12V Wiring

A) Its easier to route a +12v trunk to your major devices, and its also easier to find a ground(off the chasis). While you will still run 2 wires to the switch, you will only have to pass 1 through the gromet/harness.

B) The switch would have a slightly lower load passing through it (since it is on the ground).

C) The switch would be less likely to take on any damage if there is a short (since the short is more likely to be between the switch, the appliance, and the fuse). Changing the fuse would be alot easier than changing the switch.

Its no excuse to undersize the switch, but a nice way of keeping them a bit more out of harms way.

You answer A) has some logic to it, but C) does not.....B) is just completely wrong.....

The switching devices are placed on the positive side of most devices on a automobile not on the negative. Chassis acts as a conductor and is not an earth ground.

Correct me if I am wrong, but some European cars have the switch on the - side, while US and Asian cars have it on the + side (or do I have it the other way around).

I'm not sure about the location, but I do know there is a "double standard". I was once installing a Bosch Alarm system for my car, and it had two sets of instructions, one was if the car was wired with a "-" switching, another set of instructions if the car had switching on the "+" side. It even had a instructions on how to determine which system your car is using.

Here in Asia, most cars have "-" chassis, with the switching device on the "+" side. As to why the "double standard", all I can say is great question!

On a related issue, up to about 35 years ago (in UK) vehicle electrics were +ve earth (there may have been exceptions, but all the ones I dealt with were) Then it changed to -ve earth. During the change-over period, when installing a dynamo you had to polarise it correctly by taking a length of wire from the battery live (+ve or -ve depending on system) and flicking it on the dynamo F (field) terminal a few times, to magnetise the iron in the field windings the right way.

This was about the same time alternators were coming in and if I remember right the early ones could be set up either way, by changing some connections. Nowadays I believe all alternators are -ve earth and that's it.

I understand change to -ve earth was partly because it reduced bodywork corrosion (rusting being an electrochemical process) but I'm not sure. Any more thoughts out there?

Cheers.....Codey

I am pretty sure that you are mixing up the - or + switching with the chassis earth convention in cars.

As someone else pointed out, cars used to + earth till about the late 60s as I remember, then they went to - earth as it reduces car body corrosion if I remember correctly.....

Alarms and other components were for a time allowed to be installed on either system. Today they are all for -ve earth!!

I am have a couple of old cars and my dad was a mechanic. You sir are correct. The cars in the US use to be "positive grounded". The reason they switched was because of corrosion of the frames. The ground, (sometime called common) is not to earth, it is to the frame. That's what completes the circuit.

Traditionally on a negative ground vehicle the switching is on the positive side, horns were an exception as they run a single wire up the steering column, via a brush arrangement, to the wheel.

Switching on the negative side can make a lot of sense, where possible, as any damage to the switch wire that causes a short to ground simply activates the load (hence horns stuck on) rather than blowing fuses or allowing the smoke to escape. It also means a single wire to the switch.

Even your door lights, trunk lights & hood lights are switched on the negative side. It saves cost by reducing length of wire required and simplicity of the switches themselves.

Hi Gacmoose,

assumed you switch with a transistor or a logic chip instead of a relay your control signal has to be greater than the loads "HI" if your load is grounded.

Grounding your semicon-switches allows lower control voltages, like TTL or LVCMOS etc.
Regards Uwe

Thanks to all of you for your replies. This has been an interesting discussion, actually. I was aware of + grounded vehicles in the past but for my particular question, I was referring to - grounded (common) vehicles (or boats for that matter). In any case, thanks for the info, all!

Mark

I've been hearing the terms "earth" and "ground" being used, I thought interchangeably. Now I hear Nutwood referring to them as different. Are they the same or different???

For cars up to at least this year, earth, ground & chassis all refer to the cars metal body.

There is a strong possibility that in the not too distant future, things may change dramatically....as far as I am aware, the change has not yet been implemented, but expect also things like a 48 volt bus system as well......

Perhaps other CR4 members can throw more accurate & up to date infos into this Blog!!

