"BillW50" wrote in message
. com...
In ,
Arfa Daily typed on Fri, 28 Mar 2008 16:07:57 GMT:
"BillW50" wrote in message
. com...
In ,
Arfa Daily typed on Fri, 28 Mar 2008 12:52:58 GMT:
[...]
As for a transistor radio that uses PNP transistors, I haven't seen
one that uses transistors at all for many years, let alone PNP ones,
so I think you might be struggling to fit that into my "modern"
category, also.
Well Arfa... they still use transistors (both NPN and PNP types) in
modern equipment. The reason you don't see them anymore is do to the
magic of minturization. But they are still there, just neatly
packaged into what is known today as the intergrated circuit (IC
chip). 
And I though I was pedantic !! Yes, of course ICs contain
transistors, and yes, I would accept that some of them may be PNP
types, depending on block function within the IC, but I don't think,
with the best will in the world, that this is the level of transistor
existence that William was referring to with his "transistor radio
using PNP transistors" scenario. In any event, in the case of an IC
taking a single polarity rail, it is irrelevant whether the
transistors inside are NPN or PNP or FETs or whatever. PNP
transistors are just used 'upside down', as are discrete PNP
transistors when used in any piece of single polarity rail equipment.
The ground is still (typically for //modern// equipment) the "-" side
of the power supply / battery.
Anyway, this is getting out of hand. The OP aked a simple question,
and I gave a simple answer. This afternoon, I was in a friend's
computer repair shop. Bear in mind that he deals with monitors of all
types and makes on a daily basis. I asked him how he would go about
determining the polarity of such a monitor, and he said that he would
stick one side of his ohm meter on one of the D connector locking
screws, and the other on each pin of the DC connector. When he found
the pin that read short to the connector locking screw, it was his
contention that he would have identified the "-" side of the power
supply. So that's pretty much exactly what I said. He also frowned
and shook his head, and said that he couldn't remember how many years
it had been since he had seen a DC connector that had the pin as the
"-".
Which is also pretty much what I said ...
Arfa
That would be okay if it were a negative ground system. But like what
William Sommerwerck mentioned, we don't know that. The way I would do it
is to ohm the the power in. And the lower resistance would be the correct
polarity. Although you would need another meter to read the polarity of
the ohm meter. As they are not standardized on multimeters. You could also
use a diode (or LED) to learn of the polarity of the meter as well.
--
Bill
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.
As for your method of determining polarity, it makes no sense at all, unless
you are assuming a series diode, which is quite unlikely in most modern kit,
as it represents a waste of power due to its forward voltage drop. It may
even have a shunt protection diode, in which case, your 'test' will ensure
that the polarity is determined INcorrectly. Even if the device did have a
series diode, depending on where the supply first goes, there is still no
guarantee that there will be any reading at all on a standard multimeter on
ohms. If there is not any diode - series or shunt - any reading of ohms
obtained across the input socket, is unlikely to reveal anything meaningful.
What is your experience in fault-finding, I wonder, to have caused you to
have formulated such a bizarre method, and believe that it would
uncategorically give you a correct result ?
Arfa