----- Original Message -----
From: David Chapman
To: amphicar-lovers@yahoogroups.com
Sent: Saturday, September 11, 2004 10:11 AM
Subject: Re: [amphicar-lovers] Positive ground
> Ever wonder why the Amphicar is positive ground? I asked a mechanic
> from England and his answer seemed reasonable.With the high humidity
> in England, the automakers felt that the positive ground reduced
> electrolises within the car therefore less metal deterioation. With
That is basically correct, if using copper wire and if the wire gets wet and
it is exposed to air then the positive side of the circuit corrodes - a form
of electrolisis.
This is much worse at high currents so was a problem on 6V cars (remember
volts X amps = watts) but was an even bigger problem on pre war cars which
had fabric insulation on the cables - this let both air and water through.
Lucas were world pioneers of automotive electric systems and so set the
positive earth standard.
By about 1964 the advent of 12v electrics (mainly for better lighting and
radios), PVC covered cable and also the use of steel instead of copper cable
(because copper was very expensive in the 1960s) meant the electrolisis
problem went away. I think it was the Japanese who started using negative
earth. In the UK it was about 50/50 in the mid 1960s and most accessories
had a positive/negative switch. By about 1968 when alternators started to be
fitted as standard to British cars a decision had to be made and a standard
adopted because the early alternators would blow up if the battery was
connected backwards. I think it's also easier to make a negative earth
alternator, something to do with the diodes inside.
I'd say 80+ % of the classic vehicles here that were originally positive
earth in the UK have been converted, most so an alternator can be fitted but
also to avoid toasting things like cell phone chargers !
David Chapman in the UK
I'll just add a few anecdotal and long-winded comments to Dave's reply.
In the USA, where I doubt that Lucas had much influence, the 6 VDC, positive
ground standard was pretty universal in 1955 (as I can attest by having owned a
1955 Studebaker). However, by 1960, the standard had changed to 12 VDC, negative
grounding (my benchmark is a 1960 Chevy that I also owned).
The 6 VDC to 12 VDC migration is easily understood. Power losses are worse with
low voltages; if you want to maintain the same electrical power movement, but
also want to save money with smaller copper wires, then increasing the voltage
is the only way to do it. By doubling the voltage, you could halve the amount of
copper. Saving a few dollars on materials cost translates to even more dollars
shaved off the sales price (or a bigger profit margin at the same price).
We are now on the edge of a new vehicular standard, the 42 VDC system. New
vehicle designs need to move much more electrical power around the car (for
things like high intensity lights, adaptive suspension, transmission control,
passenger comfort), and 12 VDC is just too lossy. BTW, although battery cost
does go up slightly with the higher voltage, everything else (light bulbs,
LED's, alternators, gauges, motors & solenoids) all either stay the same or get
cheaper and smaller. Going to the higher voltage has more benefits than just the
copper savings; any weight reductions also help handling and operating economy.
Discussing polarity is pretty close to religion and politics. It is my opinion
that, in an automobile, "grounding" polarity is totally unimportant.
My first point is that an auto is a closed system, there is no "ground", no more
than in your cell phone or electronic watch. There is CONVENTION, and that is
important. Makers of gadgets like alternators and radios use polarity sensitive
parts, and they often (for cost and simplicity) make the metallic housings of
their gadgets part of the electrical system. Who would want to pay for an
electrically isolated alternator that needed TWO heavy-gauge electrical
connections?
I think the negative ground was influenced by the shift from 6 VDC to 12 VDC, in
that industry wanted to dramatize the incompatibility. In order to prevent
consumers from mixing cars and accessories, I think the polarity was flipped to
further emphasize the new system.
Everybody sooner or later cites boats for clues to the polarity standard. Well,
boats are used in a much different environment that cars; that big puddle you
float in has chemical and electrical properties that you have to consider. Boats
also spend much more time IN the water than does an Amphi, and most (personal)
boats don't have a steel hull.
Boats often use a sacrificial electrode (like a block or pod of zinc) that
forces an electrical current in such a direction that the sacrificial electrode
is corroded by losing atoms into the water. Your sacrificial electrode gets
smaller and pitted, but your monel shafts and rudders don't! (Actually, this is
an electroplating process, and the "protected" metal gets a thin layer of metal
plated onto it.) Boats can also electrically force a current from an on-board
power supply to ensure the protection of the machinery. Boat corrosion really
gets serious when you connect to shore power, and introduce the possibility of
serious ground currents through the AC power (and you usually have an AC to DC
trickle-charger for the marine batteries, so that ties your DC system to the AC
power).
The key to understanding marine corrosion is to understand that it's all related
to current flow and requires a "circuit" that completes the path for the
current. In an old-fashioned car (gee, am I talking about an Amphi?), the
electrical system (circuit) is pretty simple. The battery is connected to the
frame by a short, heavy cable. We may call this "ground", thinking that the
chassis frame is a ground, but that's not true. The car sits on rubber tires,
and these keep the car isolated from true ground (even more so when the road
surface is a non-conductive asphalt; concrete is a conductor, albeit a very poor
conductor).
The other side of the battery connects to two main points; one, the ignition
switch, and two, the starter relay. No current flows through either until you
"command" the flow (by turning the key, energizing the Amphi's circuits and
momentarily closing the starter relay to initiate cranking). With those switches
OFF, there is no current flow through the battery, so the polarity is irrelevant
to the worry about corrosion currents.
When an Amphi is in the water and running, there is a possibility that current
flow will go from various accessories back to the battery through portions of
the steel hull. This will lead to slight variations of electrical potential
along the hull, and this WILL create corrosion somewhere. But this exposure is
very slight, and happens for just a short time. I really think the probability
of noticeable corrosion is just about zero from this.
Amphis do have some dissimilar metal exposed to the water. There is steel
everywhere, but some of the bronze or brass bushings are wetted. And so are the
aluminum faces of the differential case. Aluminum, brass & steel; now we have
some (uhh sorry) potential for electrolysis. But you would have to let you Amphi
sit for days and months in the water for this to build up to a problem. (Boats
don't suffer corrosion in the boatyard!)
As I live in an arid part of the world, maybe I just don't understand the
problems faced by Amphis that live in perpetual salt fogs, so I apologize in
advance. But it seems to me that, so long as you hose the salt water off your
Amphi, and let it dry out, you shouldn't have any corrosion worries at all.
Ed
El Cajon, CA USA
67 Rust Guppy
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