Re: Checking a BC548B NPN Transistor?

On Sun, 17 Jun 2007 07:23:14 GMT, Ross Herbert
<rherber1@xxxxxxxxxxxxxx> wrote:

On Thu, 14 Jun 2007 17:07:02 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


---
<JF> The polarity switch is there precisely so that you don't have
to switch leads over. A typical scenario might be where you need to
measure voltages on, say, an audio amplifier with positive and
negative rails and various positive and negative voltages around the
amp. Normally, one would connect the black lead to 0V, (ground)
then use the other lead to probe for the various voltages, knowing
that an upscale reading would indicate a positive voltage and a
downscale reading, negative. In order to get the negative voltage
to swing upscale, one would use the polarity reversing switch in
order to keep from having to swap the probes back and forth which is
much more convenient, especially if ground is chassis and you're
using an alligator clip to connect to it.
-----

**RH** Yes, I can see where the polarity switch would have been used
to avoid swapping the leads in the example you give. I concur that it
can be an advantage in certain circumstances but even with such a
switch it does not rule out a judgement of the correct placement of
the leads in the first place to read dc voltage. If you are just going
to take pot luck on the polarity of the voltage to be observed you
will have a 50/50 chance of getting the leads the wrong way round
anyway, irrespective of which position the polarity switch is in
initially.

---
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.
---

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.

---
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.
---

For example,in the case of a -VE
chassis it is usual to permanently connect the black lead to chassis
while placing the red lead on the point of interest using one hand. To
my mind it would be quicker to simply lift the red lead off when the
pointer moves the wrong way because you don't have to go looking for
the polarity switch. I suppose its a case of "horses for courses" or
which ever method you prefer - much of a muchness to me.

---
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.

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.
---

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.
---

**RH** On the AVO the momentary switch simply reverses the connections
on the moving coil meter movement. I agree that this is not really the
best solution and it is so rarely employed by users that it is easy to
forget it is there (as I did forget).

---
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?

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.
---

<JF> I don't believe Triplett or Simpson products were unavailable
outside of the US back then, (whenever that was) just not as easily
(or possibly as cheaply) available as your own domestic stuff was.


**RH** They may not have been entirely unavailable but due to the
preponderence of locally produced meters of at least the same quality,
particularly in Europe, Triplett and Simpson would have still been
quite uncommon in this region. In Australia and NZ they would have
been even rarer - an example of Triplett military test eqpt left over
from WW2 would be about as much as we would have seen here. During the
era when analog meters were common, Australia produced her own high
quality test equipment from University, Paton and AWA etc. An example
of Paton and other Australian test gear from the 40's is shown here
(the Radio & Hobbies cro is a DIY project from the 50's).
http://www.southcom.com.au/~pauledgr/Testgear.htm

I have been involved with using meters since the 1950's and I had
never heard of Triplett and Simpson let alone seen one in Australia
until perhaps the 70's. I doubt there would be more than one or two of
these brands still in use here these days.

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.

---
Good ol' Yankee ingenuity!
---

----
BTW, on your meter, when you press the polarity reversal switch on
the resistance ranges do the probe polarities reverse?
----

**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.

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


--
JF
.

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