter should work om a positive earth car, as you originally planned, if you can electrically isolate the socket when mounting, and then connect with a two wire system to the (isolated from the car bodywork) body of the socket (-ve) and then positive to the central connector.
GuyW

James
I'm sorry to hear about the melted cable - what a complete pain! It really would pay to fuse the main supply to avoid such a catastrophe (the original wiring scheme on these cars was pretty basic to say the least). I did this with the Frogeye but then I was also making my own loom and converting to negative earth, partly because I used an alternator. Conversion really isn't that hard, only the rev counter needs attention and the dynamo in your case (and the battery and coil connections are reversed). Worth considering in my view.
Bill B

I, like James, learned the hard way what an unfused short can do to the loom and components within.

A new loom is a lot of work, if the damage is isolated to that one wire/circuit perhaps consider bypassing/replacing just that portion and leave the loom be?

During a major rewire (pos. to neg. earth and dynamo to alt) of my newly acquired 1967 project I did as Bill with his Frog: in my case an inline fuse for the brown solenoid-to-everything-else wire.

If it trips during a night drive I'll see you on the other side.



Richard Reeves

This thread was discussed between 18/08/2025 and 26/08/2025

MG Midget and Sprite Technical index

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