Re: Checking a BC548B NPN Transistor?

On Sun, 17 Jun 2007 10:10:04 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


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Well, I don't know about you, but when I use an analog multimeter I
usually have a pretty good idea of what I'm looking for if I'm using
it to measure voltages, and even more so if I'm using it to measure
current. I relegate 'pot luck' to those situations where I'm trying
to determine the polarity of an unmarked diode or the type (PNP or
NPN) and pinout of an unknown transistor.
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I don't take pot luck either when measuring voltages but sometimes
accidents do happen. I figure it out before I start probing - as most
experienced techs do.

I agree that the polarity switch would appear to find most use when
testing semiconductors. HOwever, even without such a switch I don't
find it a problem. For example, I test a loose transistor on the bench
by simply having a lead in each hand and probing between pins in order
to first determine the base lead. If I get conduction to two other
pins when the black lead is on the third I know it is NPN and the
black lead is on the base. And similarly for a PNP with the red lead
on the base. I don't make permanent connections for this simple test
since it is not necessary and a polarity switch would not be any
advantage. Only when testing to determine which of the other two pins
is emitter and collector do I make permanent connections. For this
test I connect the meter leads to the two pins other than the base pin
(pot luck principle). To check for NPN I connect a damp finger ( or
short cct) between the black lead (+ve) and the base pin and if it
conducts I know the black lead is on the collector and the red lead
must be on the emitter. If it doesn't conduct I know the leads are
reversed and I swap them. In the case of a PNP the damp finger (or
short) goes between the red lead (-ve) and the base. If it conducts
the red lead is on the collector and if it doesn't I swap the leads.
If I don't see any conduction at all I carry out further testing to
determine what device I have or whether it is faulty or has low gain
(some power devices may not give an indication with a damp finger).
The situation where the leads are permanently connected to the device
is the only real advantage I can see for having a polarity switch on
the meter. But this function alone (to avoid swapping leads) would
not be sufficient reason to include it in my opinion.


Being able to reverse the polarity with the switch (after
the event) is no more of an advantage than lifting one lead off when
the pointer swings the wrong way.

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Except that simply lifting a lead won't do it. Since you need to
swap leads, the lead on chassis ground will have to be removed and
replaced by the lead formerly used as the probe and Vice Versa.
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I suppose I was trying to say that inexperienced users who have a
meter with a polarity switch may forget to check the position of the
switch before using the meter. You will no doubt agree that because
the switch can be left in either position between uses, that unless
the switch is checked, the lead polarity can be as indicated by lead
colour (red +ve) or the reverse (black +ve) on dc and resistance
ranges. The probability of failing to check is ever present and where
not done can produce hard slam situations. I maintain that this is
less likely to occur where the polarity of the leads is fixed and
immutable and all users of such meters know this fact. Where the
switch polarity is such as to cause a reversed reading or a "hard
slam" then the time taken to use the polarity switch is excessive and
won't protect the meter pointer. I agree that using the switch for
this function is not why it has been included, but does everyone
understand this?

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Again, simply lifting the red lead won't do anything except
disconnect the red lead from the circuit. If you expect the meter
to read upscale you'll need to actually swap the leads. That is,
connect the lead that was formerly the probe, to ground, and Vice
Versa, as mentioned earlier.

Experienced users would be unlikely to make an error and in most cases
they would probably be probing with a lead in each hand. If an
unexpected polarity were encountered it doesn't take any longer to
swap leads than it does to move the polarity switch on the meter. It
will only be where both leads are permanently connected - and never in
the case of undetermined dc voltage polarity - that the polarity
switch will be of use. In fact when measuring DC voltages the polarity
switch would be as useless as the moving coil reversal button on my
AVO. The only real use for the polarity switch is when testing
semiconductors, and I maintain that its benefit even there is a moot
point.


As far as looking for the polarity reversal switch goes, it seems to
me that instead of me "making a big deal" out of this, you're
grasping at straws in order to try to prove that the polarity
reversal switch is largely fluff.
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Well that is your opinion. As I said before, because the polarity
switch can be left in either position the lead polarity can change
from use to use unless the switch is checked beforehand - an
additional function which is vital to avoid any mishaps. To my mind it
is far simpler and less confusing to have a fixed polarity rather than
having to check which polarity my leads are whenever I want to use the
meter to measure dc voltage... Horses for courses...

