would AC be better?
would a lower or higher voltage be better?

i remember when cars were positive earth... ... - .- -.




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I have a a positive earth car

Probably they are dc because the electricity was originally provided by a dynamo, prividing DC, also never heard of a AC battery. The altenators on modern cars produce AC current which is turned into DC to power stuff.

As to the voltage.

Cant see a good reason for 12v, it seems a bit arbitary..... There is nothing that can't be fixed with a hammer😜😜
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It is most likely because it was a happy medium between then current draw and voltage. Some of us will have experienced the old 6 volt cars and remember how feeble the electric system was. Nowadays modern cars are drawing huge current so everything has to had to be upgraded.
Watts = volts x amps.
I recall a few years ago manufacturers toying with the idea of higher voltages to run specific items in cars but that means two electrical systems and all the problems that can cause. Funnily enough the voltages are nearly always divisible by 12.

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OK, form what I know of this, it originally was 6 volts and the wet cell was only used to power the lamps front & rear. In the early days these replaced the gas lamps which had been used until that time. At this point the engine's ignition system remained magneto based.

6 volt wet cells became the norm up to the changeover period in the 50's/60's. Remember 6 volt Beetles?? And also the MGB/GT which went to 12 volts by putting 2 x 6 volt batteries in series.

Not long after the 12v change, a lot of cars were still positive earth but this had a detrimental effect on aiding rust and corrosion. Go read about anodes and cathodes if you really fancy a nosebleed. Anyway, negative earth then became the standard and is what we have today pretty much globally. As for DC versus AC, how many AC batteries have you seen? It's all about having a useable on board power source for starting etc.

So, about 10 years ago there were discussions about 48 volt systems for cars due to ever-increasing demands being placed upon the 12v systems, as indeed we see with the well documented battery issues in FFRR's. It came to nothing in terms of agreements although it is likely to re-surface at some point, especially with hybrids being a reality now. 48 volts incidentally is the choice of datacentres where safe voltage power supplies come from UPS to rack as it is the highest voltage which is considered to be universally safe to humans. But the underlying principles are that it is easier to use a high voltage to force a lower current through a circuit than it is to use a lower voltage to force a higher current.

So there you have it.

Im convinced.

Though i think the voltage humans can tolerate is much higher, its the amount of current which kills you.... A low voltage shock can do a lot of damage if its accompanied by high current, whereas a hight voltage low current shock can be tolerable. There is nothing that can't be fixed with a hammer😜😜
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very informative Dom..


i wonder what benifits having 24volt systems on trucks is.... ... - .- -.




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northernmonkeyjones wrote:

Im convinced.

Though i think the voltage humans can tolerate is much higher, its the amount of current which kills you.... A low voltage shock can do a lot of damage if its accompanied by high current, whereas a hight voltage low current shock can be tolerable.

northernmonkeyjones wrote:

Im convinced.

Though i think the voltage humans can tolerate is much higher, its the amount of current which kills you.... A low voltage shock can do a lot of damage if its accompanied by high current, whereas a hight voltage low current shock can be tolerable.

stan wrote:

very informative Dom..


i wonder what benifits having 24volt systems on trucks is....

Yep, so as said/surmised above, current is more damaging than voltage but the last line is that voltage provides the EMF (electro-motive force) which drives or powers a circuit. Without voltage a current can never flow.

Recalling my days at Uni we even discussed whether a current was actually electrons flowing from negative to positive, or if indeed it was holes in matter flowing from positive to negative, the properly deep philosophical theory behind electricity. Anyway, the outcome is basically identical, you create a potential difference between 2 points, thus creating the positive & negative and the voltage (PD) across those 2 points is relational to the amount of force (EMF) being applied to the circuit, i.e the number of volts and it can be thought of as the "series" part of the circuit.

So, current, or amperes if you prefer. This is the "mass" of electrons moving in the same direction at the same time, in a parallel fashion. Think of them as the momentum of the circuit, or the weight of matter being moved. Mass is a better word but more people understand weight - very similar. to put in more layman terms, which would hurt you more, a glass marble fired at you from a catapult, or a balloon fired at you from a shotgun (assuming it were possible of course) ??? The mass of the marble inflicts far more damage. This is the current, a high amperes value.

So, to try and answer Stan's question about 24v trucks etc. Big engines need a lot of power to start them, specifically a massive amount of maintained torque. Think of torque as intertia in a rotary fashion. So, to try and maintain that torque it is easier to use a "faster" series (linear) force (EMF) to drive the same parallel force of amperes otherwise the cables become thicker than your fist in order to give the parallel capacity.

In really basic terms, high voltage moves electrons fast with voltage being the energy which does the moving. High current moves a lot of electrons which are the mass being moved. They combine to form a unit of power which most of us know as the Watt but as can be seen form the above, the amount of watts is not the whole picture. 6 volts driving 100 amps won't kill you whereas 100 volts driving 6 amps probably would. Same amount of watts (600) though.

Who's asleep so far??

Right AC versus DC. DC is easier to store in a battery, or portable static power source and it also provides approximately 1.414 times the amount of energy (most correctly rated in joules) which is referred to as the RMS value (root mean square average) and this comes about because DC is constant voltage whereas AC is an alternating voltage which ranges so many times per second (the Hz value) between +ve and -ve. During this cycling there is a point where the EMF is at zero volts as the sine wave transitions from positive to negative. Therefore, using the RMS average value, you only get about 0.707 the amount of effort from 12v AC as you would from 12v DC assuming all other factors remain constant.

See? Clear as mud.

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Aye Captain, it will be a privilege Sir. Just remember, you cannae change the laws of physics!