In Arfa Daily typed on Fri, 28 Mar 2008 16:07:57 GMT:
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