Positive Earth 12v Socket

[Richy-dick]

Hi
It’s been some years since I posted on this forum so I’m glad to report that I’m still the proud owner of a 1967 S3 S/E DHC

My question is quite a simple one:
Can the “cigar lighter” socket on my POSITIVE earth car be used to power our 21st century accessories, such as phone chargers, sat navs, GoPro etc.? I’d hate to blow something up by plugging it in so I’ve never tried it.
Richard

[Andy8421]

Interesting question.

Car power supply is pretty dirty, so the cigar lighter plug in adapters have a reasonable amount of supply conditioning to clean up the car supply before dropping the voltage to the 5v required for a USB powered device.

While it would be fairly trivial to have the adaptor work on positive or negative earth (a simple bridge rectifier in the input conditioning circuit would do it) the question is whether the manufacturer has gone to the expense of doing it, or whether the input conditioning circuitry inadvertently provides this functionality.

My guess is probably not, but one of the adapters I have come to bits easily, and it is a simply job if you are handy with a soldering iron to swap the input connections over.

Alternatively, you could swap the connections over to the cigar lighter socket. In a “normal” car, the lighter socket earths to the body, but on the Elan it fixes to a plywood dash and has a separate earth. It’s not ideal to have the body of the socket live however, particularly given the rats nest of cabling behind the dash.

Another possibility is to use one of the extension cables available for cigar lighter sockets. A bit of creative soldering could easily create a polarity swapping extension plug / socket.

Edit: Depending on how the adapter works, and the type of USB lead you use, if you adopt any of the suggestions above you may find the metal outer connection of the plug at the end of the lead is “live” relative to the car earth and could short. There isn’t a lot of exposed metalwork in an Elan cockpit to short out to, but it is something to consider.

[Lotusian]

I can confirm that if you plug a standard USB adapter into a positive earth cigar lighter, the adapter will smoke and die - don’t ask how I know.
Reverse the connections as suggested.

[Richy-dick]

Thanks for your responses. I’ve decided to run a hidden 12v extension from the battery to behind the dash. I can’t really be bothered messing around changing polarities of anything.
Richard

[smo17003]

Another alternative would be to get something like a Noco GB40 jump starter/power pack. Primarily they are for jump starting, but they also have a 12v USB outlet for charging phones etc. They are quite small so would fit in the glove box, or attach with velcro in the passenger footwell.