In UK bigger vehicles have 24v electrics - battery, alternator and starter but use a split system with 12v for the lights. Because lamp filaments on 24v are fragile and gave frequent failures.

If electrics goes to 48v I assume lamps will stay 12v.

Codey

........I would assume that they are not going to be filament when 48 volts comes along....!

I'm a little skeptical about the fragile filaments. Most larger vehicles, trucks, earthmoving etc, use 24v as it halves the current draw when cranking heavy engines over. They use 24v throughout with no apparent increase in globe failures. The 24v start, 12v run system has generally been used in vehicles aimed at the domestic consumers as there are so many more accessories and spares readily available for 12v.

There have been some exceptions to this. Volvo had a range of trucks with 24v start and 12v run. They had a magic box (actually some big relays) that connected the batteries in series for starting and parallel for running. Most owners of these trucks viewed these boxes with deep suspicion as they could cause a spectacular failure if mistreated.

Hi Nutwood

I got the bit about fragile bulbs from my brother who runs a truck or two, but I don't have any more details. And it was a few years back so things might have changed.

I'm sure you're right about greater availability of accessories and spares for 12v.

Codey

It depends on where you are in the world as to wheather you call it earthed or grounded. In the US it is usually grounded, In Europe and UK it is earthed.

You've got me puzzled. Where did I refer to them as different?

Arc suppression. Often it requires a large gauge wire to carry current over a measure of distance. When the switch is on positive side, an arc takes place each time you flip the switch. A switch on the ground is routed through a relay and may or may not have a small diode for arc suppression depending on current load. With that in mind, it is to protect the switch and give it longer life because the switches only job is to turn on a relay, the relay carries the load and the switch does not need to be protected.

You seem to have mixed up a whole bunch of facts, perhaps I can straighten you out.

In DC circuits, a diode (reversed across the contacts) would only help if you were switching an inductive load, but usually a capacitor only is connected across the contacts to stop arcing at the contacts.

A diode is usually connected across the relay coil to short out back emf and protect the switching transistor or switch(it would have to be a very weak switch to need such protection!), this has nothing to do with the load contacts and arcing there.

I hope you found this helpful...

Sorry nutwood. It was jrpeck that referred to them as different. My apologies

That's OK. I wondered if you were refering to a different thread where I refered to +6v (relative to ground) as 0v for that application. Somehow I'd missed jrpeck's post just above mine. I think he was using "earth" in a very literal sense.

http://www.google.com/search?hl=en&q=define%3Achassis

I don't know about new cars but my 84 chevy truck routed

BAT+ >>> FUSE >>>> SWITCH >>>> DEVICE >>>> GROUND (BAT- OR FRAME OR BODY). I refer to the main load bearing C-channel beams that run the length of the truck as the FRAME, and the rest as the BODY, though the convention varies somewhat by design, see link. The fender wells, front clip, bumpers, etc all bolt on to one or the other or both. Since these bolted together pieces don't always make great contact with each other, you should probably have a ground cable running from the FRAME and another from the BODY back to the BAT-. The body is usually rubber grommet mounted which can insulate it from the frame. Check resistance on your connections to make sure you have a good path back to BAT-. BAT- would be the equivalent of EARTH, the FRAME and BODY merely serving as large conductors to complete the circuit. In addition, use the best connectors you can find. Bad connections in auto electric are more common than good ones and will irritate the crap out of you. Solder then connectors when possible. Put fuses as close to BAT+ as possible and minimize plastic tapes, etc. because those are the things that catch fire during a short. The copper wire will melt into but the plastic will produce a flame that can ignite all the other plastic junk the car mfgrs are so fond of. Any conductor between BAT+ and the fuse is unprotected and will melt and possibly cause a fire when shorted. Also, the computer on my old truck used a similar convention. The processor and transistors close the sensor signal circuits to ground if I recall, though the new computers might be different. No matter, I threw it in the trash. You can't do that anymore with the new cars unless you want to do some major major redesign.