Car Electrics for Dummies (well, for me!)

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Water247

Water247

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Bit of a hobo but often around the Surrey/Berkshir
Lovely Landy Owners,

I am getting to grips with my wiring in my Series 3 and realise I am just lacking the basic knowledge about car electrics. So my understanding is (please don't laugh, or if you do at least have the decency to correct me :D):

I have a negative earth system, where 12V DC flows from positive to the negative terminal (earth). The return to the negative terminal is done via the car chassis (ideally via an unprotected/unpainted part).

Because electricity requires a flow something will not work if just hooked to the live, only once leccy is allowed to flow 'through' to the earth will something show any life.

Any 12V appliance/component (terminology?) can essentially be run off this as long as it has the positive and negative hooked up correctly.

How am I doing so far? Presuming the above is correct - and please point out if it's not - I am left with a few questions I don't seem to figure out a definite answer for.

1) If electricity is using the cars very chassis as part of the circuit why do we not get electrocuted?

2) Is it correct that 12V is not really enough to do any harm? The sparks you can get from the battery at times look to me as though they could sting a bit! :( For example, if I just touch the positive terminal will I not get shocked as my feet are grounding me (give or take rubber soles). What if I touch the Pos. and the Chassis or even both polarities together with each hand??

3) Is 'shorting' the circuit effectively just giving a low resistance or highly conductive (same thing?) route from positive straight to negative? I understand this to be dangerous but how does it differ from Positive -> Component -> Chassis -> Negative? Is it just that the resistance of this path is lower or something to do with the distance??

Think I will leave that there as don't want to overwhelm/bore you all. You can probably tell I am keen as mustard and clueless :eek: ...though like to think I am perfectly capable if I take baby steps and listen your wonderfully good advice that is about to follow ;)

Cheers,

Andy.
 
Water247 said:
Lovely Landy Owners,

I am getting to grips with my wiring in my Series 3 and realise I am just lacking the basic knowledge about car electrics. So my understanding is (please don't laugh, or if you do at least have the decency to correct me :D):

I have a negative earth system, where 12V DC flows from positive to the negative terminal (earth). The return to the negative terminal is done via the car chassis (ideally via an unprotected/unpainted part).

Because electricity requires a flow something will not work if just hooked to the live, only once leccy is allowed to flow 'through' to the earth will something show any life.

Any 12V appliance/component (terminology?) can essentially be run off this as long as it has the positive and negative hooked up correctly.

How am I doing so far? Presuming the above is correct - and please point out if it's not - I am left with a few questions I don't seem to figure out a definite answer for.

1) If electricity is using the cars very chassis as part of the circuit why do we not get electrocuted? coz yu cant short the chassis to anything with yo body.

2) Is it correct that 12V is not really enough to do any harm? The sparks you can get from the battery at times look to me as though they could sting a bit! :( For example, if I just touch the positive terminal will I not get shocked as my feet are grounding me (give or take rubber soles). What if I touch the Pos. and the Chassis or even both polarities together with each hand?? A slight tingle at most, because your body is a high resistance. If you were soaking wet yu might suffer burns. DONT CHECK THIS!

3) Is 'shorting' the circuit effectively just giving a low resistance or highly conductive (same thing?) route from positive straight to negative? I understand this to be dangerous but how does it differ from Positive -> Component -> Chassis -> Negative? Is it just that the resistance of this path is lower or something to do with the distance?? tiz all in the resistance - remember V=IR

Think I will leave that there as don't want to overwhelm/bore you all. You can probably tell I am keen as mustard and clueless :eek: ...though like to think I am perfectly capable if I take baby steps and listen your wonderfully good advice that is about to follow ;)

Cheers,

Andy.
Click to expand...


ps - dont worry too much - you are perfectly safe. just make sure you have a CO2 fire extinguisher to hand.
 
12v is not high enough to give you an electric shock - you need about 80 to 90 volts DC to cause any damage and only under very exceptional circumstances. You also have to complete a circuit with your body to get shocked, from the battery +ve terminal to the -ve one. The few thousand volts generated in the ignition system won't kill you either - there's not enough current flowing.
Where 12v can be dangerous is when you short it out - connect the battery positive side to the chassis. Lead acid batteries have very low internal resistance so, in the event of a short there's little to stop a high current flow and this can generate a lot of heat. I't's a good idea to remove wrist watches and rings before working on car electrics 'cos shorting out the wiring with one of these can lead to severe burns. That's why we use fuses, a weak link in the circuit which should melt quickly if there's a short. Quite a few of your landy circuits are, however, unfused so go carefully.
Yes, as previously mentioned, it's the resistance of the components that limits the current flowing in any circuit. V=IxR indeed. Look up Ohm's law on Google for more details.
Just to completely confuse you though, the current is formed by electrons flowing through the wires. Scientists had sussed the idea of a current long before they discovered the electron and had assumed it flowed from positive to negative. Unfortunately electrons actually go the other way but by the time they found this out it was too late and to this day all the electrical calculations that are direction sensitive use this mythical "conventional current" flowing from positive to negative in the sums!
 
oxides said:
Just to completely confuse you though, the current is formed by electrons flowing through the wires. Scientists had sussed the idea of a current long before they discovered the electron and had assumed it flowed from positive to negative. Unfortunately electrons actually go the other way but by the time they found this out it was too late and to this day all the electrical calculations that are direction sensitive use this mythical "conventional current" flowing from positive to negative in the sums!
Click to expand...

You learn something new every day ;)
 
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