Positive earth - adding accessories?

[Castanley]

Hi guys,

I was wondering if anyone would be able to point me in the right direction on this one?
I'd like to add a little 12v to 5v USB adapter box to my car. It basically has a positive and negative lead that need to be wired into the car's electrics. These wires go into the converter box which has a couple of USB sockets where you can plug in your devices. I'd like this box to only become live when the ignition switch is on, rather than being on permanently. The fly in the ointment is that my car (1954 saloon) has positive-earth electrics.

Would I need to take a live feed off the chassis somewhere, then via a fuse into the box? Then would the negative connect to one pole on the ignition switch?

I know I could just take it to an auto electrician but then I wouldn't learn anything and that's all part of the fun.

Any advice from an electrickery expert would be gratefully received!

Chris.

[simmitc]

Do you want to keep the car as +ve earth? It's very simple to convert to -ve earth, and that does make using modern accessories much easier; but if you want to keep originality, then that's fine.

As regards the USB converter, check that it is fully insulated and that no part of the circuitry can come into contact with the rest of the car. I suspect that it will be fully enclosed in plastic, which is fine, but necessary to check.

If you keep the car as +ve earth, then you needs to secure the +ve wire from the converter to an earth point, anywhere on the chassis or diretc to the +ve post of the battery. The negative wire will need a fuse which is then connected to a supply that is switched by the ignition switch. If you convert to -ve earth, then the wires are reversed, but the principal remains the same.

Looking at the fuse box, you will probably see green wires on each end of one of the fuses. You need to identify which end is the feed - it's easy top say which it should be, but over the years things get changed, so it's worth checking. If you have a multimeter then use that, if not, find a small 12 volt bulb with wires attached to use as a test lamp. Connect one wire form the meter/lamp to the chassis. Remove the fuse and switch on the ignition. Now touch the other wire form the meter/lamp to the fuse terminals. With one termional the meter will show approx 12v or the lmap will light. This is the feed from the ignition switch. When you turn the ignition off then both fuse termials should have no reading.

Now you know which is the feed, you can connect the supplementary fuse from the converter to this terminal.

[Castanley]

Excellent, thanks very much for the clear advice.
It sounds like I wasn't too wide of the mark with my assumptions. Yes I was planning to keep the car as positive earth, partly for the sake of originality and partly because, well, everything works fine as it is. This little USB adapter is my only concession to modernity and will be both so I can use my phone as a satnav and also to power a little retro style radio on the parcel shelf.
It is simply a little plastic box with pos and neg terminals on one side and two USB leads/sockets on the other.

Best regards,
Chris

[Declan_Burns]

I did something similar on my MG TD but I used an automotive relay connected to the switched ignition mounted in a plastic box clearly labelled "12V negative earth supply for accessories" for the benefit of future owners(s).
Regards
Declan

[geoberni]

Personally, I'd change the car to Neg earth.
Why complicate life?
Mixing 2 completely opposite systems on the vehicle is just asking for problems in the future.
You'll not be able to do anything to the car such as fit LED lights, have a radio (or a replacement if you've already got one).
Buying things like electronic ignition is possible but they cost more.

As others have said, it's possible to do it, but then a lot of things in life are possible, they're not necessarily sensible though...

[Declan_Burns]

Geoberni,
LED's on positive earth cars is no longer an issue. Many of the latest ones are dual polarity. I have some on my MG TD for the instrument lights.

Regards
Declan

[geoberni]

Geoberni,
LED's on positive earth cars is no longer an issue. Many of the latest ones are dual polarity. I have some on my MG TD for the instrument lights.

Regards
Declan

That's interesting; thanks for the update.
I can understand how some are reversible, such as Wedge base or Festoon caps, but a bayonet cap has always been polarity conscious . That's the whole point of LEDs, they require the electrons to flow one direction.....
I suppose they must be making them with additional circuitry in the body to switch them around. You wouldn't think it was worth the cost with only a miniscule % of Pos Earth vehicles on the road....

berni

[Declan_Burns]

Berni,
Here's a link to Lew Palmer's website. He specifically mentions the polarity neutrality.
https://www.brittrix.com/
He has some interesting stuff.

This company also makes bi-polar LED's. They also have the flashing festoon bulbs for the trafficators (flashicators!). They also have them in dual polarity so it must be worthwhile.
http://www.dynamoregulatorconversions.c ... ds-etc.php
Regards
Declan

[geoberni]

Certainly interesting stuff.
I looked at a couple of items on the brittrix site.
I guess that as he talks of specific circuit boards rather than just LED lamps, his are brighter.
Considering the driving conditions in the USA, I can understand why.
Being out on a multi-lane Freeway is scary enough, without being in a classic!
Quite a high cost though. A pair for L549 rear lights is almost £80 in the basket and I couldn't see anywhere it mentioning shipping costs.

I've got trafficator flashers from dynamoregulators, they're simple enough to use as you just flip them the other way around to change priority.