"msg" wrote in message
...
Arfa Daily wrote:
snip
JHC !!! Do you not understand the word "modern" ? Do you not understand
the phrase "... deals with monitors of all types on a daily basis" ? I
repair this stuff all day every day for a living. I have done for over 35
years. I cannot remember the last time I saw a piece of kit of any
description, which employed a positive ground. My friend, who owns a
computer repair shop, and has done for many years, cannot remember the
last time - if ever - that he saw a monitor with an external power
supply, that was not negative ground with the connector sleeve as the
negative connection.
With so much interconnectivity between household items now, there has had
to be a degree of standardisation on this issue, and it has evolved
through a general concensus amongst manufacturers, that negative ground
will be the convention.
snip
I have resisted commenting but can do no longer; I don't know about the
U.K.,
but I frequently encounter negative center coaxial power equipment in my
work. These are not monitors, but a variety of consumer and industrial
portable devices. When the connector is not labeled and I don't have
docs,
I will physically inspect the internal wiring or the pcb that hosts the
power
connector and also do resistance measurements between ground planes and
the
power connector contacts to determine ground. The assumption that ground
planes are negative is a given in most instances. As for the subjective
label "modern", that is a religious issue that shouldn't be a factor in
good electronics practice 
Michael
Michael. I'm not quite sure exactly what you are saying here. I do not doubt
that you encounter equipment with a negative pin connection on the coaxial
DC connector. I have not disputed this during this thread. In fact, I
actually said in my original reply to the OP, that although these days, pin
= "+" is the common convention, it is by no means cast in stone. I'm sure
that even though you do have dealings with negative pin equipment, you would
concede that positive pin is by far the more common at this point in time,
and has been for some years. Irrespective of which pole of the connector is
the positive one, you seem to accept that ground being negative is the "...
given in most instances", which is what most of the controversy generated
within the thread, has been about. So as far as I can see, we are both 'on
the same page'.
I don't understand what you are saying about the word "modern". It is quite
a well defined word, and fits well, in this context, with the dictionary
definition
http://dictionary.reference.com/browse/modern
particularly with the entry that refers to it being something that is "not
obsolete". Whilst you are correct that it is a subjective word, in the case
of electronic equipment, I would contend that most electronic engineers
would infer something of the order of 8 -10 years to be meant, when calling
electronic equipment "modern".
And Bill.
I will now explain why your contention that your method will work under all
circumstances, is not valid. You are quite wrong with your assumption that
all of the circuits in a piece of equipment are stacked up in parallel
across the DC input socket. Whilst this might have been the case some years
back, the DC connector on "modern" (infer whatever period you like from that
word) equipment, usually connects straight into some form of internal
ancilliary power supply, or a regulator or regulators, which are often
switching types.
The reasons for this are manifold, but include the fact that most modern
equipment does not contain circuitry that runs just from 12v, which is a
typical 'standard' value for external power unit equipment, and also
efficiency, which dictates the regulators typically being switchers. The
various circuits contained within the equipment, are connected to the back
end of these regulators, and are thus not connected to the DC power socket
in any way.
Often, the input to this regulator circuitry, contained within special
purpose ICs, will not produce any meaningful ohms reading, when subjected to
the low test voltage from a multimeter. So, you are just as likely to read a
virtual open circuit across the socket, irrespective of which way round you
have your meter. This, in itself, will not help you to determine the
polarity. But worse. If the unit employs a shunt protection diode, when your
meter is connected //backwards// to the correct polarity, you will get a
reading of 700 ohms or so, but when it is connected the //correct// way
round, you may well read infinity or near. By your definition of how your
system works to determine polarity, that would give you a clear indication
of which was the correct polarity, but would actually yield the *INcorrect*
polarity.
As for external metalwork not being connected to the internal common ground,
these days, that is rare. I do come across the situation sometimes, on AV
amps, where the RCA socket sleeves are floating with respect to the chassis,
but it is the exception rather than the rule, and is done to help alleviate
potential ground loop issues when connecting to other equipment.
As far as the polarity of the ground goes, I am prepared to say that in my
considerable experience, on modern equipment designed for the consumer
market, it is always negative. And that really is about as cast in stone as
anything in electronic design ever is. I could of course be wrong on this,
but if anyone wants to correct me with specific examples - remembering
"modern" and "domestic", I'm listening, and willing to modify my position on
it.
I say again, that the OP asked a simple question, to which there was a
simple answer. I don't really believe that there was any need to muddy the
waters to the extent of all of this silly stuff that has been put forward,
but hey - ho. I guess it all makes for an interesting life ... d;~}
Arfa