|
So why did the car battery earthing convention change from positive to negative earth?
|
Anything to do with the introduction of alternators?
(Pure guess, btw!!)
|
|
I heard that positive earth caused encouraged corrosion. As soon as this was discovered, it fell out of favour.
|
Spot on - positive electrodes rust faster than negative as you may remember from school physics/chemistry.
Whether this actually happens on a large car body is probably open to debate.
|
|
|
|
Anything to do with the introduction of alternators?
Cars were negative earth long before the introduction of alternators.
|
|
Something rings a vague bell about changing polarity on dynamos by touching a live wire on a terminal on the back of the unit.
|
Something rings a vague bell about changing polarity on dynamos by touching a live wire on a terminal on the back of the unit.
>>>>>>> Something else rings a bell if you get it wrong too.
Its the fire brigade one ;-)
|
|
Didn't have you down as a southern softie, Bell boy. :)
|
Going even further back, wasn't there a change from negative to positive earth?
I'm sure some here are old enough to remember. ;>)
|
It was to prevent galvanic corrosion of the bodywork.
In the period when cars were changing over, and a postive earthed car crashed with a negative earthed car, did sparks fly?
|
:)
Probably just had to replace all the fuses!
Just out of interest can an alternator work just as well on a positive earthed car (assuming its wired up for pos earth!)?
Edited by b308 on 09/05/2009 at 12:41
|
|
|
|
Something rings a vague bell about changing polarity on dynamos by touching a live >> wire on a terminal on the back of the unit.
Yep, did it years ago and it worked. Can't remember which terminal you flick with which wire but worked fine.
|
yes, you had to connect a piece of wire from the positive battery terminal to the small (i think it is called the field terminal) terminal on the back of the dynamo until you saw a spark, then it was re polarised.
You also had to do this to polarise a new dynamo when it was fitted.
|
|
To encourage the sale of car radios.
|
|
I had a 1971 Mini that was positive earth. However I managed to acquire a negative earth radio...so I mounted it on a piece of chipboard hung beneath the dash.
|
Did something similar with my positive earth Triumph Herald.
Provided the case of the radio was not earthed to the car, it worked fine.
I later changed the car to negative earth.
Everything worked OK except the fuel gauge.
|
|
|
|
|
As stated, a dynamo is capable of outputting either polarity. It is able to do that because the voltage polarity is determined by the polarity of the residual magnetism left in the iron when it is out of use. Iron is inside the field coils [they are wound round it] and the dynamo rotor [armature] spins in that field which generates voltage in the armature windings [very small at first] that appears at the brushes and the brushes are connected to the field coils [as well as used as output] and that drives more current through the field coils, and that increases the magnetism and so produces a higher voltage, and so on. The voltage rises until it reaches the level at which it is detected as high enough and then the regulator comes in to control the field-coil current so as to limit any further voltage increase.
|
|
|
|
|
|
|
|
|
From an electronics point of view, virtually all modern semiconductors operate with a positive supply and negative 'ground'. Attempting to fit these systems in a +ve earth vehicle would be almost impossible.
The earliest 'solid state' transistor radios employed germanium-based transistors, which operated with a negative supply and positve ground - presenting no problems with the +ve earth vehicles of the period. The move to silicon-based transistors - which require a +ve supply and negative ground - naturally suited negative earth cars.
|
|
|
