In article ,
Chris Bolus wrote:
On Tue, 09 Sep 2008 18:49:43 +0100, "Dave Plowman (News)"
wrote:

In article ,
Richard Green wrote:
I've never been clear whether car electronics are made with negative
earth because they have to be, or purely through convention, most
cars having negative earth when they were introduced. Certainly some
of the early radio/cassette players and accessory rev counters were
switchable.

The first transistors were geranium and PNP so basically positive
ground.

Not all. When I was doing a lot of electronics in the 70s, one of the
most common transistors was the AC128 - an NPN germanium.

Indeed. I was generalising. But PNP transistors were the norm with
geranium in the same way as NPN is the norm with silicon. If you look at a
catalogue from the '60s you'll see that NPN were rare and expensive.

(FWIW they're
now very sought-after by guitar fx builders!)

That says much about their performance. ;-)

And very expensive. Silicon ones are NPN and negative ground - and
they're also very much cheaper to manufacture. Or were when they
arrived.

But of course, apart from where heatsinking is required, there is no
need to connect any part of the transistor to ground. So an electronic
circuit board can be made with any polarity provided insulating mounts
are used.

So by and large it's convention.

Cars use the chassis as one leg of the supply. So anything which had
peripherals - like injection etc - would have to use fully insulated
sensors with wired returns.

--
*I don't suffer from insanity; I enjoy every minute of it.

Dave Plowman London SW
To e-mail, change noise into sound.

Thank you all very much. The consensus appears to be 12 volt Positive
earth.

I appreciate the response. I am in USA and have wanted a pre war Rover
10 or 12 for years. Having seen one advertised (in the UK) I was
interested in some of the details I had not explored before before I
took the plunge abnd contacted the seller

Thanks a lot



On Sep 8, 8:03*am, JHB wrote:
Hi:

Does anyone off hand know the voltage and polarity of the pre war
Rover electrical systems?

Thanks

John Baker

Dave Plowman (News) wrote:
In article ,
Richard Green wrote:
Pre-war Rovers used negative earth until part-way through the 1936 model
year, when they changed to positive earth. All models used conventional
earth return through the chassis.

It's interesting that at least some of their models reverted to negative
earth long before electronics made this necessary. And of course Rolls
Royce stayed with negative earth.

Hello Dave,
I've never been clear whether car electronics are made with negative earth
because they have to be, or purely through convention, most cars having
negative earth when they were introduced. Certainly some of the early
radio/cassette players and accessory rev counters were switchable.
Regards,
Richard
Tuesday 09/09/2008, 16:18

Looking for something completeyly different I came across this in
"Automobile Electrical Maintenance" published in 1938.

-----------------------
Positive Earthing

Hitherto it has been the custom in single-pole systems to earth the negative
pole of the battery to the chassis frame. More recently, however, it has
become the practice to earth the positive terminal since there are certain
advantages in the latter method. In the first place the h.t. distributor arm
of the ignition unit becomes negative to the distributor contacts so that it
does not burn away as before. Secondly, the central electrode of the
sparking plug is now negative to the metal shell of the plug, so that any
corrosion, or burning away, is now confined to the metal shell and not to
the central electrode. Another advantage lies in the relatively lower
temperature of the central electrode, whereby a lower "high tension" voltage
can be employed to produce the spark.

-----------------
I understand some of the reasoning but not the bit about lower voltage.


Malcolm

In article ,
Malcolm wrote:
Looking for something completeyly different I came across this in
"Automobile Electrical Maintenance" published in 1938.

-----------------------
Positive Earthing

Hitherto it has been the custom in single-pole systems to earth the
negative pole of the battery to the chassis frame. More recently,
however, it has become the practice to earth the positive terminal
since there are certain advantages in the latter method. In the first
place the h.t. distributor arm of the ignition unit becomes negative to
the distributor contacts so that it does not burn away as before.
Secondly, the central electrode of the sparking plug is now negative to
the metal shell of the plug, so that any corrosion, or burning away, is
now confined to the metal shell and not to the central electrode.
Another advantage lies in the relatively lower temperature of the
central electrode, whereby a lower "high tension" voltage can be
employed to produce the spark.

----------------- I understand some of the reasoning but not the bit
about lower voltage.

I suppose it means if there's less erosion you don't need as high a
voltage to cope with an increased gap - or that resistance increases with
temperature. But a bit tenuous.

I've a feeling that the above article misses out that positive earth
increases body corrosion for the reasons given.

--
*The man who fell into an upholstery machine is fully recovered*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Malcolm wrote:
Looking for something completeyly different I came across this in
"Automobile Electrical Maintenance" published in 1938.

-----------------------
Positive Earthing

Hitherto it has been the custom in single-pole systems to earth the
negative pole of the battery to the chassis frame. More recently,
however, it has become the practice to earth the positive terminal
since there are certain advantages in the latter method. In the first
place the h.t. distributor arm of the ignition unit becomes negative to
the distributor contacts so that it does not burn away as before.
Secondly, the central electrode of the sparking plug is now negative to
the metal shell of the plug, so that any corrosion, or burning away, is
now confined to the metal shell and not to the central electrode.
Another advantage lies in the relatively lower temperature of the
central electrode, whereby a lower "high tension" voltage can be
employed to produce the spark.

----------------- I understand some of the reasoning but not the bit
about lower voltage.

I suppose it means if there's less erosion you don't need as high a
voltage to cope with an increased gap - or that resistance increases with
temperature. But a bit tenuous.

I've a feeling that the above article misses out that positive earth
increases body corrosion for the reasons given.

--

ISTR that the issue of body corrosion due to polarity was not generally
recognised in the thirties; it was only some years after the war that it
was confirmed as an issue and the general move back to negative earth
occurred.

Of course, BMC took a novel approach by fitting sacrificial anodes. They
called them doorhandles.

GMacK

The Rover 10, 12 & 16hp saloons & sports were 12volt positive earth
according to wiring diagrams which I have.

"JHB" wrote in message
...
Hi:

Does anyone off hand know the voltage and polarity of the pre war
Rover electrical systems?


Thanks

John Baker

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Roger
saying something like:

I thought that pre-war Beetles was a bit optimistic but a quick google
brought up the information that there were a few pre production
prototype Beetles made before war broke out. Production was actually
scheduled to begin in September 1939.

Istr reading that the very first production models were presented to
selected 'winners' of a lottery (fixed, I've no doubt) in various local
ceremonies held througout the Reich, as an incentive for the workers to
keep putting money into the VW pot. Of course, such events were
thoroughly covered by Joe Goebbels at the time and the cars were taken
back as soon as the cameras left, to be 'presented' to the next lucky
winner, on the promise that as soon as production really got going
they'd get a new car - real soon now.

There was some sort of workers' fund set up to produce the Beetle and if
the war hadn't happened the production VWs would have been filling the
streets, driven by grinning family men as they drove their flaxen-headed
Hans and Heidis to Youth Camp.
--

Dave
SE6a

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Geoff Mackenzie"
saying something like:

Of course, BMC took a novel approach by fitting sacrificial anodes. They
called them doorhandles.

Or subframes.
--

Dave
SE6a