The momentary polarity reversal switch sounds kind of inconvenient
(and dangerous) to me in that to use it you've either got to press
it when you get a downscale reading, which means you've got to go
looking for it and press it while keeping the probe on the point of
interest and getting the reading, or press it first and then go
looking for the point of interest. Either way requires doing three
things at once, which can be kind of confusing.
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Indeed and, because of that, claimed that it had no polarity
reversal switch. Kind of like a carpenter "forgetting" that his
favorite claw hammer can also be used to _remove_ nails, eh?

You are being a bit hard I think. If the function on the AVO is of
dubious value (as I have said it is) and is never used, then it is
easy to overlook that it is even there. Your criticism might be valid
if the button were frequently used but the analogy of the carpenter's
hammer is totally facetious. The claw function would be in use
frequently and would be impossible to forget, unlike the reversal
button on the AVO.


Also, because of the Wiki article making the claim that on _all_
analog multimeters (when the OHMS ranges are being used) the voltage
on the minus input will be more positive than the voltage on the
PLUS input and your buying into it without even checking it out is
the reason this thread is still going on and you're desperately
trying to defend your untenable position.

As I said before, I erred in not including a qualifier in my statement
so yes, by not including one I did make the error of meaning "ALL"
meters. Perhaps I am just guilty of being ignorant of the fact that of
all the manufacturers of non-electronic analog multimeters, the only
ones to include a polarity switch are from the USA. I am also guilty
of being ignorant of these meters because they are not commonly found
in the rest of world. The total number of users of meters with a
polarity switch in the USA would be only a relatively small percentage
of the total number of users of analog meters outside of the USA, so I
claim that my statement referred to the majority of such analog meters
found today.

And just before anyone else mentions they know of analog meters which
don't have a polarity switch but which put out +ve on the red lead
when on resistance ranges, I'll wager that in such cases the meter
will be an electronic unit a-la Philips 2503 or a modern Gossen
Metrawatt Metraport 3A
http://www.gossenmetrawatt.com/resources/tt/mport3a/ba_gb.pdf
Note that the NPN and PNP designations on the transistor symbols
diagram on p.21 are reversed (the symbols are correct). This meter is
fully electronic and on resistance ranges the red lead is +ve and the
black lead is -ve. Their non-electronic meters such as the MetraHit
1A/2A http://www.gossenmetrawatt.com/resources/tt/hit1a_2a/ba_gb.pdf
use a 1.5V battery for low resistance ranges and semiconductor testing
similar to AVO, Triplett and Simpson analog meters. Note on p.18 the
instruction that the common connection lead (which is black btw) is
+ve, as it always has been on the majority (ALL?) non-electronic
analog meters manufactured outside the USA.

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Looking at the analog products from both Triplett and Simpson, they
still produce analog meters with the polarity switch, but I have not
been able to locate any non-electronic meters of similar types from
other makers outside the US which a include a similar function. It
seems to have been a function on US brand meters only eg, Triplett
and Simpson.

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Good ol' Yankee ingenuity!

Yes, but in the eyes of manufacturers and users of such equipment
outside the USA, obviously ingenuity which is of dubious value, and
therefore not considered a necessary item.


**RH** On the AVO, No. The switch simply reverses the moving coil
meter connections. I have never had cause to use it, nor do I regard
it as necessary or desirable.

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Yes, it's probably best not to mess with it since it's only
switching 37.5µA.

I agree. I can vaguely remember my instructor telling me much the same
back in the 50's. He said "using this button can get you into more hot
water than it is worth - don't use it".

John, (and any other users of Triplett or Simpson meters with a
polarity switch), I had no knowledge of any meters with such a
function before posting and I acknowledge that you are accustomed to
the functions of your meters and think they are great. I apologise for
getting your back up but I also expect a little latitude for my
unintentional ignorance of these meter brands. US users on the other
hand also have to accept that the vast majority of users outside the
USA where these brand meters are not common, will have a different
viewpoint. The fact remains that for non-electronic analog meters the
rule "when on OHMS, black is +ve" is true. And there must be many
thousands of cheap analog meters without polarity switches being used
even in the USA today.
.